Tachometer needle jumps sometimes - cause?

[82DMC12]

I have noticed sometimes that my tach needle generally works smooth and steady but periodically it will jump up for a moment like it has a seizure or the throttle was barely blipped, and then it is fine again. This has been observed both at idle, like at a stop light, and while accelerating especially at higher RPM's. I can't predict it but it does it randomly during pretty much any drive. I don't notice any change in engine performance during this split-second jump.

I assume it means the signal is getting interference somehow but it's so brief that the needle doesn't jump more than say 20 RPM - where should I be looking? I did replace the impulse coil with a NOS coil a couple months ago when I had my distributor out.

Ignition coil is an Accel Super Stock coil and perhaps 20 years old. I have a new Bosch rotor, DMCMW repro cap, DMCMW ignition wires and coil wire, and NGK plugs, all installed this year and properly gapped.

Here's a video showing the tach jump at about 5 second in.

https://youtu.be/hBG6znVDcBU

What is the cause?

[Bitsyncmaster]

That is going to be hard to find the problem. What I would do is check the DC voltage from fuse #5 with an O-scope to see if the DC voltage is related to the jump. If it happens enough you can test if loads (headlights, AC etc.) turning on or off cause it.

[Ron]

You might try connecting a handheld tach and see if it also jumps -- Could be a loose connection feeding the dash tach (but no problem to the distributor).

[82DMC12]

Thanks guys. It was also suggested a loose PCB or the screw that holds it on. I'll actually be pulling my binnacle off perhaps next weekend for replacement so I'll check that.

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[David T]

I would start by replacing the coil. Many owners had problems with "performance" coils. This could be an indication of the coil beginning to fail.

[Jonathan]

Thanks guys. It was also suggested a loose PCB or the screw that holds it on. I'll actually be pulling my binnacle off perhaps next weekend for replacement so I'll check that.

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I'd also like to know because mine does the exact same thing in exactly the same way you've described it. I also think it's a loose connection or ground up at the console or the printed circuit, but I haven't gone back in and taken apart all of that to find out. Yet.

[82DMC12]

I'd also like to know because mine does the exact same thing in exactly the same way you've described it. I also think it's a loose connection or ground up at the console or the printed circuit, but I haven't gone back in and taken apart all of that to find out. Yet.I'm going to get a new OEM ignition coil and see how that goes first. It could definitely be the coil because this spring I realized it was hooked up backwards (!!!) but interesting to report I never had a problem all this time. This spring I reversed the wires to make it "right" and seemingly no change but I am noticing this random tach jump. Maybe related to reversing the coil? My idle hasn't been as smooth ever since I reversed the coil but I've made a lot of other adjustments lately too....

If that's not it, in about a week I'll be inside the binnacle for the first time in about 20 years so I'll see what I find there (I did replace the PCB board back then though!)

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[Ron]

I'm going to get a new OEM ignition coil and see how that goes first. It could definitely be the coil because this spring I realized it was hooked up backwards (!!!) but interesting to report I never had a problem all this time. This spring I reversed the wires to make it "right" and seemingly no change but I am noticing this random tach jump. Maybe related to reversing the coil? My idle hasn't been as smooth ever since I reversed the coil but I've made a lot of other adjustments lately too....

If that's not it, in about a week I'll be inside the binnacle for the first time in about 20 years so I'll see what I find there (I did replace the PCB board back then though!)

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$.03

Reversing the polarity on the coil will not hurt it, but it will reduce the spark. And, if the coil is causing the needle to jump, you should be able to hear the engine miss.

With all of the changes you've made, I'd suggest you go over all of the basic setting and tests.
If the (handheld;-) tach needle doesn't jump and there in no skip when using the timing light on each of the plug wires, I wouldn't purchase a new coil, except as a last resort stab in the dark.

[82DMC12]

$.03

Reversing the polarity on the coil will not hurt it, but it will reduce the spark. And, if the coil is causing the needle to jump, you should be able to hear the engine miss.

With all of the changes you've made, I'd suggest you go over all of the basic setting and tests.
If the (handheld;-) tach needle doesn't jump and there in no skip when using the timing light on each of the plug wires, I wouldn't purchase a new coil, except as a last resort stab in the dark.How do you hook up an external tachometer? I've never attempted such a thing. I see I can get a Bosch tach on Amazon for like $33.99.

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[David T]

Hooking up a coil "backwards" makes the spark jump from the ground electrode to the center electrode. It can cause the engine to not run as well as it should. Make sure it is correctly hooked up now, maybe it was right and now it is wrong. I would replace the coil even if it is good, too many stories of non-OEM coils causing problems and no one has said an aftermarket coil gave them any noticeable improvement. If it was backwards for a long time I would clean and regap the plugs or maybe just replace them (depends on how worn they are).

[Ron]

How do you hook up an external tachometer? I've never attempted such a thing. I see I can get a Bosch tach on Amazon for like $33.99.

Sent from my Pixel 3a using TapatalkA lot of D owners use an ACTRON Tach Dwell Voltmeter Model CP7605 (https://www.ebay.com/itm/255533042803?epid=1301858527&hash=item3b7ef4cc73:g:GmQAAOSwHUdiecSN) to set their dwell. (Another for $22.99 total, used (https://www.ebay.com/itm/175276540874?hash=item28cf4bc3ca:g:6bcAAOSwUWRievT I).) Like many others, you simply set the switch to "RPM", connect the Black clip to ground and the Green (or sometimes RED) clip to the coil terminal marked "-" (where the harness' White/Slate wire is connected).

[82DMC12]

A lot of D owners use an ACTRON Tach Dwell Voltmeter Model CP7605 (https://www.ebay.com/itm/255533042803?epid=1301858527&hash=item3b7ef4cc73:g:GmQAAOSwHUdiecSN) to set their dwell. (Another for $22.99 total, used (https://www.ebay.com/itm/175276540874?hash=item28cf4bc3ca:g:6bcAAOSwUWRievT I).) Like many others, you simply set the switch to "RPM", connect the Black clip to ground and the Green (or sometimes RED) clip to the coil terminal marked "-" (where the harness' White/Slate wire is connected).Oh Derp! I have the Actron meter already!

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[82DMC12]

I replaced the Accel Super Stock coil with a new Bosch blue coil considering the fact that I accidentally ran the Super Stock backwards for so long (and surprisingly it ran fine) but also because there was some red oil inside the main coil terminal making me wondering if the coil was cracked inside. Anyway, installed it easily and idled the car with my Actron meter set to RPM's. Luckily (?) the tach on my dash started jerking quite a bit at idle and I was able to confirm that the needle on the Actron was NOT jerking at the same time. So, I'm thinking I might have a bad connection at the binnacle. I'll be taking the binnacle out soon anyway so I'll give it a good lookover.

[82DMC12]

Update on my jumpy tach needle -

1) Confirmed the needle on my Actron external tach is not jumping when the dash gauge jumps, so it's not the coil
2) Replaced the coil anyway with a blue OEM coil because my Accel Super Stock seemed to have red oil leaking out of the main coil wire terminal
3) Pulled the binnacle off and found a bad connection on the main harness. Someone had cut a wire and tried to splice in another wire with a butt connector. The new wire was already cut off and gone but the remaining butt connector was not very tight. Cut it out and repaired the wire with a marine heat shrink butt connector
4) While the binnacle was out I cleaned the two plugs with electronic cleaner and tightened all the PCB nuts.
5) Started the car and no more jumping observed!...... until last night I was idling the car in my garage and saw it jumping every now and then. Damn!

What else should be I checking??

[DMC-Ron]

Since it's easy to check, connect up a voltmeter to the 12 volts and, with the car running, see if the alternator regulator is spiking. This is when having an old analog voltmeter is handy. Observe the tach and the voltmeter simultaneously and see if there is a coincidental twitch.
Ron

[Ron]

Curious as to what color the spliced harness wire was.

[David T]

What you really want to see is pulsating DC due to one or more shorted or blown bridge rectifier diodes. Not all meters are sensitive enough to see that on an AC scale. You can see it on a scope or a good alternator tester. That is why I recommend going to an auto store and have the alternator checked for free. Many will test the battery and alternator for free and they usually have good testers.

[82DMC12]

Curious as to what color the spliced harness wire was.

I will be back in there next week. I replaced my binnacle (future post coming on that!) and have it just set in and only the back studs are attached to the dash while I wait for a new plexi lens for it to arrive next week. I'll check the wires again and let you know...

[82DMC12]

What you really want to see is pulsating DC due to one or more shorted or blown bridge rectifier diodes. Not all meters are sensitive enough to see that on an AC scale. You can see it on a scope or a good alternator tester. That is why I recommend going to an auto store and have the alternator checked for free. Many will test the battery and alternator for free and they usually have good testers.

Hmm well I'm actually replacing my alternator next week because it's making some noise, is probably 20 years old, and only making 12.4V when the engine is hot and fans and headlights are running.

[82DMC12]

Hey guys, update time. I actually replaced my alternator, and problem didn't improve, in fact it got worse! Here's a video after about 30 minutes of driving. I still have a slightly elevated idle problem I need to sort out, but as you can see the tach is going nuts. It doesn't show up on my Actron meter.

I will check the connections on the back of the binnacle one more time. It's been said the tach itself is probably bad (the resistor on it perhaps). Is there anything else to check short of replacing the tach gauge?



https://youtu.be/W3Y_2wEeZ_M

https://youtu.be/W3Y_2wEeZ_M

[Jonathan]

Hey guys, update time. I actually replaced my alternator, and problem didn't improve, in fact it got worse! Here's a video after about 30 minutes of driving. I still have a slightly elevated idle problem I need to sort out, but as you can see the tach is going nuts. It doesn't show up on my Actron meter.

I will check the connections on the back of the binnacle one more time. It's been said the tach itself is probably bad (the resistor on it perhaps). Is there anything else to check short of replacing the tach gauge?



https://youtu.be/W3Y_2wEeZ_M

https://youtu.be/W3Y_2wEeZ_M

I had the intention of going in and checking the ground for the instrument cluster. I believe it is tied into the ground bolt that is on the corner of the centre console, but not 100% sure. I was going to trace that out better once I got in there. But check the grounds for the instruments is what I would suggest.

[82DMC12]

Curious as to what color the spliced harness wire was.The spliced wire was the black ground wire on the main plug. Maybe someone tried to install a backup ground? Anyway the wire was no longer there by the time I got the car. I don't think the ground could be the problem because all my other gauges and instrument lights are working fine and not flickering. What do you think? I'm thinking the problem is probably somewhere on white slate or green or the tach itself is bad.https://uploads.tapatalk-cdn.com/20220529/5ae79978ace201e611f419225cadb3b9.jpg

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[Ron]

Hey guys, update time. I actually replaced my alternator, and problem didn't improve, in fact it got worse! Here's a video after about 30 minutes of driving. I still have a slightly elevated idle problem I need to sort out, but as you can see the tach is going nuts. It doesn't show up on my Actron meter.

I will check the connections on the back of the binnacle one more time. It's been said the tach itself is probably bad (the resistor on it perhaps). Is there anything else to check short of replacing the tach gauge?



https://youtu.be/W3Y_2wEeZ_M

https://youtu.be/W3Y_2wEeZ_M
I'm leaning toward the W/S or tach as well.
Maybe coincidence, but it getting worse after having the binnacle off and other work might indicate a bad connection.

Anyway, if you don't find anything when you check out the connections again, remove the tach and hook it up to the coil, eliminating the wire harness and connection problems... (Connections are the same as with the Actron, except you will need to connect a hot (+) wire also.)
68509
If you are uncomfortable doing this with the binnacle off, try on another vehicle...


EDIT: Forgot to answer: I think the resistor could make it read wrong but not vibrate. You would be hard pressed to find another one...

[82DMC12]

I'm leaning toward the W/S or tach as well.
Maybe coincidence, but it getting worse after having the binnacle off and other work might indicate a bad connection.

Anyway, if you don't find anything when you check out the connections again, remove the tach and hook it up to the coil, eliminating the wire harness and connection problems... (Connections are the same as with the Actron, except you will need to connect a hot (+) wire also.)
68509
If you are uncomfortable doing this with the binnacle off, try on another vehicle...


EDIT: Forgot to answer: I think the resistor could make it read wrong but not vibrate. You would be hard pressed to find another one...

Thanks Ron. I will have my binnacle apart next week installing a new plexi lens. Maybe I'll try this tach hook up trick and see what I find. What do you think about the "trigger / stator air gap" - have you seen such a thing out of spec on a DeLorean and possibly cause this? I had my distributor apart this winter and replaced the pickup coil but it seems like there's only one way to put it back together and I didn't notice anything that can go out of adjustment during that process.

[Ron]

Thanks Ron. I will have my binnacle apart next week installing a new plexi lens. Maybe I'll try this tach hook up trick and see what I find. What do you think about the "trigger / stator air gap" - have you seen such a thing out of spec on a DeLorean and possibly cause this? I had my distributor apart this winter and replaced the pickup coil but it seems like there's only one way to put it back together and I didn't notice anything that can go out of adjustment during that process.YW!
Yeah, they can act odd if not installed to specs, but it seems it would be more consistent anyway....Unless the dash tach is way more sensitive than the Actron, I wouldn't think the trigger would be the culprit. And even if, I would expect the engine to be cross/misfiring, ether of which should be noticeable when listening to the engine.

[David T]

A distributor with bad gaps on the impulse coil will mess up the idle. Very important to get it gapped right. Any time you take the distributor out you should check the gaps. The rotor is soft metal and bends easily.

[82DMC12]

A distributor with bad gaps on the impulse coil will mess up the idle. Very important to get it gapped right. Any time you take the distributor out you should check the gaps. The rotor is soft metal and bends easily.David,

What would the symptoms be of this "rough idle"? I definitely have something weird going on with the idle quality, aside from this jumpy tach. I assume the idle quality would be "rough" all the time and not intermittent?

I actually ordered a smoke machine so I can try to finally zero in on my idle issues after a valley dive this spring.

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[82DMC12]

Tomorrow I'll take video of the idle so you guys can hear what I'm hearing.

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[82DMC12]

OK, I followed Ron's advice and installed my tachometer directly to the coil. I found that the needle continues to jump sometimes when hooked up that way. I then double checked with the Actron external tachometer and I don't see the same jumpiness happening. I think that pretty well proves that the tachometer electronics are faulty.

Any comments?

https://youtu.be/btd9xXf0YAk

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[Jonathan]

OK, I followed Ron's advice and installed my tachometer directly to the coil. I found that the needle continues to jump sometimes when hooked up that way. I then double checked with the Actron external tachometer and I don't see the same jumpiness happening. I think that pretty well proves that the tachometer electronics are faulty.

Any comments?

https://youtu.be/btd9xXf0YAk

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So... does one rebuild a tach? Or can a new one be purchased from one of the vendors?

[82DMC12]

So... does one rebuild a tach? Or can a new one be purchased from one of the vendors?

Looks like NOS tachs are certainly available, or for half price you can get a rebuilt main board. Remember the gauges are standard GM of the era. I decided to order this replacement board for half the price of a new tach so I know my faces will continue to match. A couple owners I know mentioned they bought this board and it works perfectly. Will report back once I get it installed next week.

https://www.ebay.com/itm/144016092576

[David T]

David,

What would the symptoms be of this "rough idle"? I definitely have something weird going on with the idle quality, aside from this jumpy tach. I assume the idle quality would be "rough" all the time and not intermittent?

I actually ordered a smoke machine so I can try to finally zero in on my idle issues after a valley dive this spring.

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On that one car, we tried everything, the owner then tried a 1200 ohm resistor in the impulse coil circuit and that helped. Then he told me the impulse coil was replaced and that was when I told him we had to check the gaps. They were all not right but one was way off causing the rough idle. As for your tach, some tachs are a lot more sensitive than others, others are damped a lot so you would not notice any 'jumpiness". Vacuum leaks are the main source of a bad quality idle and we actually did smoke test the car and fixed all the leaks and the idle was still cr-ppy. Do the smoke test and if that doesn't fix the idle (and you have good patterns from the fuel injectors) pull the distributor and check the gaps.

[82DMC12]

On that one car, we tried everything, the owner then tried a 1200 ohm resistor in the impulse coil circuit and that helped. Then he told me the impulse coil was replaced and that was when I told him we had to check the gaps. They were all not right but one was way off causing the rough idle. As for your tach, some tachs are a lot more sensitive than others, others are damped a lot so you would not notice any 'jumpiness". Vacuum leaks are the main source of a bad quality idle and we actually did smoke test the car and fixed all the leaks and the idle was still cr-ppy. Do the smoke test and if that doesn't fix the idle (and you have good patterns from the fuel injectors) pull the distributor and check the gaps.Very interesting, thanks for the input. Did you have to bend each reluctor tooth separately to close the gap? I think I have brass feeler gauges for this. Never done it before, any pictures anyone has of this process would be well received.

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[82DMC12]

All right. I'm going crazy! New tach board - no help. Pulled the distributor and checked the reluctor air gap - made a couple small adjustments and it's definitely good now - but no help with my problem. I thought for sure I had confirmed the tach was bad by hooking up directly to the coil but no dice. Still jumping all over the place randomly. I also bought a smoke machine and there are NO leaks - so I guess the random ramp-up of my idle and the crazy tach are related, possibly the idle ECU is crapping out? Here is a summary of where I'm at now (also posted to Facebook)

I've been dealing with an idle that randomly creeps up to around 1,200 rpm accompanied by my tach needle bouncing everywhere and freaking out. In this video, I started the car cold and it started at 775 RPM for a few minutes and then started to get unstable and creep up as if someone is slowly turning a dial up. I am not positive but I think it's associated with going into closed loop.

https://youtu.be/3KJ1Y-QwbSo (https://youtu.be/3KJ1Y-QwbSo?fbclid=IwAR0qevVUvCYBDRWpWmsvUOzlgTwe76OtEbs yKLde7vWc2eeDqcUlKEFggmw)

Here is what I am positive of:


Throttle plates are totally closed
Used a smoke machine and confirmed NO vacuum leaks - already replaced all o-rings, gaskets, vacuum lines, etc.
Idle speed switch confirmed electrically closed by checking the wires for +12 vs 0 at the idle ECU
Ignition timing is dead-on at 13 deg with no advance when cold
CO is set to exactly swinging over 20 deg on the 8 cyl scale
Pulled injectors again and confirmed no drippage, spraying evenly with good pattern
Fuel pressure is normal and control pressure ramps up as expected
New impulse coil in the distributor and just checked the reluctor air gap today - perfectly set. pickup is outputting ohms as per shop manual
New distributor cap, wires, rotor, plugs (NGK)
New O2 sensor
Thermistor checked and OK
New ignition coil (Bosch blue NOS)
New tach control board installed today - no dice even though the old tach board was jumping even when hooked directly to the white/slate on the ignition coil and wired directly to +12 and Ground. I thought that proved the tach was bad but apparently not!!
Removed binnacle and cleaned and checked all connections and speed nuts
Idle motor tested and clacks full rotation each direction when applying +12 to each side pin.


So, as you can see I've spent a lot of time working on this but no luck yet. Since the idle is ramping up without touching the throttle from cold, and sometimes it returns to normal WITHOUT touching the throttle, I can only assume at this point the issue is the Idle Speed ECU or maybe the idle motor? Has anyone seen this before or have anything else I should be checking?

[Bitsyncmaster]

Your idle creeping up is probably due to bad deceleration valve springs. I had the same problem with my car and it would creep up to 1000 RPM when hot.

I still think you may be getting voltage issues (electrical noise). Now that may be caused by a bad connection, the alternator or marginal battery.

[82DMC12]

Your idle creeping up is probably due to bad deceleration valve springs. I had the same problem with my car and it would creep up to 1000 RPM when hot.

I still think you may be getting voltage issues (electrical noise). Now that may be caused by a bad connection, the alternator or marginal battery.

Hi Dave, were your decel springs collapsed? Here is what mine look like. I just had the throttle out yesterday and checked them again. Both spring seem to have equal adequate tension and they are closing properly.

68556

As for bad connections and noise, I have a new Interstate battery installed last fall and a new DPI alternator installed just this week! No difference from my old alternator.

VERY perplexing.

[Bitsyncmaster]

Hi Dave, were your decel springs collapsed? Here is what mine look like. I just had the throttle out yesterday and checked them again. Both spring seem to have equal adequate tension and they are closing properly.

68556

As for bad connections and noise, I have a new Interstate battery installed last fall and a new DPI alternator installed just this week! No difference from my old alternator.

VERY perplexing.

Those springs do look good.

Maybe your tach does need dampening so it could be that part has failed in the mechanical part.

[Jonathan]

Your idle creeping up is probably due to bad deceleration valve springs. I had the same problem with my car and it would creep up to 1000 RPM when hot.

I still think you may be getting voltage issues (electrical noise). Now that may be caused by a bad connection, the alternator or marginal battery.

Hey Dave or others...

Besides our batteries being the group 78 for physical size and the side post terminals, is there a rating in cranking amps that's right? I see some batteries have more cranking amps, but wasn't sure if more is always better in things like this, like maybe there is a limit where that's too much.

[Bitsyncmaster]

Hey Dave or others...

Besides our batteries being the group 78 for physical size and the side post terminals, is there a rating in cranking amps that's right? I see some batteries have more cranking amps, but wasn't sure if more is always better in things like this, like maybe there is a limit where that's too much.

I've always use the smaller size. Thick it's 75 in my D. Only thing with battery size would be how long it would run if the alternator died unless you have sub zero temps.

[David T]

Unusual symptoms are generally caused by an unusual problem. If you have ruled out all of the common things you now must recheck all of your work and look for an unusual cause. New parts are particularly suspect. I would concentrate on the secondary ignition system but an increasing idle is going to be fuel system related, either the decel springs, the throttle plates moving (play in the throttle linkage) and/or a problem in the idle systems. Can also be a vacuum leak sensitive to temperature. The idle ECU does not commonly fail and it is not known to fail in this manner. Fluctuating system voltage can cause weird symptoms so check the battery and the alternator and all of the connections.

[82DMC12]

David,

I could try running my smoke machine while the engine is hot, that should reveal any heat-related vacuum leaks, don't you think?

Regarding the decel springs, you can see the photo of the springs above. They are not collapsed. Is there any way to test the decel spring theory without replacing the throttle body?

I did just try a spare idle ECU that painterdave had - no change, it's not the ECU itself. Could it be the idle motor?

I am going to put a timing light on all six plug wires one at a time and see if I can find if any of the cylinders is crapping out (for the tach issue)

[Rich]

Besides our batteries being the group 78 for physical size and the side post terminals, is there a rating in cranking amps that's right? I see some batteries have more cranking amps, but wasn't sure if more is always better in things like this, like maybe there is a limit where that's too much.

I'm unaware of a factory spec for a DeLorean battery aside from the Group 78 size. (Group 78 batteries are all side terminal type)
As mentioned, a smaller side-terminal battery can work.

Higher cold cranking amps (CCA) or cranking amps (CA) are better, mainly in terms of cranking/starting power as the term implies. Higher ratings typically come with a longer battery warranty.

No limit. A higher rating won't hurt anything besides your wallet.

[David T]

David,

I could try running my smoke machine while the engine is hot, that should reveal any heat-related vacuum leaks, don't you think?

Regarding the decel springs, you can see the photo of the springs above. They are not collapsed. Is there any way to test the decel spring theory without replacing the throttle body?

I did just try a spare idle ECU that painterdave had - no change, it's not the ECU itself. Could it be the idle motor?

I am going to put a timing light on all six plug wires one at a time and see if I can find if any of the cylinders is crapping out (for the tach issue)

I was thinking more along the lines of a system that changes as the engine warms up. You could try running the motor in the dark and looking for any sparks or blue glow that would indicate a problem in the secondary ignition system. A system wide problem would not single out one cylinder. A misfire on one cylinder would give you the jumpy tach and the bad idle. I think you are looking for two separate problems, one causing the fast idle and one causing the bad idle/jumpy tach.

[Jonathan]

I'm unaware of a factory spec for a DeLorean battery aside from the Group 78 size. (Group 78 batteries are all side terminal type)
As mentioned, a smaller side-terminal battery can work.

Higher cold cranking amps (CCA) or cranking amps (CA) are better, mainly in terms of cranking/starting power as the term implies. Higher ratings typically come with a longer battery warranty.

No limit. A higher rating won't hurt anything besides your wallet.

Thanks for the explanation. The extra warranty/extra cost sounds a little like how they price power surge bars for home theatre systems. More expensive version = longer/higher warranty.

Question about battery disconnect switches... could a wonky disconnect switch in some way cause irregular voltages and the like? I'm just thinking out loud as to what other things might affect things intermittently that haven't already been discussed.

[82DMC12]

Hey guys, I checked a few more things today.

1) Cleaned with a wire brush and electronics cleaner all of the bulkhead connectors since white/slate and some of the wiring for the idle parts go through these connectors.
2) Cleaned Fuse 1
3) Tested the thermistor at the Idle ECU (black/yellow and black/slate on the inner white plug, disconnected from the ECU) while the engine was still cold. On the 20K scale I get about 8.00.
4) Started the car and let it idle up until hot. After a certain point the idle get a bit rough and then slowly ramps up to around 1,200 RPM just as it had been before. As it climbs up, the tach starts to freak out and shown in my previous video.
5) While this is taking place, I hook up my Actron external tach to monitor directly off of the white/slate on the coil. The Actron is not bouncing all over like the dash tach does.
6) While the car is idling, I hook up my timing light and check each cylinder while the tach is freaking out to see if there is any change to the flashing. There is not. All six plug wires seem to be firing normally with no drop out or monkey business
7) Unplugged the inner plug on the idle ECU and the RPM's jump up to about 2,000 RPM as expected. The tach is still bouncing around so this tells me the tach bounce is not related to the Idle ECU.
8) Shut off the car now that it's hot (fans on) and checked the thermistor again at the Idle ECU on the same pins. Now my 20K scale is reading only 0.86.

Very interesting because I'm reading some diagnostics information online that suggest the thermistor should read 8K to 12K when cold and 3K to 5K when hot. Clearly I'm not interested in replacing the thermistor unless I am positive my readings are bad and they mean the thermistor is dead/dying.

Wondering if anyone can confirm and even better if someone whose car is running correctly would repeat the same thermistor test?

Would a thermistor reading these values result in a randomly high hot idle? Perhaps the tach is still a different issue but maybe the thermistor explains the idle.

[Bitsyncmaster]

I have been helping another owner with his idle problem and he got about the same resistance as you on the idle thermistor. But mine at 75 deg. F (cold) is 20.7 Kohm and 1.12 Kohm when my fans turned on and I shut the engine off.

I would also like to see what some other owners are reading on their thermistor. I used my reading when I designed my ECU software but I don't think that difference makes much change.

[82DMC12]

I have been helping another owner with his idle problem and he got about the same resistance as you on the idle thermistor. But mine at 75 deg. F (cold) is 20.7 Kohm and 1.12 Kohm when my fans turned on and I shut the engine off.

I would also like to see what some other owners are reading on their thermistor. I used my reading when I designed my ECU software but I don't think that difference makes much change.

Thanks Dave. I appreciate the measurements. I'd be forever grateful if a couple more owners would do the same test and report back!

[Jonathan]

Andy, I'm not sure if you're still chasing this fluttering tach issue, but thought I'd share some recent progress with the same issue on my own car.

About a week ago I got to reinstalling the alternator belt, one-handed tensioner bracket. I replaced my alternator a couple seasons ago and forgot the part where you had to drill out slightly the one attachment hole to get the securing bolt to go in completely. I had everything all back together when I realized this and so just put the original back in instead.

The alternator had to come out completely to get the proper angle on drilling out that hole. That part was easy and when I reinstalled everything, I put the new tensioner bracket on as well as a new belt. Now, while I don't think it was related to the previous belt being too slack, that work has eliminated the fluttering tach needle.

The step in there that I think did the trick was to really focus on making sure the brown cables were tight on the alternator post. I snugged up the post on the alternator first and then made sure the two main brown cable jumbles were aligned well enough to come together flush and snug. I got everything tightened back up and have over 100 miles on the car and the tach needle has not fluttered a single time.

I'll keep an eye on it still, but wanted to share in case all of that was just traced back to an alternator post electrical connection not being good enough.

[Azar]

Hmm well I'm actually replacing my alternator next week because it's making some noise, is probably 20 years old, and only making 12.4V when the engine is hot and fans and headlights are running.Hi, I have same problem with the needle but very rarely in idle, mostly at 3000-3200 RPM.
Btw, my alternator is also delivering 12,5V with AC and Fans on and 14.0 with everything switched off.
Can you confirm that with the new alternator it is now improved? Thanks.

Just another observation: I think it got worse after the engine overhaul and I can see that checking the alternator cables should me on my list (after I fix the engine).
Interestingly, almost all of my instruments are now reading lower than before engine overhaul:
- Voltmeter is showing 1,5V less than actual (same as my aftermarket radio which has screen mode showing voltage)
- fans kick in on 190 instead of 210 before and switch off at 150 instead of 160 before.
- oil pressure showing less than of measured with the manual gauge
- fuel is showing 1/4 less than in reality

[82DMC12]

Andy, I'm not sure if you're still chasing this fluttering tach issue, but thought I'd share some recent progress with the same issue on my own car.

About a week ago I got to reinstalling the alternator belt, one-handed tensioner bracket. I replaced my alternator a couple seasons ago and forgot the part where you had to drill out slightly the one attachment hole to get the securing bolt to go in completely. I had everything all back together when I realized this and so just put the original back in instead.

The alternator had to come out completely to get the proper angle on drilling out that hole. That part was easy and when I reinstalled everything, I put the new tensioner bracket on as well as a new belt. Now, while I don't think it was related to the previous belt being too slack, that work has eliminated the fluttering tach needle.

The step in there that I think did the trick was to really focus on making sure the brown cables were tight on the alternator post. I snugged up the post on the alternator first and then made sure the two main brown cable jumbles were aligned well enough to come together flush and snug. I got everything tightened back up and have over 100 miles on the car and the tach needle has not fluttered a single time.

I'll keep an eye on it still, but wanted to share in case all of that was just traced back to an alternator post electrical connection not being good enough.


Interesting observations! I'm sure power problems can cause all sorts of whack stuff.

I see I forgot to update this thread. I had a couple threads going at once :-) Anyway, not sure if I mentioned it or not but a few months ago I had the distributor out so I could replace the o-ring on the shaft (while doing a valley dive). At that time I also replaced the impulse coil since I had an NOS one still in a bag, sealed up, that I bought like 15 years ago and never installed. Since I had the distributor out, I replaced it. After that valley dive is when my jumpy tach got so much worse. I ended up pulling the distributor out and putting the original impulse coil back in AND I replaced the black "tach fly lead" that snaps into the distributor (and then goes to the 2-pin Bosch connector in the firewall). It must have been one of those two parts causing my issue because my tach problem went away and hasn't come back since.

You mentioned the brown wires on the alternator. A few weeks ago I pulled them all out of the car and replaced them with 4 awg welding cable and new hydraulically-crimped copper terminals. I think that helped my voltage issues too.

[Bitsyncmaster]

Hi, I have same problem with the needle but very rarely in idle, mostly at 3000-3200 RPM.
Btw, my alternator is also delivering 12,5V with AC and Fans on and 14.0 with everything switched off.
Can you confirm that with the new alternator it is now improved? Thanks.

Just another observation: I think it got worse after the engine overhaul and I can see that checking the alternator cables should me on my list (after I fix the engine).
Interestingly, almost all of my instruments are now reading lower than before engine overhaul:
- Voltmeter is showing 1,5V less than actual (same as my aftermarket radio which has screen mode showing voltage)
- fans kick in on 190 instead of 210 before and switch off at 150 instead of 160 before.
- oil pressure showing less than of measured with the manual gauge
- fuel is showing 1/4 less than in reality

Sounds like you have a bad power connection somewhere. One common problem are the two feed through 12 volt and ground in the back of the relay compartment. You have to drop the rear plywood board to get at those. Maybe just tighten the nuts up.

[Jonathan]

Sounds like you have a bad power connection somewhere. One common problem are the two feed through 12 volt and ground in the back of the relay compartment. You have to drop the rear plywood board to get at those. Maybe just tighten the nuts up.

Thanks Dave. I'll look at these connections on my own car as well.

[Jonathan]

Sounds like you have a bad power connection somewhere. One common problem are the two feed through 12 volt and ground in the back of the relay compartment. You have to drop the rear plywood board to get at those. Maybe just tighten the nuts up.

I got the connections cleaned up behind the rear board. They didn't look terrible nor were they loose to the touch. I took them apart and cleaned/sanded off any surface corrosion on the cable lugs and then reinstalled with new nuts and washers.

Car started right up but it still seems to have voltage issues. Like the voltage doesn't go up to it's usual spot right from the beginning. I noticed it hovered a little under the 13 volt marking for a few seconds and then eventually came up above the 13 volt marking. The belt doesn't seem to be loose or slipping, but maybe it is?

I turned on some accessories, including the headlights and stereo and AC fans and the voltage dropped a small amount with each new thing added. With those running it was right at or just below 13 volts. No cooling fans running up front mind you as the engine was cold still.

Then after turning all those accessories off again and just letting it idle in the garage, I noticed the voltage "hunt" a little. Or vary might be a better word. Not hunting like our idle can do sometimes, but the voltage needle moved some up or down and then back like it was having a hard time adjusting itself and finding it's spot.

I would describe it as having a problem with regulating the voltage, but then I don't know what that might mean exactly. Help me understand, is the "voltage regulator" function something going on inside the alternator and the only connection it has to our car's system through that single yellow/brown output wire? I'm going to go double check that wire and any connections it has are all good still.

Is there anything else to check regarding this voltage not being stable? I can tighten up the belt some more, but I just don't think that's all it is.

[Bitsyncmaster]

Yes the alternator regulates the voltage internally in the alternator. There is an external connection to remotely sense where the voltage reference it uses but most just leave it open and it uses the battery terminal on the alternator. However my DMCH alternator I used that remote input jumpered to the battery terminal on the alternator because I could see a dip every 18 seconds. That jumper also fixed the error code flash it did before starting the engine.

[Jonathan]

Not sure I follow what you mean about the ground, Dave.

I was under the impression the alternator got it's ground from the mounting bracket and bolts that secure it directly to the passenger side engine manifold. Are you saying there is another way to provide a ground to the alternator? And it uses this as it's ground reference?

[Bitsyncmaster]

Not sure I follow what you mean about the ground, Dave.

I was under the impression the alternator got it's ground from the mounting bracket and bolts that secure it directly to the passenger side engine manifold. Are you saying there is another way to provide a ground to the alternator? And it uses this as it's ground reference?

Not talking about any ground in past reference. I assume your alternator has a good ground via the mounting bolts. I was taking about the +12 reference that the regulator uses to provide the correct voltage.

FYI:
The alternator should hold a higher voltage when it is cold. This was always designed into the regulation because a lead acid battery requires a higher voltage when the battery is cold. Real cold you should almost get 15 volts but hot can drop to as low as 13.5 volts. Normally my car puts out 14.6 volts but I don't drive it on cold winter days.

[Timeless]

My car has this intermittent issue as well. It doesn't happen at idle only when cruising at 2000-3500 RPM. (auto trans) Voltage is ok and it doesn't matter if A/C, lights, stereo are on.

To my knowledge the binnacle/cluster have never been removed judging how original everything looks under the dash. Id did replace the battery today so I'll continue to monitor it.

[Timeless]

Ignition coil replaced with DMC NOS Bosch and tach still erratic/jumping while cruising. With a recent trip in the VOD and everything in there solid I'm starting to think it's tachometer age issue.

[Ron]

Have you tried a hand held tachometer?

[Timeless]

Have you tried a hand held tachometer?
I have not. If I had one I would test with it.

[82DMC12]

I have not. If I had one I would test with it.

If you have the actron dwell meter it also does tach and volts. Unfortunately I don't know if a handheld tach is reliable way to rule something out since my tach jump did not occur on the actron but it did with the dash gauge. I think the actron has too much damping so it sent me down the wrong rabbit hole.

[Timeless]

UPDATE: Replaced the battery and alternator recently, about 3 months after the DPI engine mount ground cables. Tach jumping has reduced by ~90%.

[82DMC12]

UPDATE: Replaced the battery and alternator recently, about 3 months after the DPI engine mount ground cables. Tach jumping has reduced by ~90%.

Hard to say what the source is. I still had jumping even after a new battery and alternator! But I haven't had it ever since replacing the tach lead on the distributor and putting my old impulse coil back in.

[Jonathan]

I had been watching this thread previously because my tach needle fluttered a lot. It wasn't fluttering with the actual speed of the engine. It was just the instrument.

Over the winter, after seeing mention of the brown cables on the back of the alternator and that nut not being tight, I focused my attention there in the Spring before I got the car out. The nut I had on there wasn't staying tight. Too much wiggling possible with those cables pinched on top of each other. So I replaced that simple nut with an equally simple one, but this time I used a nyloc nut. And after 400+ miles so far this season, the needle has not fluttered a single time. So that was the problem for me. Those brown cables on the backside of the alternator were not maintaining good, solid contact.