Tachometer needle jumps sometimes - cause?

[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.

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A lot of D owners use an ACTRON Tach Dwell Voltmeter Model CP7605 to set their dwell. (Another for $22.99 total, used.) 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 to set their dwell. (Another for $22.99 total, used.) 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