Low voltage at idle

[mluder]

This is a new one... Or at least I never noticed it before.

The dash voltage meter will read around 13 when I'm stepping on the gas. When I start to decelerate and engage the clutch it drops in the neighborhood of 8 and stays there until I accelerate again. It has even died a couple of times as I'm coasting to a stop light.

Battery is less than a year old. Not sure about the alternator. I haven't hooked a meet up yet. Is it the battery unit holding charge? Can a loose belt cause the alternator to under perform?

Argh?!?

Steve
#4456

[Ashyukun]

Very much sounds like it's the alt- once the car is running, it should be providing the lion's share of the power to the car. A loose belt can DEFINITELY cause exactly what you're seeing. Check that it's good and tight. If you're running an aftermarket alternator like some of the ones Hervey (SpecialTAuto) sells, you should be running a slightly shorter belt than stock- I didn't realize this and was having similar though less extreme issues than you're experiencing, and after replacing the belt with the appropriate length one I've had no more issues with low voltage at idle.

[TTait]

First - check your alternator belt - a good press on it should deflect it maybe half an inch. If it was slipping you would probably hear it.

Then, check the battery lamp in your dash - just turn the key to run (not start) and watch the lights in the binaccle light up. IIRC the battery light is second from the bottom on the right. If it doesn't light up, the bulb is bad. If the bulb is bad the alternator won't charge.

Your problem is probably neither of the above, but you can check them both in less than 2 minutes.

Now, get a digital volt meter, pretty cheap and easy to use. Set it for DC voltage, it it has a scale that you need to select then pick 20 volts. Put the red on the + side of the battery, and the black on the -. A good battery at rest should show 12.75 volts or there about. If its less, don't panic yet.

Now go to the engine compartment and check voltage there, put black on the metal ring below and forward of the throttle spool, and the red on the + jump start post just forward of the coolant bottle on the right. Note the voltage there.

Then start the car, and check the voltage again. At idle you should see 13.5 or so and at 2200 rpms you should see more than 14.

Turn on the high beam headlights and max AC with the fan on #4, note the voltages at idle and 2200, and let us know all the voltages you measured.

Your problem is likely either a bad alternator, dirty electrical connections, or both.

Your alternator is kind of like 3 generator circuits in one. The alternator generates alternating current as 3 sine waves, each offset from each other so they are evenly spaced. One part will be producing more than 12 volts, the next part will have voltage starting to rise and the other has voltage starting to fall, so one of them in theory is always above 12 volts. The there are diode packs in it that only allow the positive voltage from each of the 3 circuits to come out of it, so its no longer alternating current (AC) like in your home, but direct current (DC). Then the voltage regulators in the alternator cut off the voltage above around 14.5 volts from each of the circuits in the alternator. The result is a constant output of 14 volts, made up from 3 overlapping wave forms with their bottoms cut off and everything above 14 cut off flat.

When the alternator starts to fail one or even two of the 3 circuits fails. This provides you with current that runs for 14 volts for a few milliseconds, then it drops, then comes back up. The voltmeter in the dash and even your multimeter don't read the voltages fast enough to see the pulsing, but the net effect is you don't have enough power under load.

The other contributing factor is all the connections for the battery, the ground, the alternator, etc. As those oxidize over time the resistance in the system builds up and the voltage goes down.

No matter what you should get into the battery box and clean up those connections with an emery cloth or a dremel or similar. Leave the battery disconnected and then go under the car and remove the bolts at each step of each circuit along the way and clean up all the mating surface, 1 at a time. This includes the plus cable going to the starter and then up to the battery post, the - cable to the frame, the grounding strap from the frame to the engine, the and the connections at the alternator. Don't use grease, get them clean, dry and tight again. Also check for looseness and corrosion where the eyelets crimp onto the cables too.

Let us know your voltages and clean your grounds, and then we can talk new alternators, battery lamp, and batteries and such.

[mluder]

First - check your alternator belt - a good press on it should deflect it maybe half an inch. If it was slipping you would probably hear it.
As I mentioend - there was some whining but not high pitched squeeking like out-right bad belt. This morning when I was testing, the engine was cold and had been in the garage oernight. Yesterday when I heard the noise, the car had been parked outside in near 80 degree temps.

Then, check the battery lamp in your dash - just turn the key to run (not start) and watch the lights in the binaccle light up. IIRC the battery light is second from the bottom on the right. If it doesn't light up, the bulb is bad. If the bulb is bad the alternator won't charge.
Light is fine

Your problem is probably neither of the above, but you can check them both in less than 2 minutes.

Now, get a digital volt meter, pretty cheap and easy to use. Set it for DC voltage, it it has a scale that you need to select then pick 20 volts. Put the red on the + side of the battery, and the black on the -. A good battery at rest should show 12.75 volts or there about. If its less, don't panic yet.
Rest voltage was 12.38

Now go to the engine compartment and check voltage there, put black on the metal ring below and forward of the throttle spool, and the red on the + jump start post just forward of the coolant bottle on the right. Note the voltage there.
12.53 Here

Then start the car, and check the voltage again. At idle you should see 13.5 or so and at 2200 rpms you should see more than 14.
Idle was at 14.5
2200 RPMS was 14.4

Turn on the high beam headlights and max AC with the fan on #4, note the voltages at idle and 2200, and let us know all the voltages you measured.
Idle was at 12.5 though on the dash it read much lower - around 10
2200 RPMS was 14.4


Your problem is likely either a bad alternator, dirty electrical connections, or both.

Your alternator is kind of like 3 generator circuits in one. The alternator generates alternating current as 3 sine waves, each offset from each other so they are evenly spaced. One part will be producing more than 12 volts, the next part will have voltage starting to rise and the other has voltage starting to fall, so one of them in theory is always above 12 volts. The there are diode packs in it that only allow the positive voltage from each of the 3 circuits to come out of it, so its no longer alternating current (AC) like in your home, but direct current (DC). Then the voltage regulators in the alternator cut off the voltage above around 14.5 volts from each of the circuits in the alternator. The result is a constant output of 14 volts, made up from 3 overlapping wave forms with their bottoms cut off and everything above 14 cut off flat.

When the alternator starts to fail one or even two of the 3 circuits fails. This provides you with current that runs for 14 volts for a few milliseconds, then it drops, then comes back up. The voltmeter in the dash and even your multimeter don't read the voltages fast enough to see the pulsing, but the net effect is you don't have enough power under load.

The other contributing factor is all the connections for the battery, the ground, the alternator, etc. As those oxidize over time the resistance in the system builds up and the voltage goes down.

No matter what you should get into the battery box and clean up those connections with an emery cloth or a dremel or similar. Leave the battery disconnected and then go under the car and remove the bolts at each step of each circuit along the way and clean up all the mating surface, 1 at a time. This includes the plus cable going to the starter and then up to the battery post, the - cable to the frame, the grounding strap from the frame to the engine, the and the connections at the alternator. Don't use grease, get them clean, dry and tight again. Also check for looseness and corrosion where the eyelets crimp onto the cables too.

Let us know your voltages and clean your grounds, and then we can talk new alternators, battery lamp, and batteries and such.

See numbers above - It's not really exhibiting the same simptoms as the other day but like I said... it was a cold start and in a cool garage.

Steve

[Bitsyncmaster]

With everything on your voltage is going to drop at idle speed of 775 RPM. As long as your voltage at the jump terminal is above 12.5 volts your not draining the battery.

Don't go by the dash gauge but would be interesting if you installed my ground jumper to see if that fixes the volt gauge.

[Canon20DFan]

Remind me - you gained voltage on the dash gauge by cleaning/isolating the ground to the dash cluster, right?

With everything on your voltage is going to drop at idle speed of 775 RPM. As long as your voltage at the jump terminal is above 12.5 volts your not draining the battery.

Don't go by the dash gauge but would be interesting if you installed my ground jumper to see if that fixes the volt gauge.

[DMCMW Dave]

The other one to try - replace/clean the fuse that powers the meter. #5.

[TTait]

Cleaning the connections mentioned above is always a good idea, its not going to get you in trouble so long as you disconnect the battery first.

Another thing to do is keep the meter handy in the car. If you are at a light and the needle reads low, pop down and disconnect your dome light and if the probes from your meter are small enough you can hook them up quickly to the purple and black wires by plugging directly into the female terminals. Takes 10 seconds - then you have a more accurate voltage gauge coming off a different circuit. Useful any time your suspicious about the dash gauge.

[Bitsyncmaster]

Remind me - you gained voltage on the dash gauge by cleaning/isolating the ground to the dash cluster, right?

You could temporarily check this if you made a jumper to plug into the inertia switch connector. That jumper would plug into the center pin (black/purple) and you would need to attach the other end to a frame ground.

That connector uses a smaller flat terminal, not the standard 1/4" one. I would just trim a 1/4" one down to fit.

A lot of cars have this low reading dash volt gauge. It would be nice to find a fix for that. I think mine reads just what the jump terminal is but it was pretty close even before I did the new ground jumper.

[David T]

Alternators cannot put out a lot of power at idle. It just isn't spinning fast enough. I don't care what the output rating is. If the battery is low the alternator will try to charge the battery but if it is very low and the motor is idling there just is not enough "oomph" in it to do much. Especially if you are running accessories like lights or A/C. Load-test the battery and charge it up. Check the belt on the alternator. Clean and tighten all of the "big" connections between the battery, frame, starter, and alternator. If the battery is over 5 years old just replace it. Use a Battery Tender if you are not regularly using the car to keep the battery fully charged. At least one day each month. Some auto stores will test your battery and alternator for free in the hopes of selling you stuff.
David Teitelbaum

[mluder]

Alternators cannot put out a lot of power at idle. It just isn't spinning fast enough. I don't care what the output rating is. If the battery is low the alternator will try to charge the battery but if it is very low and the motor is idling there just is not enough "oomph" in it to do much. Especially if you are running accessories like lights or A/C. Load-test the battery and charge it up. Check the belt on the alternator. Clean and tighten all of the "big" connections between the battery, frame, starter, and alternator. If the battery is over 5 years old just replace it. Use a Battery Tender if you are not regularly using the car to keep the battery fully charged. At least one day each month. Some auto stores will test your battery and alternator for free in the hopes of selling you stuff.
David Teitelbaum

Thanks, David.

I drive the car pretty regularly and the battery is less than a year old.

I will work on tightening the alternator belt.
Is there a run down and location map to the "big" electrical lconnections you are reffering to other than the obvious ones at the battery?

Steve

[Ron]

Alternators cannot put out a lot of power at idle. It just isn't spinning fast enough. I don't care what the output rating is....

+1
-----

FYI- There are third party "high amp" alternators going around that have too small of a pulley on them. I understand that the ones being shipped now have the correct size on them.
(Most were GM/Delco Remy style units.)

[Ashyukun]

+1
-----

FYI- There are third party "high amp" alternators going around that have too small of a pulley on them. I understand that the ones being shipped now have the correct size on them.
(Most were GM/Delco Remy style units.)
*raises hand*

The high-amp one I have would be one of those. It requires a smaller belt by a modest bit to be properly tensioned- something I didn't realize until recently (since my car ate the belt that came with the alt), and switching to the smaller belt helped things out a good bit.

[Ron]

Yeah, loose belts don't get it. Sounds like you were one of the lucky ones tho...

You made me re-read my post...thanks.
-----

:banghead: OOPS!!!

I said too "small" of a pulley but meant too say too "large" of a pulley.

...the main drive pulley on a D is smaller than anything GM used to (directly) drive their alternators (71xx, 1xSI, etc.), so you need a smaller pulley on those alternators to correct the ratio.

I'm going to bed...

[content22207]

Alternators cannot put out a lot of power at idle. It just isn't spinning fast enough. I don't care what the output rating is....

I disagree:
3364
(Note that idle speed was purposefully dialed down for that pic -- I typically idle between 700-800 RPM. Alternator is a generic Autozone 100 amp unit).

Bill Robertson
#5939

[Ron]

I disagree:
3364
(Note that idle speed was purposefully dialed down for that pic -- I typically idle between 700-800 RPM. Alternator is a generic Autozone 100 amp unit).

Bill Robertson
#5939

This really doesn't prove anything, Bill. To show the power you have to show the voltage (here ~13V) and the amperage (?). Remember P = I x E (Watts = Amperage x Voltage).

Your alternator should maintain voltage at 14.2/14.5 (v/s 13.5) depending on the series. With all of the mods you have done, your D should draw less power than most, so why is so low -- don't your lights brighten when you break idle?

Here ya go, I remembered where this was-
Note: This is true amperage, alternator shaft RPM (you can measure and figure the ratio for yourself ; )

3378


...surely you don't trust a Delorean Volt gauge (no mater how many times you have compared it to a "real" one :)).

[content22207]

Dash gauge accuracy has been verified with a handheld meter in real time (I drove around with it plugged into a window switch harness connector).

My car does indeed draw less than most DeLoreans:
3383
(taken at 2,000 RPM, with everything on except the power windows), but I think the reason my dash gauge reads so much more accurately than everyone else's is because it is grounded directly to battery via 4 gauge cable.

I should mention that my engine block (ground point for the alternator) has two 4 gauge ground wires in addition to the crappy factory braided strap -- one to the frame (other motor mount), and one directly to the battery.

Bill Robertson
#5939