Voltage drop problem Options

[jvmarino]

Driving the 914 today I noticed an issue with voltage drop for the first time. Car starts up fine, runs good as usual, but when rpm goes over about 3600, the volt gauge has a noticeable drop down to about 12v indicated. Generator light also comes on. When rpm goes back below 3600, light turns off and volt gauge jumps back up about 1.5 volts.

My initial thought was a loose alternator belt, as I haven’t touched it since rebuilding car almost 30 years ago. Felt the belt and it seemed too slack. So tonight I tackled this project and after about an hour of cursing at the lower 13mm bolt, I finally found a way to loosen it. I get the belt tightened and button everything back up. Fire car up and it still does the same volt drop thing.

Anyone here have any thoughts on what the problem is. Car still drives fine, but I don’t want to take it anywhere until this is solved as it would stink if it completely dies on me while using it.

Jim

[technicalninja]

Full field the alternator at 2500 rpm and see what happens.

Check basic wiring. Power feeds (the alternator harness can be a problem area).

Check powers and grounds before changing any parts.

Add additional ground from close to alternator on engine to negative battery post (can use one side of a set of jumper cables).
This is just a test to see if it changes anything.

It's easy to change the regulator and hard to change the alternator.

I'd change to a new style solid state regulator as well.

@914sixer was a good source for these.

[jvmarino]

Well I picked up a good deal on a solid state regulator (the kind with the pig tail) and popped it in. The old unit (original) had some corrosion on bottom so I had high hopes, but alas it made no difference to the problem. I will next do the full field check on the alternator. I fear that it will be the problem.

[GregAmy]

Sounds like this one from last week:

http://www.914world.com/bbs2/index.php?sho...=371725&hl=

Any time I read "GEN light gets brighter on with more RPM" I immediately go straight to "bad diode in the alternator".

[jvmarino]

I am by no means an expert on car charging systems. It seems the more I read about troubleshooting them, the more confusing it gets with possible problems. I did see some other internet discussions where people mentioned these same symptoms that was due to worn brushes in the alternator. It is odd that the light comes on and the volt meter reading drops like a switch when I hit that magic RPM level, and then it reverses when the rpm comes back down below that level.

Really dreading having to take it out of the car. Just tightening up the belt was a major PIA as the pivot bolt nut was nearly impossible to get a tool on. Also finding that there are no rebuilt units available to purchase. I need to see if there are any good shops in the Baltimore area to rebuild an alternator.

This is one of those times when you appreciate the simplicity of a front engine and a vehicle with lots of parts available. On the other hand, I guess 50 years from an alternator is pretty good.

[Spoke]

was due to worn brushes in the alternator.

It almost sounds like this is the issue. I had a '74 Dodge Colt (Mitsubishi) and it stopped charging on a long drive. I stopped to check things and at idle it would charge. Rev the engine and charging stopped.

Pulled the alternator and put my ohmmeter across the armature. Spun the alternator by hand and observed the resistance increase then decrease as the armature slowed down.

Brushes were well worn. Cleaned the brushes and the commutator rings and all was good.

Do the full field test by pulling the VR and shorting D+ to DF.

Measure both battery voltage and D+ voltage at different RPMs.

[jvmarino]

was due to worn brushes in the alternator.

It almost sounds like this is the issue. I had a '74 Dodge Colt (Mitsubishi) and it stopped charging on a long drive. I stopped to check things and at idle it would charge. Rev the engine and charging stopped.

Pulled the alternator and put my ohmmeter across the armature. Spun the alternator by hand and observed the resistance increase then decrease as the armature slowed down.

Brushes were well worn. Cleaned the brushes and the commutator rings and all was good.

Do the full field test by pulling the VR and shorting D+ to DF.

Measure both battery voltage and D+ voltage at different RPMs.

Looking to do more testing this weekend and perform the field test you mention.

When you said the brushes were well worn, did you need to replace them or just clean them?

[Spoke]

was due to worn brushes in the alternator.

It almost sounds like this is the issue. I had a '74 Dodge Colt (Mitsubishi) and it stopped charging on a long drive. I stopped to check things and at idle it would charge. Rev the engine and charging stopped.

Pulled the alternator and put my ohmmeter across the armature. Spun the alternator by hand and observed the resistance increase then decrease as the armature slowed down.

Brushes were well worn. Cleaned the brushes and the commutator rings and all was good.

Do the full field test by pulling the VR and shorting D+ to DF.

Measure both battery voltage and D+ voltage at different RPMs.

Looking to do more testing this weekend and perform the field test you mention.

When you said the brushes were well worn, did you need to replace them or just clean them?

In my case I used the original brushes because they didn't make replacements. Do the tests first before any swapping of parts

[StarBear]

Yep; same thing with my DD 2009 Camry. Fortunately in the front and replacement easy to get. Less than $200 if I recall. (IMG:style_emoticons/default/biggrin.gif)
No air-cooled sound, though! (IMG:style_emoticons/default/mad.gif)

[jvmarino]

Well I finally got around to some testing after some delays. First I jumpered DF to D+, hooked meter to battery and started car. Voltage started above 14v and running rpm up over 4K the voltage went up over 18v. I then removed jumper and plugged VR back in and car was running about 13.6v and at high rpm dropped down between 13.1 & 13.3. The GEN light still came on at high rpm’s.

Did not do what I thought it would when using the jumper. Any thoughts from those with better experience than me?

[jvmarino]

Read through some more old threads on the forum and realized I missed a testing step.

So today I ran it with VR installed and measured 13.6v at battery and D+ when idling. When running up to higher rpm, battery stays above 13v (drifts between 13 and 13.6), but D+ drops down to about 6v. From what I have read it sounds like a bad alternator.

[Spoke]

Well I finally got around to some testing after some delays. First I jumpered DF to D+, hooked meter to battery and started car. Voltage started above 14v and running rpm up over 4K the voltage went up over 18v. I then removed jumper and plugged VR back in and car was running about 13.6v and at high rpm dropped down between 13.1 & 13.3. The GEN light still came on at high rpm’s.

Did not do what I thought it would when using the jumper. Any thoughts from those with better experience than me?

The voltage at the battery seems in the correct range for maximum alternator output which is the point of the DF to D+ test. Was the GEN light on during the test at all?

That you have tried 2 different VRs seems to point to the alternator as the issue.

[Spoke]

Read through some more old threads on the forum and realized I missed a testing step.

So today I ran it with VR installed and measured 13.6v at battery and D+ when idling. When running up to higher rpm, battery stays above 13v (drifts between 13 and 13.6), but D+ drops down to about 6v. From what I have read it sounds like a bad alternator.

D+ and battery voltage should be within about one volt from each other. The battery voltage is driven by 3 diodes off of the stator and D+ is driven by 3 other diodes also off of the stator. The battery diodes pass the high current of charging the battery and powering the electronics of the vehicle. The D+ diodes only pass current enough to power the VR. So the only difference between D+ and battery would be the difference of voltage drops of the diodes.

Based on your test results, it appears the alternator has issues.

[jvmarino]

The GEN light comes on when key is in before car start-up, and then only comes on again when engine rpm gets around 4k. When I did the follow up test to check both the battery and D+ with VR in, the voltage at the battery held steady, but the D+ dropped in half at the higher rpm's.

[davep]

While the alternator may have diode issues, please take a close look at the alt harness. I have examined over 20 and not found one that did not have problems. Now I just rebuild the harness with upgraded wires.