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Alternator Intermittent Overvoltage

86 Diesel Suburban

Lately the voltage gauge sometimes starts to wander upwards toward the 18v red zone. When it does it wanders up and down a lot, but mostly well above the center 13v. Sometimes I left of the throttle a little and it goes back and stays at normal, and sometimes it doesn’t and bounces up again as soon as I press the throttle. Doesn’t happen on every drive, just sometimes.

I thought it unlikely this was a bad voltage regulator, since it’s a pretty new alternator, 2 years tops. So I thought I’d see what other possible causes could be. (Why do they make the voltage regulator internal to the alternator now anyways, since they seem to go bad more often than the core? Can I just add an external regulator like the older trucks had?)

I’ve heard over-voltage can ironically sometimes be caused by a short, since the regulator dumps more voltage into the field coils because the main coils are showing they aren’t delivering enough current (because it’s all being soaked up by the short), resulting in an overall over voltage as the alternator tries to compensate for the voltage drop of the short.

How do I tell if this is the cause of the overvoltage or if it’s something else instead? If it is a short, where could it be? I’m assuming it must be somewhere weird, because almost any normal place I could think of for a short would simply blow a fuse. Or is there such a thing as a partial short-- high enough resistance to not blow a fuse, but low enough to drive the alternator nuts?

Could it be related to overheating? I initially thought that was a possibility, because it began around the time the weather turned warm here, but on any given day it seems pretty random and not tied strongly to excessive speed or going up hills.

Just because the alternator is 2 years old is no reason the voltage regulator couldn’t be going bad. But before I replaced the alternator I would want to verify that your voltage gauge is correct. You can get a cheapo voltmeter that plugs into the cigarette lighter and gives a digital readout of the system voltage.

If that readout matches the “wandering” readout from the dashboard voltage gauge, then you’ve ruled out a faulty voltage gauge and you can proceed with further troubleshooting.

I also agree with that thought. I had a replacement alternator do ghe exact same thing some year or so after I put it in. Testing confirmed the voltage regulator was at fault. I used the warranty on the alternator for a replacement and all was fine.

The dash-mounted voltmeter is the more likely problem…As jesmed said, verify the problem with a second voltmeter…

My money is on a faulty regulator. Over voltage can damage the battery and other things so you should fix this ASAP. Some folks have had a replacement alternator go bad right out of the box so having one fail after two years isn’t a surprise really. Hopefully it has a good warranty behind it.

My money is on the voltage regulator too. Stuff happens. be thankful the problem is so minor.

Another possibility is a poor, intermittent connection between the alternator and battery.

Will add an additional voltage gauge first and see how that goes and let you all know, as I’ve been meaning to put in a more accurate one anyway.

In the interest of being prepared if that isn’t the problem, it sounds like everyone is saying a bad voltage regulator is more likely than the remaining possibilities. Since I don’t want to spend a lot of money only to find out that wasn’t actually the problem, how do I be sure? How can I confirm a bad voltage regulator before buying a new alternator? How do I rule out or confirm the other possibilities? (i.e. a short, or a bad alternator connection as ok4450 pointed out)

And maybe this is just more for my own education than practical, but I was curious about an answer to my other question: can you add an external voltage regulator to the system? Or will that just cause the alternator to overload itself and burn out more quickly when there’s a problem? Just seems if voltage regulators are prone to go out, I’d rather have an extra layer or protection instead of being stuck on the road somewhere, or continuing on with a blown regulator and having that cause something even more expensive and hard to replace blow out from overvoltage, or having to carry a whole extra alternator for emergencies when a voltage regulator is just a tiny little solid-state thing.


Keep it simple . . . attach a digital multimeter to the battery posts and monitor the voltage while it’s idling. If your leads are long enough, you could even snake the meter into the car, so that you can periodically monitor the voltage while driving.

If you do this, I believe many of your questions will be answered

My bet is on a bad voltage regulator.

But, if you want to rule out a bad connection between the alternator and the battery, hook an external voltmeter between the alternator output and the + battery terminal (not the clamp) and measure the voltage drop.
Should be a very low voltage. If that jumps around by an amount similar to your dash gauge , then a bad connection is indeed your problem.

To answer your other question: it is not impossible to add an external voltage regulator. But it is an almost insane undertaking. Among other things, you would have to remove the alternator, take it apart, remove the existing regulator, and bring out a new control terminal before reinstalling the alternator and new regulator to test your handiwork. You can’t just hook two in series.

That’s assuming your new regulator was even compatible.

I guess I’m lucky with voltage regulators.
The last one I had fail was in a 1962 Plymouth Valiant in 1969, when I jumped someone’s dead battery at Woodstock who probably had a shorted cell.