Alternator over-voltage

September 15, 2018, 11:28 AM

Standard GM 3 wire alternator on a 350 and I’m getting voltage at the dash gauge from 13,5 (idle in Drive) to about 16v (normal city driving) to over 18v (freeway+ speeds). The 2 wire plug is firmly seated. Is there any reason to run the wires down and double check them or is it safe to assume the internal regulator is dead and I should just replace the whole assembly?

Oh, abd there that white powdery corrosion junk forming on the positive terminal. I assume that also indicates over-charging?

September 15, 2018, 11:32 AM

Some Shot

Sounds like the regulator isn't regulating. Might check the water in the battery too (not as a cause, but an effect).

September 15, 2018, 11:41 AM

egregore

First verify with a volt meter on the battery, at the same engine rpm where the gauge shows overcharging, what the real voltage is. Dash gauges aren't always accurate. If it really is overcharging, then the internal voltage regulator is bad.

What kind of car? A pre-~1985 GM alternator, for example, is relatively easy to replace the regulator/brushes/diodes in.

September 15, 2018, 12:13 PM

jimmy123x

Verify with a volt meter. Revving the engine up to about 2500 rpms in neutral should be similar to the speed the alternator is turning at on the highway. If it truly is that high. You have a bad voltage regulator (could be a diode if it's a fairly new vehicle instead of a regulator).

September 15, 2018, 01:09 PM

220-9er

Check and clean the connections at the battery and the ground, then check the battery itself.
If you can open the caps on your battery, check individual cells with a specific gravity hydrometer for variation.
If the battery is at or past its warranty period, I'd say it's suspect too.

September 15, 2018, 01:09 PM

bubbatime

My first car did that. Man, those headlights worked GREAT at 16-17 volts!!.

September 15, 2018, 03:02 PM

thumperfbc

It’s the 73 Nova but a GM Zz4 engine manufactured around 2001. I’m pretty sure I replaced the alternator once before, 10+ years ago. Given stage of life then I’m sure it was the cheapest replacement I could find at Kragen or Auto Zone. I’ll check the battery fluids once I square the alternator problem. Direct replacements are under $40 locally or a high output is available on amazon for about $60. Given how cheap that is I’m not sure it’s worth taking the time to replace the internal regulator, especially since I’ve never done it.

September 15, 2018, 05:31 PM

shovelhead

GM Part number 1116387.......at least the last time I looked one of those up, it's been awhile........

Once a GM parts person, always a GM parts person

Most likely the regulator is toast. BUT! First, have the battery tested. A wonky cell can cause some strange things also. Also, depending on where you go for your alternator they may be able to bench test it first.

September 15, 2018, 06:00 PM

.38supersig

You should have a D shaped hole on the back of your alternator. Ground the tab inside the hole to the housing. Note any changes in voltage output. If the voltage increases, it is not the regulator.

September 15, 2018, 06:24 PM

sns3guppy

You should be running 1.5-2.0 volts above battery voltage, to keep the battery charged. There shouldn't be a voltage increase when the alternator takes a load; the only means for a voltage increase is a bad field, or bad regulator.

Don't assume that onboard voltage indications are correct. Verify by running a regulator across the battery or battery to ground, and it's always best to load test and verify what's really happening.

September 15, 2018, 07:34 PM

hrcjon

98% dead regulator, 1% bad measuring device, 1% battery. So eliminate the last two with a simple voltage test and you are left with the issue. Don't run the car like this you will kill the battery if you haven't already.

September 15, 2018, 10:03 PM

thumperfbc

quote:
Originally posted by shovelhead:
GM Part number 1116387.......at least the last time I looked one of those up, it's been awhile........ Once a GM parts person, always a GM parts person

Most likely the regulator is toast. BUT! First, have the battery tested. A wonky cell can cause some strange things also. Also, depending on where you go for your alternator they may be able to bench test it first.

It seems illogical that a bad battery would cause an increase in voltage, but I know O’Reilly will do load testing for free, so I can do that.

quote:
Originally posted by .38supersig:
You should have a D shaped hole on the back of your alternator. Ground the tab inside the hole to the housing. Note any changes in voltage output. If the voltage increases, it is not the regulator.

Thanks, I’ll check this.

quote:
Originally posted by hrcjon:
98% dead regulator, 1% bad measuring device, 1% battery. So eliminate the last two with a simple voltage test and you are left with the issue. Don't run the car like this you will kill the battery if you haven't already.

Yea, that was one of the first thoughts I had. Won’t be surprised if I have to change the battery. It seems every time I lose an alternator on any vehicle it takes the battery with it.

September 15, 2018, 10:30 PM

shovelhead

When cars were equipped with ammeters if the charge rate was constantly high that was a possible sign of a failing battery. The regulator would sense the need for more charge due to one cell not maintaining a full charge. Once the regulator senses that the battery is fully charged it shunts excessive current produced to ground. The alternator puts out "unlimited" power subject to design limitations, the regulator is the valve between it and the battery.

I know this is apples to oranges comparison but the alternator on my '82 Harley if I remember correctly can put out fifty or so volts without a regulator in the system during testing for output. And with my '02 Harley when the battery was failing the voltmeter was reading high,15-16 volt range until I replaced the battery.

September 15, 2018, 11:47 PM

thumperfbc

quote:
Originally posted by shovelhead:
When cars were equipped with ammeters if the charge rate was constantly high that was a possible sign of a failing battery. The regulator would sense the need for more charge due to one cell not maintaining a full charge. Once the regulator senses that the battery is fully charged it shunts excessive current produced to ground. The alternator puts out "unlimited" power subject to design limitations, the regulator is the valve between it and the battery.

I know this is apples to oranges comparison but the alternator on my '82 Harley if I remember correctly can put out fifty or so volts without a regulator in the system during testing for output. And with my '02 Harley when the battery was failing the voltmeter was reading high,15-16 volt range until I replaced the battery.

I see what you’re saying. I’ll have both tested.

September 15, 2018, 11:50 PM

hvyhawler

I had a bad battery in my 00 Jeep XJ. The overcharge light would come on, I would stop, pull out my meter, test my alternator, by then it would show normal, finally it wouldn't start, replaced the battery and the problem went away...

September 16, 2018, 06:20 AM

pbslinger

Check voltage while using a jumper cable to bypass the battery wire on the alternator. Clamp on the battery+ terminal on the alternator and the other end on the positive battery terminal. This will check for a voltage drop in the main power wire from alternator to battery.

September 16, 2018, 07:46 AM

henryaz

I have a small Blue Sea digital voltmeter mounted to my dash. It is connected directly to my battery, so it reads alternator output when the engine is on, otherwise, just the battery voltage. I have it switched using a switched wire and a relay. It is interesting to see how the alternator/regulator changes voltage output based on the battery's state of charge and engine RPMs. After running for a while on a trip, it settles in at about 13.5V, which is a float charge level.

September 16, 2018, 08:01 AM

charlie12

quote:
Originally posted by bubbatime:
My first car did that. Man, those headlights worked GREAT at 16-17 volts!!.

You're right I had a 1984 Chevy truck and one night I turned on a country road that had just been overlaid. I said damn those new reflectors are bright. Then looked at my volt gauge and it was about pegged out.

September 16, 2018, 08:04 AM

charlie12

My Yaesu ham radio in my truck will display the DC voltage. It comes in handy

September 16, 2018, 08:31 AM

shovelhead

quote:
Originally posted by thumperfbc:
quote:
Originally posted by shovelhead:
When cars were equipped with ammeters if the charge rate was constantly high that was a possible sign of a failing battery. The regulator would sense the need for more charge due to one cell not maintaining a full charge. Once the regulator senses that the battery is fully charged it shunts excessive current produced to ground. The alternator puts out "unlimited" power subject to design limitations, the regulator is the valve between it and the battery.

I know this is apples to oranges comparison but the alternator on my '82 Harley if I remember correctly can put out fifty or so volts without a regulator in the system during testing for output. And with my '02 Harley when the battery was failing the voltmeter was reading high,15-16 volt range until I replaced the battery.

I see what you’re saying. I’ll have both tested.

Sorry, sometimes I don't explain as clearly as I should. I'm a lousy teacher!