Thread: Different voltage at meter and battery

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I can't find any reported problem exactly like this, so I'm starting a new thread. I've executed all of the test procedures in other posts and found that with the ignition off, I have a .4ma (.0004a) drain from my radio, which I can live with; otherwise, the drain is not measurable.

The problem is that while I have 13.86v at the battery, my voltage gauge only shows 12-12.5v at the same time at any engine speed. As above, leaving the car unattended isn't the problem - driving the car for 20 minutes or so drains the battery completely, even though it appears that the charging circuit is working. Turning the lights on drops the voltage meter reading to less than 8v, but then it comes up to 12v at highway speed. I seem to recall that the meter used to read 13.5 or so at highway speed and 12.5 at idle or around town last season.

I'm going to clean and re-seat every ground that I can find. Any other ideas?

Put an amp meter on it and see what the charging system is really doing.
What engine? What alternator?

Good questions, thanks. Ammeter shows no to little current. 400ci GM small block, standard GM 60amp alternator (not one-wire). Classic Instrument gauges, about 15 years old.

Evan with no charger, your battery should last longer than that. There is a draw on it somewhere. Check amps with the engine off, key on. If no draw then, it must be the charging system.The voltmeter just tells you that the alternator is trying to charge, the ammeter says how much. If it isn't coming up, the alternator or regulator is toast.

What are you using for an ignition switch? Many moons ago I had a 40 sedan that would discharge the battery with the engine running and the culprit was a worn out ignition switch.

The alternator has an internal regulator. The ignition switch is part of a Ron Francis wiring kit. Both of these are about twelve years old and have less than 20,000 miles on them.

There was a really good auto electrical shop around here a few years ago that would bench test alternators, generators, and starters for free while you waited, but they're gone. That was always my first stop after running sanity tests myself. I suppose that I could run the car down to AutoZone, but the battery is inaccessible under the car and they want to make direct connections at the battery terminals (don't blame them). Also, I know that even that short trip will kill the battery so I'd be stuck there.

My first thought was "bad regulator". If it were mine I would pull the alternator, carry it to AutoZone or your equivalent, and have them bench test it. My bet is that your internal regulator is toast.

Even though it's reading 13.86v at the battery? I thought that if the regulator (internal or external) were bad, the voltage level would also be low. I'm still confused as to why the reading is higher at the battery terminals than at the voltmeter.

Originally Posted by Dorsey

Even though it's reading 13.86v at the battery? I thought that if the regulator (internal or external) were bad, the voltage level would also be low. I'm still confused as to why the reading is higher at the battery terminals than at the voltmeter.

It's called "voltage drop" and it depends on where your volt meter is wired into the system.

I understand voltage drop, but why all of a sudden and not at the time I initially wired it? Nothing has changed in the wiring since 1995. Also, why is the drop not proportional to the battery voltage? The battery voltage stays at 13.86v with the lights on, but the gauge reads 8v or so.

I suspect a grounding issue or that the internal regulator is at fault as suggested above, and will let this thread know as I learn more. There's not a bench-testing facility within 26 miles of me anymore (a one hour drive in this area) or I would have already confirmed or eliminated the regulator. I hate to just change parts until the problem goes away.

The AutoZone web site is filled with helpful test procedures, so that's my next step. The only problem I've found is that with our creations is that we have to interpret and adapt those standardized procedures to our particular cars.

Thanks for all of your comments.

I would first pull the battery and insure that it will hold a charge and that it does not have a weak and/or dead cell. I would then have the alternator and starter, including solenoid, checked to insure that neither has an internal short to ground. I would isolate all other components by pulling fuses to make sure that noting is grounded that's not supposed to be. It's then merely a matter of tracing the ignition/charging circuit looking for wires that could have chaffed and grounded.

An local automotive electrical shop that I thought had disappeared is still in business, so I took my alternator there for testing after pondering some of the comments here. The owner has a cool test rig (that his grandfather built) for off-vehicle testing. He told me that it was not "putting out", so he opened it up and found that the regulator was bad. $40 and about 20 minutes later, I had my alternator repaired and re-tested to assure that it works.

I re-installed it and now all is well again - I'm now getting 14v+ at the battery and 12.5v at idle. Also, my ammeter shows positive current flow.

If anyone lives in my neck of the woods and wants a great recommendation and the name and address, I'll be happy to provide it.

Hi,
This might be a touchy subject, but I would advise that you remove your Ampmeter and replace it with a voltmeter at the dash. There are several reasons to do this but the primary one is safety. I do know of an electrical fire caused directly by the ampmeter. If you notice, hardly any of the guage manufacturers offer an ampmeter anymore. Besides... you get more information from the Voltmeter than the ampmeter.
Just trying to help.

I have both ammeter and voltmeter. The former is a 60a S-W and is mounted on the firewall (don't ask). The latter is Classic Instruments, mounted in the dash. I agree that the voltmeter is more useful.

I had a similar problem with my daily driver a few years back. Had to replace the alternator, it wasn't charging the battery as indicated. After a short while the car didn't have enough power to restart. Battery was ok, just a bad alt in my case.