randallsteel

Hello everyone,
some background: I bought a 1975 corvette at a cheap price due to some major electrical problems. The car has a electrical ground that causes the battery to be discharged when the car is off. If you disconnect the battery and hookup a ohmmeter from the pos batt cable to ground, the reading is 13.8kilo ohms. I have hunted and hunted for the cause of this ground and so far I have found that if you disconnect the plug on top of the alternator, the ground disappears. My understanding is that the two wire plug at the top of the alternator is for the voltage regulator.
Does this mean the voltage regulator is the source of the fault? Does anyone know where those two wires go? Do those two wires have tap offs later in the circuitry and the ground could be caused by one of those taps?
Other things that don't work that could be the issue:
no interior lights. The warning lights on the center console don't work. The ammeter has never moved. Headlights are intermittent.

Any thoughts or help on the situation would be greatly appreciated. Thanks

MelWff

could be me but I have never heard of anybody using an ohmmeter from the positive cable to ground to determine a parasitic battery drain.
usually you use an ammeter to determine the drain.
nevertheless it sounds like you may have a bad diode in the alternator.
I assume you have had the battery load tested?

randallsteel

the great thing about electricity is that if you know two pieces you can solve for the third. A 12 volt battery with a 13.86kilo ohm resistance attached yields a current draw of 865miliamps. Thanks for your thoughts, I think I'll go ahead and replace the alternator. The battery is 3 months old so it should be good. I've never heard of an alternator failing like that, but I'm open to anything if it'll fix the issue!

lionelhutz

13.8k ohms would cause a 0.9mA drain, which is not the issue unless you consider the battery going dead after months of not driving as being a problem. So, whatever is causing the issue, you're not measuring it with the ohmmeter so it's not going to work for the troubleshooting.

First off - how long can the car sit before the battery goes dead?

Disconnect the negative cable only and connect an ammeter between the negative cable and the battery post. Then, make sure the current draw is under about 10mA (0.01A). If not, start disconnecting things until you find whatever is causing the higher current draw.

If you think it is a large drain then that could damage the meter. So, first connect a headlight bulb in between the negative cable and the battery negative post. If the car has no aftermarket electronics, then the bulb should not light at all. Anything from partial to full brightness indicates excessive leakage current.

lionelhutz

Hmm, you replied after I opened the thread. You forgot that k = 1000.

randallsteel

the battery definelty suffers after it sits for around two weeks in between start ups. I temporary solved the drain issue by putting a knife switch to the pos terminal of the battery. When the car is off I open the switch and it stops the drain. But I would rather actually fix the issue.
What do you think about the fact that when I disconnect the two wire alternator plug the resistance changes from 13kilo to 4 meg? Bad alternator? Or are those two wires grounded elsewhere and the alter is fine?
Thanks. And yes I forgot the 1000 factor haha

lionelhutz

Well, there is a rather simple electronic circuit in the alternator between terminal 2 and ground, so I would expect it to have some resistance and leakage current all the time. As I already posted, 13k ohms of resistance across the battery is not causing the drain unless you leave it parked for months at a time. Well, unless the battery was way down on reserve capacity.

Hammerhead Fred

Try this thread

lvmyvt76

You could just rebuild the alternator, a lot cheaper!!!

randallsteel

I bought a new one for $50. Now the system is at a 11.8kilo ohm resistance. I was doing some checks on the old alternator and one of the voltage regulator connections had a 41 ohm resistance to ground. The new alt has a 10kilo ohm for that same plug. So I think the volt reg circuit was messed up. Probably why my battery was having trouble.

lionelhutz

Which terminal had 41 ohms? The wire on terminal #2 is switched off with the key meaning it's not that wire causing a drain unless you have other issues. You didn't measure 41 ohms with your tests when the key was off so that resistance wasn't connected.

sunflower 1972

The voltage regulators for the GM CSI alternators are notorious for going bad. If one is having battery charging problems, 9 times out of 10 it's the regulator.