I have a battery drain on my 70 cutlass when the car sits overnight. I put a test light between the negative battery terminal and the negative battery cable. It lights up when the car is off. When I disconnect the alternator plug the light goes out. I replaced the alternator( which is functioning fine when running btw), voltage regulator, battery and checked the wiring harness. I also removed each fuse one by one and tested for a drain and found nothing. I’m lost!

 

Every time my car did that it was a bad diode in the alternator. I don’t know if any other failure (such as voltage regulator) would act the same.

 

Is the draw to the point that the car won't start after sitting overnight? Does the alternator or the wires that go to it feel warm after the rest of the engine is cool?

I agree it does sound like a bad diode in the alternator even if the alternator has been replaced. Alternators aren't difficult to disassemble to check the diodes with a multimeter.

 

Fundamentally, you need to "measure" & express alternator output at the battery w/ the engine running w/ all accessories off. Should neither be below 13.8VDC nor above 15.0VDC. Average/Nominal VDC output should be ~14.3VDC - 14.6VDC.

 

Completely drained in the morning if I don’t disconnect the alternator plug. Alternator is putting out around 14 when running. Alternator does seem to be exceptionally hot .

 

I'm sorry but around 14 says nothing. Did you know 10 is around 14 on a scale of 1 to 100?

 

FYI, it is quite possible a bad battery is more than capable of demonstrating a voltage w/in specifications only to completely fail when internal plates are faulty - thus, causing an alternator to work extraordinarily harder and longer if a battery cannot keep up with and maintain a charge.
A brand new battery directly out of the box which has been prepared for the first time should demonstrate a voltage of ~13.3VDC. That same battery should demonstrate ~13.3VDC for ~1 week sitting on a wood platform no installed into any vehicle or device.

 

1) Remove battery from vehicle and charge battery on a wooden platform until it is fully charged ~13.3VDC. Battery should maintain that specific full charge (~13.3VDC) continuously for a minimum of one week.
2) If the battery is capable of maintaining a full charge, place battery into vehicle w/o attaching either the ground cables or power cable to the battery (just the plain battery). Battery should maintain a fully charged state.
3) If the battery has held its fully charged state, hookup the negative (-) ground side of the battery w/ all accessories off - engine not running. The battery should be able to maintain a fully charged state of ~13.3VDC overnight.
4) Hookup the positive (power) cable to the fully charged battery - engine not running. A brand new battery should be able to maintain ~13.3VDC overnight.

If both (3) & (4) demonstrate no reduction in battery charge, the battery is good. If either (3) or (4) demonstrate a reduction in battery voltage you have most likely a parasitic draw. How significant a draw will be dependent on measured battery voltage overnight. If you have a significant battery draw (note: you have not tested the working ALT, as yet) you have a parasitic draw - somewhere.

 

You need to test for a parasitic draw w/ a DMM in order to isolate the circuit which contains the parasitic draw (if you have one).
The most you should see is ~50mA (milliamp). Anything >50mA signifies a significant parasitic (AMP) draw.

Test For Parasitic Draw w/ Digital Meter

 

Is the alternator warm/hot when the rest of the engine has cooled? If yes it is defective.

 

Yes I think. I will confirm tomorrow. Thanks to everyone for your help!

 

Norm, that is a lot of good battery information, but when a parasitic drain goes away when unplugging the alternator, I highly suspect it’s not a faulty battery issue.

 

Kenneth, did I miss some vital statement(s) indicating he unplugged the alternator and a parasitic drain went away?

 

Kenneth - I re-read what I think you're getting at & I missed it.

It does speak of a parasitic drain doesn't it? Although, we still have no validation it is in fact a parasitic drain. But, I did miss that connection.

 

To the OP.

A draw on the battery only occurs if there's a problem with either a component in the alternator itself or connections to it. It's actually a rare event to have a new ALT which you stated was putting out/delivering >14 VDC create a parasitic draw when the car is not running (i.e. in a engine off overnight condition). I suggest you look for a ground short in the ALT wiring. Begin by evaluating your ALT wiring matches the CSM wiring diagram.

 

Waiting for the response to sugar bear's question-- is it warm after sitting overnight? You could try putting a small screwdriver or hacksaw blade against the rear bearing to see if it has a magnetic pull. If it does, pull the regulator plug to see if it goes away. Reconnect the test light like you had it and pull the regulator plug to see if the light goes out. I suspect a regulator problem.

 

Unplugging the regulator is definitely worth a try.

 

Norm, it’s in the very first post.

 

Replaced the voltage regulator. Problem solved. Thanks again to all that responded.

 

Excellent 👍

 

So this is the second "new" voltage regulator? As in the first "new" one was faulty?

 

No. I swapped alternators. Both tested good so I replaced the voltage regulator and the drain stopped. It now holds a charge overnight. When I start the car when it’s cold it’s putting out about 16v but quickly drops to 14.3 when fully warmed up. Is that normal?

 

^x2^ Yeah, baby.

 

But in your first post you said you had replaced the alternator, voltage regulator, and battery and still had the issue. Now you're saying you didn't replace the voltage regulator at that time?