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Vintage 140 weird battery drain

AndrewNance

AndrewNance

Amateur hour!
Joined
Oct 24, 2011
Location
Winston-Salem, NC
Alright, I'm stumped on trying to find a mysterious battery drain in my '71. Car has been back on the road for over a year and this just sort of started randomly a few weeks ago. I'm getting a draw of about 150 milliamps on a fully charged battery. No components have been added to this car at all. No clock, no stereo (aside from the non-op factory AM/FM one). I've tried pulling fuses one at a time and haven't found anything that stops the draw. It drains the battery pretty quick, like overnight.

Battery is brand new (I exchanged it last night), alternator is a Denso 100A and was recently serviced with a new rectifier bridge and brushes. The charging system seems to be working as it should and the battery reads ~14v while the engine is running.

Kinda out of ideas but hopefully I'm just overlooking something simple.
 
Try disconnecting the Denso alternator and see if the battery drain stops. A bad diode in an alternator will continuously draw power through the alternator.
 
So I had convinced myself that was the issue because I had a Denso 80A on it before and disconnecting it did stop the drain. I ordered a new rectifier for it and put some new brushes in it at the same time but the drain never stopped, which kind of confused me. So I bought a reman Denso 100A and now disconnecting the alternator makes no difference. The draw remains the same whether it is connected or not.
 
Accessory light in engine/interior/glovebox/trunk??? 150 milliamps sounds like a light draw to me. I know some of those pull direct power with key off with no fuse from the ignition switch, at least in the Amazon...
 
I just went back out to check and make sure I didn't miss anything last time. With the new alternator on the car, the draw DOES stop when I disconnect it. But that's two different alternators I've tried now - both with new rectifiers.

R32RennSport said:
Accessory light in engine/interior/glovebox/trunk??? 150 milliamps sounds like a light draw to me. I know some of those pull direct power with key off with no fuse from the ignition switch, at least in the Amazon...
Click to expand...

Hmm. There aren't any engine bay or trunk lights and I'm pretty sure the interior light hasn't even worked for a long time. The glovebox light used to but isn't working now that you mention it.

fatcatbestcat said:
Did the '71 140s have the seatbelt indicator lights? Make sure yours aren't pinched on or something weird like that.
Click to expand...

No seatbelt indicators in this one
 
You need to go back and repeat your measurement of the parasitic current. A 150 mA current on a 12 volt battery is equivalent to 1.8 watts, about the rating of one of those little dash indicator bulbs.

You said:
It drains the battery pretty quick, like overnight.
Click to expand...
If that means that the battery is drained to the point that the car won't start you have a problem with your battery (unless you have installed one of those Braille mini batteries). 150 mA should not kill a normal battery overnight. That said, 150 mA is an unusually high parasitic load.

Post #4 versus post #7 - is it now confirmed that the parasitic current goes away when the alternator is disconnected? Did you disconnect the alternator at the terminal on the alternator to do this test or did you disconnect the alternator using the cable connection at the distribution block on the inner fender (if you are still using that). The test should be done by disconnecting the alternator to confirm that the parasitic load is in the alternator. If the problem is inside the alternator, you may have contamination inside the alternator leading to leakage current and you may be able to fix that with cleaning (dust and a high humidity environment is a bad mix, particularly if you ever get close enough to the Atlantic for a little dose of salt). Also, reman does not mean new so you may have issues with the one or more of the diodes. If the problem is not inside the alternator, check the wiring between the alternator and the battery distribution block. If you are using the original 140 alternator wiring it was rated for a 35 amp alternator and may have been cooked by a larger alternator. If you retained the distribution block check if for damage and contamination. Its not 'sealed' and in its location on the inner fender it can accumulate dirt which may lead to a parasitic current loss to ground. Contamination type parasitic losses can be highly variable, low when everything is dry and increasing under humid conditions.
 
The last battery may have not been fully charged when I left it to sit overnight but the following day there wasn’t enough charge to do anything at all. Not even a dash light. I’ll inspect the distribution block and check it again. The alternator to starter cable was replaced with a larger one when I updated it to the Denso. Battery to starter and ground cables are still original.
 
Removed and cleaned up the distribution block. No change but it was pretty funky. I disconnected both cables from the starter and bolted them together, essentially bypassing the starter and that made no change either. Still, disconnecting the alternator is the only thing that eliminates the draw. I really can’t imagine I had a bad alternator, rebuilt it with a bad rectifier, and then installed another bad alternator all in sequence.
 
Is the Denso a 1 wire alternator or does it have a separate excitation terminal that you have connected to the dash charge light? If it has an external excitation terminal disconnect the terminal to see if that eliminates the parasitic current. If it doesn't, thenthe problem has to be something inside the alternator. A 150 mA parasitic current at 12 volts is equivalent to an 80 ohm load which is quite easily measured with even a cheap digital multimeter. Time to pull apart the alternator and use the ohm meter to find exactly where you have this parasitic connection to ground.

If disconnecting the exciter wire makes the problem go away then you likely have multiple issues. First off the effect of the alternator primary diodes should block any current flow from the battery through the excitation wire and second there should be no path to ground in the excitation circuit. That would definitely require more investigation.
 
AndrewNance said:
Removed and cleaned up the distribution block. No change but it was pretty funky. I disconnected both cables from the starter and bolted them together, essentially bypassing the starter and that made no change either. Still, disconnecting the alternator is the only thing that eliminates the draw. I really can?t imagine I had a bad alternator, rebuilt it with a bad rectifier, and then installed another bad alternator all in sequence.
Click to expand...

Just disconnect the exciter circuit wire and see if the alternator drains the battery. If it does, you know the answer, even if you don't want to know.
 
I mean it definitely points to having a bad diode in my alternator. It just seems unlikely that I had a bad one, replaced the rectifier with a new one with a bad diode, and then installed a reman alternator that also has a bad diode all back to back to back. I’m definitely not insisting that it’s not my problem but I have to be a little skeptical that buying another one is going to fix the problem.
 
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