Mysterious parasitic drain

[corndoggydogg]

Originally Posted by Chucks72Longbed

Don't know if you saw this or not from VettesVet but it's great info on how the system works: thanks to VettesVet!! Here is the message that has just been posted: *************** You should not need a diode in the brown wire from the cab. The factory put a resistance in the wire of about ten ohms which is enough to keep the diodes in the alternator from feeding voltage back to the ignition switch and keeping the engine running when the key was shut off.

The idiot light dashes have a charging light which gives the alternator the ten ohms resistance it needs and also protects the diode trio in the alternator from shorting to ground. The resistance wire from the key switch is used as a backup to the light bulb in case it blows and it also provides the resistance in the wiring to the alternator for the gauge dashes which do not have the light.

If one were to run a 12volt hot wire to the alternator to the no. 1 terminal without this resistance, it might blow the diodes and allow the battery power to drain to ground over time. If someone has wired your truck like that it may explain why you've still got the drain after changing the alternator. It's simple to tell if the alternator is shorted if you have a ohm meter. Just run a continuity test on the no. 1 terminal of the alternator to the alternator case then reverse the leads. If you get zero ohms both ways then the diodes are shorted to ground. Do the same thing on the large terminal on the back of the alternator. I suspect that the no.1 terminal is shorted since you stated that the drain went away when you unplugged the alternator. The no. 2 terminal should be checked also since it is hot all the time. It would drain without the key on.

Refer to this link and you should get all you need to know about the wiring to convert to the internal regulated alternator.

..........<a href="http://67-72chevytrucks.com/vboard/showthread.php?t=417872" target="_blank">http://67-72chevytrucks.com/vboard/s...d.php?t=417872</a> *************** There may be other replies also, but you will not receive any more notifications until you visit the forum again. All the best, The 1947 - Present Chevrolet & GMC Truck Message Board Network

Thanks for the detailed info. Very informative. Just tested the alternator. 41ohms and continuity both ways on terminal 1. No continuity either way on terminal 2. Mean anything ?????

[Chucks72Longbed]

Man I'm sorry, I just don't know enough about the charging system. I would PM vettevet and ask him. He seems to know a lot about this subject.

[corndoggydogg]

After months of frustration, I finally decided to tear apart all of the under-hood wiring, and rerun everything. Also cleaned up the wiring under the dash. Didn't find any glaring grounds or problems. BUT the drain is now GONE. Thanks to everyone for the input. Moral of the store: when all else fails...rip it out and start over.

[jakewalter]

My problem was that the light in the glove box was staying on.
Jake

[OldBalls]

Originally Posted by 71 super

Try unplugging horn relay under the dash. I have had several that had a horn issue and ended up being the drain

I know this feed is super old found it really helpful. I randomly had a parasitic draw that pulling fuses didn't resolve. I'm not an electrical wiz so I started to freak out a little. After reading this I pulled the horn relay and BOOM, draw is gone. I guess I should've known I could hear something clicking under the dash.

Only had the Blazer for a month and looking at the POs crappy wiring skills I thought I had a complete rewire on my hands. Luckily I can hold off on that until I'm ready.

Anyway, wanted to say thanks for the helpful tip.