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.

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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 ?????