Parasitic drain

[machomocha]

Hey guys. I have a 1995 Mustang GT. If it sits overnight the battery will be completely drained in the morning. I checked for current and there is a 3 amp draw when the alternator is attached. I thought it was the voltage regulator so I changed it, tested it, 0 amp draw. Great I thought, until I had the alternator back in the engine and the serpentine belt on and tested it again. once again a 3 amp draw. I went through 3 voltage regulators until I realized that the draw only occurs if the alternator is grounded to the car, (touching metal and completing the circuit I guess). I think I have a short somewhere. My question to you is where could the short be? could the alternator be shorting out on the inside? is this something that can be fixed by replacing the alternator or is it probably a wire somewhere running from the alternator that's touching metal somewhere. I appreciate any input. Thanks.

 

[boomerang]

Start pulling fuses one by one until you see a drop in the current draw. That will isolate it down to one circuit. IMO, unless you are extremely lucky, or some work has recently been done to the car that would assist you in knowing where to look, you're going to need a wiring diagram at that point to nail down where the problem is. There are a lot of parasitic draws on modern cars. You will be looking for a big drop not a minute drop like you'd get from pulling the fuse that powers the clock in the radio.

 

[machomocha]

thanks for the reply Boomerang. I guess I'll get out there and start pulling fuses. maybe I'll get lucky and it's something simple.

 

[phucheneh]

The alternator isn't 'drawing' so much as it's being supplied. As in, you're trying to fix the result instead of the problem. The problem, I'd wager, is that the alternator's rotor is being supplied with 12v when it shouldn't be. Giving the alternator a ground only causes it to draw current because it's completing a circuit; but that ground is always going to be constant. The issue is with the voltage feed.

Now, this is just a hypothesis, but I don't see any other option, given that you've already changed alternators and have pretty much confirmed that it's not some kind of strange backfeeding through the output terminal (big red wire).

To test: pull the two-wire plug and look for +12v. One wire is going to be the sense wire...I honestly can't remember if that's supposed to be switched or not. I think it will be on all the time, but I'm not positive (hey I made a pun). The other wire is your likely culprit- it feeds the windings of the rotor, which causes the electromagnetic field that in turn generates a much larger current when it spins. But when stationary, it's basically just a big resistor, discharging energy from your battery as heat.

If that it indeed the problem, you'll have to find a wiring diagram to figure out how that circuit is fed. There should be an something in the fusebox labeled 'ALT,' which should only have 12v with the key on. I would suspect it is fed directly off the ignition switch (no relay). So said ignition switch is probably your culprit. You may find that the back side of it (where all the contacts are) is melted, cracked, or otherwise damaged.

 

[bryanl]

One of the alternator's internal diodes could be shorted. Diodes and diode packs (trios) can be replaced.

 

[ford#1]

mabe try the starter relay sometimes that will create a big draw