GPCM killing batteries?

[Fantom]

So I've been chasing this electrical problem for a while. If I disconnect the batteries they're good to go, if i leave them hooked up something is killing them. Hooked up the multimeter on the driver's side battery, between the cable and battery post. It's not showing any amp draw. No radio hooked up, trailer brake controller unplugged so nothing else to be drawing power. After a short while, (5-10 minutes) The batteries are down to about 12.1-12.2 volts and won't crank the truck. Jumping it with my other 6.0 I have to charge the batteries for a couple of minutes before she'll start.

Batteries are 3 months old, alt is less about a year old and charging properly. FICM is a 58v swamps about 2 years old.

I've read a few threads about the GPCM making short work of draining the batteries.

So is it possible that's my problem, or is there something else that can drain the batteries that fast?

[dannyboy950]

The GPM main circuit if makeing contact can indeed draw a battery down in just a few minutes. So can the GPM battery wire if touching ground. So can the main wire from the battery to the starter if grounded. Actually any wire that gets it's power directly from the battery if grounded will draw a battery down. The bigger the wire the more amps it can draw the faster it goes down.

[forddieseldoctor]

Disconnect the wire that is bolted onto the passenger side positive battery terminal. This is the wire for the gpcm and the alternator. If you draw is gone then disconnect the big wire on the top or back of the alternator and reconnect the wire at the battery and see if your draw is back. Or you can unplug the 2 connectors on the gpcm that is located on the right hand valve cover.

[Fantom]

Originally Posted by forddieseldoctor

Disconnect the wire that is bolted onto the passenger side positive battery terminal. This is the wire for the gpcm and the alternator. If you draw is gone then disconnect the big wire on the top or back of the alternator and reconnect the wire at the battery and see if your draw is back. Or you can unplug the 2 connectors on the gpcm that is located on the right hand valve cover.

I'm going to try that tomorrow. Would a bad alternator draw current from the batteries?

[forddieseldoctor]

Yes they can.

[Fantom]

Went out today, batteries were at 12.51 volts, tested separately with cables disconnected.
Turn the key on, voltage dropped to 10.5, then dropped to 7 volts while cranking, even while jumping with another powerstroke. Cranked up and had a smell of diesel fuel coming from the exhaust.

[forddieseldoctor]

Have you had your batteries tested by a load tester? I'm wondering if you have a bad battery.

[Fantom]

Yep, had them load tested. Both tested fine and were replaced in November of last year.

[dannyboy950]

If you have checked the wires to the starter and they are not grounded, then you have a bad starter. You do have all the wires hooked up right on the solenoid?

[Fantom]

I haven't checked the starter, but I was wondering about it. If it was bad and demanding more juice than the batteries can produce. About a year ago, I went to start the truck and the starter made a grinding sound, but never did it again. I'll have the starter checked next