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Parasitic Battery Leak - Traced to fuse 15

1997 - 2003 F-150

Parasitic Battery Leak - Traced to fuse 15

Old 11-07-2018, 09:41 PM
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Parasitic Battery Leak - Traced to fuse 15

HI All-

My son has my old 2002 F150 XLT with the 4.2L v6 with about 260k miles. The other day he told me his wipers were acting strange. After switching from one mode to another, it didn't actually change for a while. For example going from super high / fast wiper to intermittent it stayed in high / fast for 30 seconds or more before moving to intermittent. Then when he got home he said the truck was doing a lot of beeping and shuddered when he turned it off and odometer was all dashes and he thought the theft light was on when it wasn't previously. About a year ago, we replaced the alternator, but not the battery. The alternator was bad but the battery wasn't too old. Anyway, truck was fine until recently, but inspecting the battery it was about 3 or 4 years old and terminals were covered in gunk. Multimeter shows batter is low, so we go ahead and replace it. All the previous strange issues seemed to be gone.. Figured problem solved.

Next morning truck won't start. Brand new battery is dead as a doornail .. 6.3 volts. Read about the procedure to find a parasitic battery leak and decided the issue is on circuit 15. However, based on this thread:


I'm not sure I did the test correctly, but in the process of finding the offending circuit / fuse I did notice some unusual behavior .. at least unusual to me so maybe it's normal and I've just never noticed this condition:

When I removed the fuse for the "Alternator Field" there were several things clicking in various places and the light under the hood started blinking. I've heard of a diode on the alternator causing a parasitic leak, so not sure what's up with that. So I thought I would remove the alternator all together to see what's up. I tested by removing the negative terminal from the battery and putting my multimeter between the negative post and the negative lead. Draw was about 3.5 amps before. With the alternator completely removed, performing the test caused the same clicking and flashing of the hood light.

None of the tutorials I read mentioned anything about expecting any strange behavior when I pull the "Alternator Field" fuse.

OMG .. sorry for all the rambling. I guess the bottom line here is I think I have it narrowed down to the Alternator Field circuit or the circuit the GEM is on (fuse 15 in the cab) .. but there are some other things on that circuit, and the GEM seems to be a source of problems (with windshield leaking). I guess what I'm asking for is how can I test the alternator field and or the GEM to further narrow this down? Am I headed in the right direction?

Sheesh I type too much.


Last edited by chackett; 11-07-2018 at 09:46 PM. Reason: I said "ECM" when I meant "GEM"
Old 11-07-2018, 11:37 PM
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The way I check them is a little country but it works and it's easy. Leaving the battery connected, remove the main wire going to the alternator. If there is any arcing at all between the wire and the alternator, the alternator is bad. There should be zero voltage jumping from the wire to the alternator.
Old 11-08-2018, 12:20 PM
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OK .. well with a new day and a fresh set of eyes, I think I can safely say I think I'm still not sure what is going on But I do think its pretty likely I am dealing with one of two things:

1. Faulty Alternator - About a year ago I replaced the alternator with a remanufactured one I got from Orielly. From what I've read those things are often very faulty .. so bad it might even be bad right out of the box. The "Alternator Field" fuse caused some strange behavior during my parasitic leak test. But I don't think I did it right, so who knows.

2. Faulty GEM - Since some of the strange behavior started when it was raining (see above with wipers) and there is a lot of info on the problems with leaky windshield onto the GEM, this seems like a likely culprit. Additionally the amp draw went way down during my leak test. But again .. since I'm not sure I did the test correctly, I don't know that I can make any decision about the GEM or the alternator.

I guess what I need to do now is 1) do the leak test correctly. 2) Test the alternator (and maybe specifically the alternator field circuit) .. and 3) test the GEM somehow.

I'm open to ideas and or further reading!

Old 11-08-2018, 01:22 PM
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Check to make sure you don't have any water damage to the GEM.
Old 11-08-2018, 01:59 PM
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Originally Posted by ManualF150 View Post
Check to make sure you don't have any water damage to the GEM.
Thanks. I'll definitely try to get some eyeballs on it .. are there any real ways to test the GEM other than to visually inspect it for possible water damage? Or replacing it and hoping that was the issue?
Old 11-08-2018, 04:41 PM
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Well, you could unplug it and see if the drain stops. That might help narrow it down.
Old 11-08-2018, 07:09 PM
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If you unplug the GEM, you automatically remove the cause of the expected draw until the battery saver times out. That's the draw via fuse F15. However, with the GEM disconnected, then the alternator's circuitry is easier to check.

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