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Weird I mean WEIRD electrical problem


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deep_weeds
Novice

Jul 1, 2012, 1:10 AM

Post #1 of 6 (915 views)
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Weird I mean WEIRD electrical problem Sign In

So I've just finished my long block swap into the 1997 Jeep Grand Cherokee 4.0 auto. It's up and running, if a little roughly - tuneup to come.

History: Last November, after nearly 500k km, the engine blew. Then it sat until last week when I tackled the swap. So it was without battery for about six months. Also, before the motor went I'd had no electrical problems with this thing, weird or not.

Now: A bunch of seemingly-unrelated things don't work. The hazard flashers, turn signals, radio, dashboard computer, interior lights, and power seats are dead. Perhaps the oddest of these is the hazard lights, as they should work even when the key is out. Everything else works. But I made a strange discovery.

If I hold the headlight dimmer switch in (that is, pulled toward me) when the high beams are on, all of the dead things work. However, toggling the dimmer and holding the switch (again, pulled toward me in the low-beam position) makes all those things fail.

I've checked every fuse and swapped every relay possible.

Weird. Consistent, but weird.

Any hints? My first thought was the multifunction switch itself, but so many of these things (the radio and power seats, for dog's sake) have nothing to do with it. So is it the Body Control Module? I'm an EE, and this one's got me stumped.

Thanks.

Jonathan

p.s. The reason I mention the time it's been stored is that I'm wondering whether there's some kind of battery backup in the BCM that might have gone dead and caused it to lose rational settings, just like a PC losing its BIOS settings when the motherboard battery goes flat.


Tom Greenleaf
Ultimate Carjunky / Moderator
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Jul 1, 2012, 2:45 AM

Post #2 of 6 (881 views)
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Re: Weird I mean WEIRD electrical problem [In reply to] Sign In

My first guess with an engine swap especially is you have one or more ground issues either broken, not tight or poor connections,

T
_________________________________________
Long retired now


deep_weeds
Novice

Jul 1, 2012, 10:40 PM

Post #3 of 6 (836 views)
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Problem solved. Though I have strong resistance to believing in coincidental failures, that's what happened here.

Not a ground problem at all. One of the fused but unswitched +12 wires running from the Power Distribution box (next to the battery) to the Junction Block (down in the passenger footwell) had a nick in its insulation where it passes through the firewall. The nick let in moisture, which eventually (concluding sometime in the last six months) corroded the wire in two.

If I hadn't spent the $100 on the factory manual for this thing when I bought it seven years ago, I wouldn't have stood a hope of finding the fault.


Tom Greenleaf
Ultimate Carjunky / Moderator
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Jul 2, 2012, 1:47 AM

Post #4 of 6 (825 views)
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Glad you found it. Stuff happens like that,

T
_________________________________________
Long retired now


nickwarner
Veteran / Moderator
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Jul 2, 2012, 8:23 PM

Post #5 of 6 (803 views)
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Smart move getting the right data and knowing how to use it. Congrats and thanks for posting the fix.


Discretesignals
Veteran / Moderator
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Jul 2, 2012, 8:48 PM

Post #6 of 6 (799 views)
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Even though this doesn't fall under your problem it goes to prove that when you punch holes in the insulation on wiring to make electrical checks or you splice circuits, you should reseal those holes and use the correct splicing methods. Seen a lot of electrical problems because someone has been there before and didn't clean up their mess.





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