not_entirely_clueless

So I've just finished my long block swap into the 97 GC 4.0 auto. It's up and running, if a little roughly.

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.

Derision

I know, I know, everyone hates those "I don't know what the problem is, but I had a xxx that did the same thing!" posts, but I'm going to do it anyway. If just because it might help.

So I don't know what the problem is, but I had a '98 Neon that did the same thing. All kinds of bizarre electrical things. Sometimes the dashboard lights wouldn't come on except for the right directional and high beam indicators, and none of the gauges would function... unless I opened the driver side door -- then the whole dashboard worked until I closed it again. Other times, it would all work fine. Sometimes when I'd hit the brakes, the left front parking light would engage. It would go back to normal when I clicked the turn signal on for a second. The backup lights wouldn't work unless I turned the interior lights on.

I went through a boatload of searching. I tried following electrical connections, trying to figure out if something was shorting out somewhere, but could never find anything. Ultimately, what it turned out to be, was that the solders behind the gauge cluster were cracked. Sometimes it'd get warm and expand and make connections again, sometimes it didn't. Apparently, all kinds of bizarre things happen.

So yeah. I'm not entirely sure how a Cherokee is wired up in the dash, but having failed to locate those issues in the Neon for two years, I would probably start my search there, were it to happen to my XJ.

AJ

ETexas XJ

You have a ground issue. Some place a ground is loose or disconnected.
It may be in a connector with corrosion causing high resistance in a ground.
Voltage will always find a path to ground, if the correct one is having a problem it will hunt it through other circuits.

w-stev29

Check all grounds especially after swapping the engine. You have had every ground loose during the swap I would def. Start there.

not_entirely_clueless

Yeah, I believe this has all the hallmarks of a ground problem, but "every ground" just wasn't many, and I got them all hooked up. When I pulled the motor I left the harness in place. So besides the alternator and starter, I think there were only three ground lugs bolted to the engine, and they're all back where they came from.

Btw, Ramblers: 65 Ambo ragtop w/327 4bbl, 63 4-door Ambo w/327, 64 2-door Classic w/287, 64 4-door Classic w/6 that's gonna get one of the 327s.

not_entirely_clueless

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.