Strange Electrical Issue

[Daskuppler]

1 hour ago, jonathan909 said:

Wow - this has been dogging you for more than a year?  Awful.

This is not going to be easy.  It's easy to say "look for a bad ground".  It's quite another to find it.  Get the drawings for your exact model and year, because here minor variations count.  Then trace every ground and +12V wire, and make sure that both the wire itself and the terminations are good.  Here's the example I usually cite:

I had a 1997 Jeep Grand Cherokee before moving to Subarus.  Ran great for years, until the high-miles motor blew (rod through block).  Then it sat over a winter until I dropped in a new engine in the spring.  When I started it up again, all of the electricals were wonked out.  Turning on the stereo affected the wipers.  The lights affected the heater.  All of this unrelated stuff was suddenly functionally coupled in the most chaotic manner.  I bought the big thick book for that year's model from Chrysler and started tracing wires.  Eventually I found that where the big harness passed through the firewall (in the most inaccessible spot, of course), a tiny pinhole in the insulation on a +12V wire had let in enough moisture over time to corrode clean through the copper conductor.  Absent that supply line, a bunch of stuff found alternate supply paths - through other things, the result being that both were semi-powered and acted nuts.  The lesson is that because the effects were so random and crazy, it would have been a waste of time to try to think them through - you just have to check every wire that can have that kind of global effect, what we call "exhaustive search".

The other example is a simpler one:  After I had the motor out of my (first) '99 Outback for I-forget-what, the AT got all kooky - the shift points were all over the place.  Everything else in the car was fine.  Turned out that I hadn't tightened down that big, most-obvious-ground-in-car ground lug on top of the intake.  Tightened it and all was well.  So the effects can can be globally insane, or just localized to some weird thing.

 

I'm hoping it's something more simple and not damage I caused when replacing the HVAC box and had the dash out.

 

I did find some more corrosion on another ground, so I'll get that replaced along with the other wires in the big kit.