Under the dash and just behind and up above your aldl port is a bracket that should have a small ground wire screwed into it.
That bracket from the factory is bare steel and it rusts.
The weak little screw is some kind of a zinc plate self tapper but it is a ring flange self tapper. As are most of those small ground screws.
In other words, when you pull it and look at the bottom of the screw head.
It is not flat like you would expect.
It only contacts through a very thin ridge that runs around the outer diameter of the screw head.
No matter how good that thing looks, pull it and clean the base metal of that bracket and install a better screw.
No matter how good it looks
All kinds of strange random instrumentation problems are caused by that one little ground.
@thinger2 is on the money with his guidance. Here's a couple of illustrations from the '93 FSM
to give you a visual clue or 2 about what he's talking about.
By the way, the ground that affects nearly everything in the cab was named
G202 by the General.
Physical location illustration:
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And after all these years the factory splice where all the fanout is performed is also of interest
if the bad actor circuits still act like they are victims of a sketchy ground while the rest of them
continue to work as advertised:
S(
plice)
207
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The wiring diagrams for these trucks are pretty decent, and take a lot of guessing out of
repairing electrical gremlins like the ones you are suffering from. If you haven't downloaded the
ones for your year already I'd highly recommend you give yourself this unfair advantage.
Hope this helps. Happy hunting!