Electrical gremlins 91 FJ80 (1 Viewer)

[GORDA91]

3FE engine. 229k miles. Mostly done baselining and so starting to get into the minor nits.

1: When I bought the truck the dome light closest to front was out. I replaced bulb and it didn’t work. Knocked hand against the cover and it flickered and then eventually worked. Has worked fine ever since.

2: lights in gear selector on floor never worked so assumed bulb was burned out and added to long list of to do’s.

3: on a long drive the other night and the gear selector lights came on out of nowhere. It was a glorious moment. But I also knew that meant a gremlin lurked somewhere…

4: got home; opened door; no dome light. Smacked it around as before, light kicked on, flickered and stabilized.

5: gear selector lights go out and no longer work. Gremlin materializes. Same scenario played out against last night. Gear selector lights come back on (glory) then go out if messing with dome light.

So they obviously are connected somehow. My fuses all seem ok. I pulled kick panel on d/s and there was an old Motorola cell phone speaker setup that was piggybacking off a couple fuses. I took the unit and all the wiring out. Nothing else obvious from visual inspection. I’m electrically challenged. For some reason have a hard time following wiring diagrams. I have a multi-meter and have used it before to test some switches at my house. So I can get up to speed if I need to. Appreciate any thoughts on best way to try to tackle this in a linear, systematic fashion.

Thanks.

 

[jonheld]

The dome lights and gear selector light are 2 separate circuits, fed from 2 different power sources with 2 separate ground paths.

All dome lights recieve a constant +12 from the 10 amp DOME fuse in the relay box on the left fender. The lights are triggered by grounding one of the door switches or from the switch on the dome light housing which grounds one side of the bulb socket.
The dome light housings are grounded through one of the mounting screws to the roof. They will be intermittent if the screws are loose or not making good contact with the ground trace.

The gear selector lamp is fed from the TAIL relay/fuse combination which feeds all interior dash illumination, and is only active when the headlight switch is in either the TAIL or HEAD position. It has a variable ground through the rheostat which controls the brightness of all internal dash illumination.

 

[GORDA91]

This is helpful. Thanks. Bizarre coincidence, I guess.

 

[GORDA91]

I took off both dome lights. They are getting 12v from the wiring harness coming from the front. I unscrewed from the roof and could not see anything that the mounting screws should be connecting to as a ground. They just screw into the ceiling. I cleaned everything with electrical cleaner and they work fine in door mode once they stabilize. The door switches both work fine. But they are intermittent if I flick them and it is hard to get them to work in ON mode without some series flicking. What am I missing? Could it be something inside the switch? That doesn’t seem like it is serviceable from what I can tell.

 

[GORDA91]

Unrelated gear selector gremlin seems to have been a bulb that was dying. I got it to work for awhile by fidgeting with the old bulb then it burned itself out for good. New one seems to be working reliably. TBD.

 

[jonheld]

I took off both dome lights. They are getting 12v from the wiring harness coming from the front. I unscrewed from the roof and could not see anything that the mounting screws should be connecting to as a ground. They just screw into the ceiling. I cleaned everything with electrical cleaner and they work fine in door mode once they stabilize. The door switches both work fine. But they are intermittent if I flick them and it is hard to get them to work in ON mode without some series flicking. What am I missing? Could it be something inside the switch? That doesn’t seem like it is serviceable from what I can tell.

In your first picture, the two left hand Phillips screws are the ground for the dome light. They're ground is created by screwing into the roof. If the screws are not tight to the roof, or not making good contact with the two traces on the left, the lamp will be intermittent.

 

[GORDA91]

In your first picture, the two left hand Phillips screws are the ground for the dome light. They're ground is created by screwing into the roof. If the screws are not tight to the roof, or not making good contact with the two traces on the left, the lamp will be intermittent.

Got it. They are both screwed in very tight and seem to be making good contact with the traces. I’ll take it off and try to see what the issue is. Maybe the headliner material is preventing good contact with the roof.