Interesting electrical problem I fixed.

[Mark Nixon]

This is more of a general story, but the final fix was a very simple, yet interesting one.

A local kid brought his truck down, he was having problems with taillights not working, dim taillights and funky turn signals.
So I do the usual walk around tests, tails, turns, brake, left & right, front & rear.
We determined that 1 rear bulb had a shaken filament and the driver's side brake/turn wire was broken ahead of the gas tank, fix that.
Back to the taillights, figure out that the wiring for the driver's side taillight was faulty, fix that.
Man! we're doing GREAT!
20 minutes later this right rear taillight has me wondering, I've traced everything front to rear, continuity test from the plug to the socket, it all checks good.
Through out all of this the passenger's side dash signal lamp is glowing with the parks on, indicating a fault in the passenger's side running lights somewhere, which we were already onto with the rear taillight not working.

Then I get to thinking, "Hell, it must have shaken the passenger's parklamp filament loose", because, sure enough, the passenger's side parklamp was really bright, like the high filament was on.
Now, I've fixed a LOT of electrical issues where a bulb has shaken a filament loose and tangled it into the other filament and caused erratic operation of the lights, so this wasn't so unusual.

So I take the suspect bulb out FULLY expecting a shaken filament and....WTH!?!?
The filament is where it's supposed to be! Plug the bulb in, still bright , pull the bulb out again, check the taillight, DAMN! It's working right! Plug the front bulb back in AGAIN and......dead taillight AGAIN! WTF is going on?!?!

Remove the front bulb YET again, THEN I put in ANOTHER bulb and....PROBLEM SOLVED! Everything works correctly!
It turns out that this kid had put NEW bulbs in both of the front lamps and one was DEFECTIVE.

I later dissected the bulb and the metal collar had somehow gotten twisted during assembly and the low and high filaments were touching together.
In all of the 30-odd years I have worked on any variety of electrical and mechanical things, I'd NEVER actually seen a bulb where this has happened.

I know I'll never forget it.

Mark.

[Hansen01]

Weird, my left turn signal came on constantly for a while but turned out my ground to my core support was starting to get freyed, its still faintly on once in a while but oh well

[edoskocil]

Mark, I'm new on here but read that you may have some underhood harnesses. I have a 90 D250 auto trans and I'm looking for ALL of the underhood and dash wiring. The previous owner butchered mine. Thanks!

[mhuppertz]

Good catch Mark! Those electrical issues with seemingly conflicting symptoms give me heck!