I fixed my truck, after the mechanics couldn't!
Naturally, I need to begin from an earlier point in the timeline.
For some time mow I have had a bizarre electrical problem going on here. The truck runs fine and its external lights necessary for safety are good. Its the non-essential interior circuits that seemed to be messed up.
At arbitrary times, for no apparent reason, I would lose power to the cigarette lighter circuit, and all interior dome and map lights, as well as the utility lamp on the back of the cab that helps me to see when I'm hooking air lines to the trailer. Then after an equally arbitrary period of time, they would come back on. Also for no apparent reason. After a while I realised that there was a click from the fusebox when this happened, and until recently I had been of the opinion that the sound was from an automatic breaker that was re-setting.
What was odd, and of course maddening because of its oddity, was that these items that lose power should belong to at least two separate circuits. There is a fuse for the cig lighter outlets, and a fuse for the dome light, etc. Being separate circuits, they should have nothing in common save that they get power from the same batteries.
I had the truck in the shop twice with this complaint, and after the first time I thought that they had fixed it. Perhaps they did fix the problem, but the issue developed again. I noticed it again during a heavy rain. There were incidents of rain around the first occurrence, so I wondered if there was a short in a ground wire outside at the battery box. When I picked up the truck from the shop the second time, the interior lights were on, and I had cold Pepsi in the cooler. Within 10 minutes, the lights were out again.
So I got a styrofoam cooler out of the car, and put 6 pounds of ice in it to keep my sandwich meat from spoiling. I discovered a few hours later that there was a hole in the bottom of it as evidenced by the squish of my stocking feet on the rug. So now I have to rely on whatever arbitrary thing is doing it to work in my favor. Not likely.
Then I remembered that there is a system that shuts off power to non-essential accessories when battery voltage is low. It didn't occur to me earlier that this could account for the multiple and ostensibly separate circuits that I was having problems with. So if theres a short there, or it just isnt reading the voltage right, that would result in the current situation. [Hah! "current" situation! I kill me!] The remaining problem was; where is this relay? [I assumed a relay since it is on in one condition and off in others]
So. Finally I found it. Its a relay actuated by an electromagnet of apparently unusual designs for most relays. This stands to reason as it must be sensitive to the voltage of battery current and open or close the relay appropriately. And one of the 4-5 wires that feed this relay is loose. Hold relay this way, lights go out. Hold relay that way, light goes on.
So now I just need to have them re-wire that relay so the connections are secure. But I prolly have to explain to them which one and why it needs to be re-wired... Until then I need to leave a light on. That way I can know when to go and wiggle my relay.