I have a little mystery I hope you guys could shed some light on
So I had a brand new Peterbilt come into my shop the other day, and all the clearance and daytime running lights weren't working. First things first, I jammed a new fuse in to see the result, and it was a hard pop. I had a wire somewhere that had clear access to a ground. I removed the cover from the rear junction box that supplied the power to all the rear lights, found the terminal for the marker/clearance and disconnected what I thought was the supply voltage.
I returned to my fuse panel and replaced the fuse. Tada! I had power to all my front running lights again. So I had to assume one of the lights on the back had a bad circut.
before testing individual wires for a ground fault I used my test light on what I assumed was the supply voltage to the marker lights in the junction box. But, I had no power.....
the wire I disconnected from the junction box had no power, yet it saved the fuse from popping when I disconnected it. And yes, I tested the other wires and the terminal in the junction box for power. alas, nothing.
So, assured that a wire with no power would not cause the fuse to blow, I placed it back onto the junction box terminal.
Shazam, all my lights came on with no blown fuses,
Im wondering if there is a computer that controls the lights, I know of the headlight computer, senses burnt bulbs and etc. but its as if the short break in continuity allowed for a relay or a fuse to reset or something. perhaps the computer was diverting voltage improperly to a ground. I have no Idea!!!
I removed a wire from the brown terminal (clearance/marker) which i tested for power, it had none, the rest of the wires bundled on that brown terminal had no power either. ( and yes my test light works, i checked)
Yet when I reattached this seemingly pointless wires all the lights turned on.
Is this perhaps a sensing wire? low voltage to the ECM or vice versa? Im boggled