New highway truck wiring

This topic contains 1 replies and was last updated by james-taylor 7 years 10 months ago
November 16th, 2013 7:06am
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
November 17th, 2013 5:58pm
It is possible to be a low voltage sensing wire, which at a 5 volt signal would generally make your test light glow a dull orange or could be a signal sensing wire. Without seeing which one I can't tell you for sure but have seen this myself many times. Pull the wire or fuse, flip the ignition and the ecu resets. I had the exact same problem on an international 2 days ago. No rear lights, pull a fuse on the light control module, flip ignition and voila! all body lights work again!

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