2010 Peterbilt 387 High Beam Relay Location

[runningman777]

After much digging thru the truck I found the turnstalk controller with the DRL module piggybacked to it.

The module has >PC/ABS< on it so I "assumed" (we all know what that word means) that this was the ABS module... but the part number P21-1019-103 shows it's the turnstalk lighting control which apparently controls the headlights, turn signals etc. There are no relays controlling the headlights.

The ">PC/ABS<" is the material the blue case is made out of.

After more searching using "387 Model Family Electrical", I FINALLY found the proper schematics for my truck: 94-6145. What a difference the proper documentation makes!

The lighting control is the source for the RH high beam and it was easy to verify as working.

Checked at the RH harness and found the same 12V, and the grounds were good.

The whole issue was someone replaced the pigtail before I did and swapped the ground and low beam connections. Which means the low beam worked but when I turned on the high beam, the two separate filaments were powered up in series which caused the dim high beam. Whew!!!

Now to put my dash back together... ugh!