2010 Peterbilt 387 High Beam Relay Location

[runningman777]

On my 2010 Peterbilt 387 I have a dim passenger side high beam. Not critical unless some DOT officer wants to zing me for it.

I replaced the pigtail and bulb, but the problem persists.

Further checking I discovered that the voltage from the wiring harness is low if the bulb is connected. I believe the relay or relay connector is the likely issue, but I cannot locate the headlight relays or fuses. The schematics show the fuses are located on the chassis load center behind the glove box, but the 3 panels located there do not contain any relays pertaining to the headlights.

If I cannot find the relays, I will wire in a temporary relay to supply 12V to the high beam when the high beam wire is activated. I'm sure with the low current draw of a relay coil that I can make this work, but I really need to find those relays.

[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!

[Rawze]

What a mess. Sounds like you figured it out though.. thanks for sharing.