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.

[runningman777]

I just got a good chuckle... looking for another headlight answer and I started reading this post and realized it was all too familiar. Oh wait, this was MY post! Lol

About 2 months ago I installed some 60W LED bulbs and new housings. Pretty good improvement and nobody is flashing their brights at me so I'm not blinding anyone.

When I use the turnstalk to flash the headlights to let a passing truck know he can get over, my high beam indicator starts flashing. Seems the system thinks I have a bulb out. The 60W LED bulbs draw less than half the amperage of the halogens.

Tonight I installed 120W LED bulbs and they draw about 20% less that the halogens so I figured they would be a better match. Just looking at the light hitting my garage door I can tell they are somewhat brighter but not a huge change.

The bad news is the high beam indicator is flashing all the time that the low beams are on.

Tomorrow I will check voltages. When I replaced the housings, I installed new harnesses that run from the frame to the individual bulbs as there were just too many repairs to those wires over the years. Who knows what's wrong between there and the module that runs the lights.