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I need to haul a trailer tomorrow and I hooked up to the trailer to verify everything works and I have zero power going to the trailer. No running lights/turn/brakes, nada. I checked the trailer with a different rig and the trailer worked perfectly. So the problem is with my truck.
I replaced fuse #6 that was blown with a new 20A fuse. Still doesn’t light up anything. I checked fuse #38 and it is in good condition. All lights on truck work as they should.
where do I need to go next? I have heard people mention the relay boxes that are near the master cylinder under the hood, but I don’t know what each of these relays do. Is one of them for the trailer lighting perhaps? Here’s a picture of the two relay boxes I have.
The 02 is a bit different than the 01 when it comes to the electrical system the I know, so just a few thoughts.
I am not sure if you have a fuse block under the hood but that is where I would start. Pull out the manual for your truck and read each fuse and location and verify they are good. Like I mentioned, the 01 has the trailer package under the hood, your location may vary.
If the fuses check good look at the relay blocks next. Again the book will tell you what is what.
I pulled the grey 12-pin harness plug (8 small 4x2 pins and 4 larger ones on the outside) that is located behind the spare tire and used a multimeter and a test light on each of the pins and nothing was getting power.
If all your truck lights are working correctly, and none of the trailer wiring has power, while the lights are on... then you need to trace up, from those connectors. Because, at least some of those wires, should share a common feed with the truck lights.
In checking for power at the pigtail connector did you use the ground in the harness or ground to vehicle? Do both to see if results change and if they do you'd know your ground is bad.
I went “upstream” of the rear 12 pin plug to the next plug which is located right behind the transmission. There’s a 12pin plug here where the truck light wires and the trailer light wires come together into one harness. I tested the plug at this point and got the following results:
Key Off, Lights Off - 1 pin lit up.
Key On, Lights Off - 3 pins lit up
Key On, Lights On - 6 pins lit up.
Flashers On - no pins were flashing
Brakes Applied - no additional pins lit up
so this leads me to believe that I don’t have brake light power up to this point and that I don’t have either blinker power at this point. Signs would point towards one of the 2 smaller relays that are in that small block by the master cylinder? I believe both of these relays are in good shape as they click and sound normal as you would expect.
These wire diagrams should help isolate your issue. The connector "upstream" of the 12-pin appears to be C410 per diagram 95-2 below. 2002 trailer wiring 1 2002 trailer wiring 2 2002 trailer wiring 3 2002 trailer wiring 4
I'd swap the underhood relays. I think they are the same. Include under dash relays in the rotation if they are the same. Idea is to work the connections in case they are dirty. Hit the sockets with a can of contact cleaner, let dry completely before putting relays back in.
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