Tech Tip: Understanding the Relationship Between the Body Control Computer and Trailer Harness – UnderhoodService

Tech Tip: Understanding the Relationship Between the Body Control Computer and Trailer Harness

A body shop dropped off a 2005 Nissan Titan XE for a trailer light problem. It was hit in the rear and had folded the bumper under the body. I have known this particular body shop for years and they pride themselves on doing a top-notch repair on every vehicle that they work on.

A body shop dropped off a 2005 Nissan Titan XE for a trailer light problem. It was hit in the rear and had folded the bumper under the body. I have known this particular body shop for years and they pride themselves on doing a top-notch repair on every vehicle that they work on.

This truck was being a problem though. All the systems were working except for the trailer turn signal lights. To be thorough, they went back thru each and every part that they had replaced or disturbed. Nothing, not a thing. Two days of checking it out led to a dead end.

That’s where I came into the picture. As always, the first thing I want to see is the wiring diagram. You know, I’ve always said change is a good thing. This time, I should rephrase that to “it’s a good thing that things change.” And, I hope this does real soon, because, this was about the stupidest thing I’ve ever seen.

It’s not the first time I have ran across trailer lights going thru computer systems, but it’s the first time I ran across only the trailer turn signals running thru the BCM (body control module)  not the trailer park lights, not trailer brakes — just the turn signals.  

Right there on the print, plain as day, R-trailer turn and L-trailer turn. Oh please, what were you dudes thinking? (Referring to the engineers) What was the reasoning behind this?    

I checked the rear wiring and all the circuits involved along the underside of the truck… all of which were in impeccable condition. I couldn’t help but stare at that BCM on the wiring diagram. I said to myself, “You mean to tell me I’m going to have to change an expensive BCM just for trailer lights?”

The BCM is attached to brackets just above the gas pedal.  Once I managed to get into position to see the BCM, I moved the wire harness out of the way to get a better look at it. It all looked great. Another glance at the print showed that pin 51 and 52 were the trailer turn signal wires, one yellow/black and the other green/black. Using a test light I checked the leads output signal directly at the BCM connector. I was so glad to see the test light flashing with the turn signals — at least it’s not the BCM. It’s wiring in between the BCM and the rear connector.

I took another long stare at the print with my head under the dash, all wedged in between the driver’s seat and my feet dangling out the door. One hand was holding the wiring harness out of the way, the flash light was by my right ear, the test light and the wiring diagram were all getting cramped in what little space that’s provided. But, I still couldn’t see the problem.  Since I already checked the wiring running into the truck and the signal was coming out of the BCM, I had to be close — real close. I was starting to understand why the body shop spent two days looking for the problem and couldn’t find it.  

I knew what I had to do. I had to start hand tracing the two leads again from start to finish, one-more-time. At this point, anything would be better than spending another minute “sardined” in this truck.  As I moved my arm out of the way and was about to slide out from under the dash, I noticed right where I had my hand holding the harness out of the way were two small in line fuse holders, almost opaque in color right on the green/black and the yellow/black wires leading away from the BCM.  

AHA! THERE IT IS! Two in-line 10 amp fuses to the trailer lights! Fixed, done and works perfectly. The prints did not show any fuses in the circuits. They only showed the wiring leading from end to end. No wonder the body shop couldn’t find the problem. I even missed it until I moved my hand out of the way and knew the correct wire colors.  It’s not the sort of thing I’d expect to find — factory fuse holders in-line with the BCM, but there they were. You can be guaranteed that I made a note about this one.  I won’t forget next time.   

It doesn’t help that the prints were wrong, and it doesn’t help that the manufacturer ran the trailer turn signals leads thru the BCM. So, I guess I can officially change my status from “change is a good thing” to “good thing it changes.” Now do me a favor there engineers … Change it!

Scott “Gonzo” Weaver is the owner of Superior Auto Electric. He is the author of the book “Hey Look! I Found the Loose Nut”, that can be purchased online at Amazon.com or at gonzostoolbox.com  

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