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Discussion Starter #1 (Edited by Moderator)
Ok so 2008 duramax, Drw dump body run off dash pto, integrated trailer brake, just over 100k.

Earlier this summer was getting a service trailer brake warning on dash. I ended up putting new relay in, new relay harness pigtail, and new fuse in under hood box 30amp j fuse. Everything was good for a while. Now I got the warning again, so I put another new relay, new itbc computer and a new hand switch in the dash. Still nothing. Dealer reflashed the new computer, did nothing. Then they checked out all the wiring (after they told me it was fine) and said they found a bad wire. Not sure what they did but I’m assuming cut and butt splice. My business partner picked it up and when we put a trailer on it, any tap of the brake foot pedal gives FULL output to trailer brake, instead of rising the output the harder you push the pedal. Don’t know what the hell is going on but that lasted about an hour into the ride to the job site when the service trailer brake warning showed up again, and no output whatsoever. Almost a thousand bucks into parts and reflashing and dealer diagnosis and back to square one. Any thoughts? It’s my busiest time of year (excavating) and I need this truck to tow my 12.5k lb excavator. Please help!
 

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Thread relocated over to "Electrical" for more appropriate responses

:thumb:
 

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Firstly, is the truck equipped with the UY2 RPO?
Open glove box and look at production sticker. Is UY2 listed?
2ndly, since it has a dump body, what type of 7 pin plug assy is on there. I mean the back side. Does the plug have screw terminals or a weather seal connector?
 

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Discussion Starter #4
The 7 way looks like the first pic, the weather sealed one. I also changed that earlier this summer I forgot to mention that. As far as the rpo code, I’ll have to get it tomorrow. Truck is at the dealer since I paid all this money their married to it. They have to guarantee their work and they charged me and didn’t fix the problem so now since they didn’t thoroughly check their work they get to do it for free. However I have a feeling I’ll get it back not working with a shrug from them saying computer diagnostic checks out and says all is good but aren’t physically able to apply real world mechanics to figure out the problem.
 

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Discussion Starter #5
Just googled the rpo code you mentioned and it made me think back, when we bought this truck it had a 5 pin trailer plug we had swapped out for the 7 pin at the dealer we bought a trailer from. All was good and trailer brake worked great since 2017 up until this summer. The reason I changed the relay pigtail harness is because two different wires were exposed through the jacket and completely corroded. Then they found one more in another part of the harness inside of wire loom protector. I’m thinking there is more corroded wires in there somewhere.
 

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The 1st one pictured comes apart and the wires are screw terminal attached inside.
The second one unplugs from the harness and the harness connector looks like this.
 

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Discussion Starter #7
So just got the truck back from dealer... turns out they replaced the abs module and everything is working good now. Output is corresponding to how hard you press the pedal. Thanks for any help. Now time for an oil change and some new rear tires for the plowing season.
 

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Great that they found the problem. I wonder if the ABS light had been displayed on your instrument panel and if there were any stored codes and what they were??
 

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Discussion Starter #9
There was no light on the dash. Not sure if there were any stored codes. They claimed that either I or they may have shorted something out when swapping parts or reflashing the itbc computer, or when they were cutting and splicing wires
 
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