LMM: Trailer Brakes

[SRR Max]

Hey everyone, new here and have a question. I have tried searching but haven't found exactly what I need. And I'm not sure if I'm in the right place, but here is my problem. I have hooked up to different trailers that have working brakes and get the same result--the factory trailer brake menu on my DIC shows output from my brake pedal, I can adjust the gain, but the trailer brakes are not working. Would this be the relay on the back of the truck or what? Thanks for all the help.

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[millerman04]

Bad ground maybe? Getting power back that far?

 

[SRR Max]

Well I'm not even sure which wires I'm supposed to be checking. I just read that the relay is back there near the bumper but haven't seen it yet either (haven't looked real hard).

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[GMC04DURAMAX]

Take a test light and have some one step on the brakes and see if the bottom left prong on your trailer plug in on the truck is getting power if its not check fuses and wiring if it was a ground issue you wouldn't have lights or turn signals on the trailer. Do the lights work just no brakes?

 

[SRR Max]

Yeah I should have explained more. I have checked all the fuses I could figure had anything to do with this. The lights do work on the trailers. Ok I need to do the test light thing and see if it is something else. minus a write being cut or something obvious, is there something else that could cause loss of power to that prong?

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[Harbin_22]

The factory brake controllers are very touchy. Everything has to be 100% for it know a trailer is hooked up. In my findings, it usually is a ground issue on the trailer

 

[SRR Max]

It really acts like it knows a trailer is hooked up because when hooked up, the output increases as I press the brake pedal. I will look at the ground on the trailer too though.

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[Harbin_22]

If the output increases, it knows they are hooked up. They usually have to be ran up pretty high to work like a normal box. If you slide the bar over, do they work? These trucks have a brake pedal position sensor, so the harder you hit the brakes, the more it applies the trailer brakes

 

[chevyburnout1]

May want to see if your ITBCM (Integrated Trailer Brake Control Module) needs an update. I had a customer that stated the trailer brakes all worked they just didn't work with very much power. I updated his module and so far he says it works great.

Could also be the solid state relay failing on the driver's side rear frame rail.

 

[SRR Max]

A dealer can look up if I need that update I guess? So if that relay is failing, it would still look like everything is working on the cluster but the trailer would not be receiving power?

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[chevyburnout1]

The dealer will need your truck to hook up to GM's programming system and see what calibration is in it.

I believe the system would act like it was operating correctly even if that solid state unit isn't putting out the correct voltage. However don't quote me on that last part as I have not had to deal with them for a long time. What you need to do is hook up a test light to the trailer wiring output, then connect a DMM to the test light probe as well. Then have someone squeeze the manual brake override and see if it goes to 12-14 volts. The test light creates a 'load' on the system and will make the ITBCM think a trailer is connected, thus allowing it to send braking voltage.

If you cycle the key and watch the test light you may even see it flicker for a second as the ITBCM sends a short pulse to see if a trailer is connected.

 

[Harbin_22]

What is a DDM, excuse my ignorance lol

 

[chevyburnout1]

Digital Multi-Meter

 

[Harbin_22]

Haha got it :thumb:

 

[trailerpro]

Take a test light and have some one step on the brakes and see if the bottom left prong on your trailer plug in on the truck is getting power if its not check fuses and wiring if it was a ground issue you wouldn't have lights or turn signals on the trailer. Do the lights work just no brakes?

Just for the record it's the lower right prong in the truck plug.

 

[SRR Max]

Ok thanks I will try that. And to answer someone's question, the trailer brakes do not react at all to me manually moving the slide bar on the brake controller.

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[Blacksox]

Help

I have the exact same problem as you
I've replaced the relay,checked fuses,replaced the IBC
I cannot reset the service trailer brake warning the IBC allows me to control that up-and-down I have trailer lights when I hook it up Still no trailer brakes and cannot clear the service trailer brake warning message
Anyone able to help here with more suggestions ?
Anyway to find out if my truck has been updated
And if I have to go to the dealer I suppose that this is not covered for free is that correct?

 

[Blacksox]

Service trailer brake warning

I still have not figured this out but I did notice without the trailer connected if I start the truck with my foot on the brake the warning light does not come up until I leave my foot off the brake. I don't know if this means anything
Maybe the sensor in the master cylinder?
Any thoughts ?
I have an appointment with the dealer on Monday

 

[Blackmax123]

My lmm was doing the same thing. I found the pig tail for the bumper mounted plug where it connects to you wiring harness under the bed by the rear tire was all corroded. I just got replacement pig tails for both sides and used heat shrink weather tight connectors and spliced them back in. They also make a really nice y plug set up you can plug in between the two pig tails if you want to run a plug up onside the bed for a gooseneck or fifth wheel.

 

[Blacksox]

I was having the same problem and could not clear the service trailer brake warning, did the relay first the IDC next and still was the same. Finally replace the pressure sensor in the master cylinder cleared the service trailer brake warning message. I will test the trailer brakes tomorrow, hopefully they will work
I read that sensor does not last very long, several people have replaced them several times already