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Discussion Starter · #1 ·
I know there are a ton of threads on this issue but so far, no luck.

Symptoms - "Service Trailer Brake System" on the DIC upon startup. On my scanner, I get a code - C0131 ABS/TCS System Malfunction (Symptom 2). This is from the Snap On Solus Ultra scanner. On my cheap handheld scanner, I do not see the "Symptom 2" message. I cannot find reference to what Symptom 2 means. There are no ABS lights on the dash. No codes are stored except when I scan the ABS side. A couple months ago I replaced the left front wheel bearing due to the ABS light and a "service trailer brake system" message on the DIC. At that time, I had no other issues after I replaced the wheel bearing. The truck has 78k miles and is a 2007.5 GMC LMM 4x4.

Since earlier today, I've replaced the sensor at the master cylinder brake pressure sensor, the sensor at the brake pedal (brake pedal sensor?), the integrated trailer brake control module (located on driver's side frame at rear of truck) and the relay for the trailer brake control module located right next to it.

I've pulled apart ALL the connections at the rear of the truck, used electrical cleaner and applied dialectic grease to all connections. I've checked all the trailer and abs related fuses for continuity. I've pulled the fuse panel located in the engine bay off and inspected for irregularities. I've checked the fuse panel on the driver's side dash. I've pulled the wiring harness off the brake controller located inside the cab. I've visually inspected the wiring harness from the rear of the truck to the engine bay and up into the fuse panel. I could not see every inch of the harness. Yesterday when I replaced my headlight bulbs, I had the batteries disconnected and even performed the "hard reset" as suggested in another thread where you touch the POS and NEG cables together once the batteries are disconnected. No luck there either.

I called and spoke to a tech at Galles Chevrolet in Albuquerque and he told me to check Terminal B at the ITBC. I could not find terminal B on the ITBC but I did find a terminal B located on the relay next to the ITBC. I checked it with the truck on and off and did not register any voltage. He told me that Terminal B should read between 4.8 and 5.2volts (higher or lower and it's bad). My guess is that I have a bad wire assuming that I have the correct Terminal B. The first week of January, I put on a new headache rack from Magnum Engineering and used their plug and play wiring harness. I put the extra wiring in a loop and placed it on the fuel tank. I ran the loom along the existing loom and fastened it with zip ties all the way to the back of the truck where the other end is connected into the existing trailer wiring plug. In case you haven't seen their wiring harness, you unplug the end located inside the bumper and plug in their wiring harness and connect the existing end into a female end on their harness. It's a good setup and I saw no issue with the wiring. In fact, I took it off and plugged the wiring back in the original factory way and it did not change the symptoms. I did not have any issues with the brake controller until a couple of weeks ago. Work was crazy and I just got around to working on it.

I'm out of ideas at this point and thinking of throwing in the towel and taking it to the dealer. I thought I'd appeal to my fellow gear heads on here and see if anyone has ideas or sees something I might be overlooking.
 

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Discussion Starter · #2 ·
I should also mention that I have an Idaho Rob EFI Live tune with the DSP5 switch. I have not had any issues with the EFI live setup since I installed it in 2012.
 

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Try re-calibrating the Brake Pressure Sensor with a Tech II.
 

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Discussion Starter · #5 ·
I don't have a Tech 2 but I posted up in the Southwest forum. Is this common with the brake pressure sensor?
 

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Have this same thing going on, thinking its moisture related as its more frequent with rain/melting snow.
 

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Is there any chance it could be an actual trailer brake wire or worn or damaged magnet causing a short in the system and causing your issue. Do you have another trailer available to plug into your truck and see if the issue is the truck or trailer?
 

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Discussion Starter · #8 ·
Is there any chance it could be an actual trailer brake wire or worn or damaged magnet causing a short in the system and causing your issue. Do you have another trailer available to plug into your truck and see if the issue is the truck or trailer?
It occurs when there isn't a trailer hooked up.

I am leaning toward it being a wiring issue. Does anyone have the procedure for troubleshooting in the GM Tech manual?
 

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Try cleaning the contacts and putting dielectric grease in the main plug
 

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Rara Avis
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I know there are a ton of threads on this issue but so far, no luck.

Symptoms - "Service Trailer Brake System" on the DIC upon startup. On my scanner, I get a code - C0131 ABS/TCS System Malfunction (Symptom 2). This is from the Snap On Solus Ultra scanner. On my cheap handheld scanner, I do not see the "Symptom 2" message. I cannot find reference to what Symptom 2 means. There are no ABS lights on the dash. No codes are stored except when I scan the ABS side. A couple months ago I replaced the left front wheel bearing due to the ABS light and a "service trailer brake system" message on the DIC. At that time, I had no other issues after I replaced the wheel bearing. The truck has 78k miles and is a 2007.5 GMC LMM 4x4.

Since earlier today, I've replaced the sensor at the master cylinder brake pressure sensor, the sensor at the brake pedal (brake pedal sensor?), the integrated trailer brake control module (located on driver's side frame at rear of truck) and the relay for the trailer brake control module located right next to it.

I've pulled apart ALL the connections at the rear of the truck, used electrical cleaner and applied dialectic grease to all connections. I've checked all the trailer and abs related fuses for continuity. I've pulled the fuse panel located in the engine bay off and inspected for irregularities. I've checked the fuse panel on the driver's side dash. I've pulled the wiring harness off the brake controller located inside the cab. I've visually inspected the wiring harness from the rear of the truck to the engine bay and up into the fuse panel. I could not see every inch of the harness. Yesterday when I replaced my headlight bulbs, I had the batteries disconnected and even performed the "hard reset" as suggested in another thread where you touch the POS and NEG cables together once the batteries are disconnected. No luck there either.

I called and spoke to a tech at Galles Chevrolet in Albuquerque and he told me to check Terminal B at the ITBC. I could not find terminal B on the ITBC but I did find a terminal B located on the relay next to the ITBC. I checked it with the truck on and off and did not register any voltage. He told me that Terminal B should read between 4.8 and 5.2volts (higher or lower and it's bad). My guess is that I have a bad wire assuming that I have the correct Terminal B. The first week of January, I put on a new headache rack from Magnum Engineering and used their plug and play wiring harness. I put the extra wiring in a loop and placed it on the fuel tank. I ran the loom along the existing loom and fastened it with zip ties all the way to the back of the truck where the other end is connected into the existing trailer wiring plug. In case you haven't seen their wiring harness, you unplug the end located inside the bumper and plug in their wiring harness and connect the existing end into a female end on their harness. It's a good setup and I saw no issue with the wiring. In fact, I took it off and plugged the wiring back in the original factory way and it did not change the symptoms. I did not have any issues with the brake controller until a couple of weeks ago. Work was crazy and I just got around to working on it.

I'm out of ideas at this point and thinking of throwing in the towel and taking it to the dealer. I thought I'd appeal to my fellow gear heads on here and see if anyone has ideas or sees something I might be overlooking.
A C0131 can get set if the ECU is loosely attached to the hydraulic unit. The mountings should be checked for the proper torque.
 

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I just got this message this morning. No trailer hooked up. Message disappeared when I pushed check button. This was the first time I started the truck in a week. The humidity this morning is 100% - Florida dripping wet but not raining.

Is this an intermittent thing? When I move the manual lever I get no output on the Trailer Gain display - I can't remember if I'm supposed to without a trailer hooked up. I'm going to plug in my 5er a little later and try it to see if it works. On EDIT: the display on the DIC is on solid now and there is no brake output with the trailer hooked up.

In 2 weeks I'm going to haul my trailer back north (over 1000 miles) and I don't want the trailer brakes giving out especially going through the mountains in PA. Should I be taking to to a dealer? I hate the thought of them trying to chase a phantom. I don't know if it's a warranty item.
 

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Discussion Starter · #13 ·
On my 2007.5, the progressive ads do not show unless something is hooked up. Presently, I cannot see any bars when I am hooked up. I am about to break down and take it to the dealer.

Would returning it to stock and then later putting EFI live back on be helpful? I guess it can't hurt but I've never nada y issues with EFI live (Idaho Rob's tune and dsp5 switch).

Since you are under warranty I would definitely take it in. Get it documented in case the issue is recurring.
 

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OnStar just ran diagnostics and told me my code is C0131 which I believe is "master cylinder pressure sensor intermittent".

ON EDIT: I've slipped a cog - it's out of warranty by 4 months
 

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Discussion Starter · #15 ·
The master cylinder pressure sensor is easy to replace. It sits at the bottom of the reservoir and is held in place by two prongs. Depress the prongs and it will come out. I also drained and replaced the brake fluid when I did this.

My next step is to see if I can recalibrate the sensor bc I still have the same error code.

The sensor ran me $22 at the GM parts counter.
 

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The sensor ran me $22 at the GM parts counter.
Ouch! They hit me for $89 for that part. And 1.5hrs labor and (hopefully) my story is over. For once, it was exactly what the computer said it was.
 

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Discussion Starter · #18 ·
Is it the part that plugs into the master cylinder under the reservoir? Maybe he rang it up wrong but I paid $22 plus tax. I have no reason to bull$hit you. Since it was cheap, I decided to buy the brake pedal sensor/switch as well.
 

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Discussion Starter · #19 ·
Joebedford,

Do you have the part number? I ran my part number and it calls it a "level indicator" and was $20.79 with tax. Maybe I didn't get the right part. I checked online and the price is even cheaper for part # 18012741. This must be why we have such a large disconnect on the price of the part.
 

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The level sensor is the one you can see looking into the engine compartment from the left fender.

The one that caused my trouble is on the RIGHT side of the master cylinder - I had to stand on the hole in the bumper to get at it.

The GM invoice says FP Number 15838718 Sensor 4.663 - $38.69 on Amazon
 
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