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Discussion Starter · #1 ·
Summary - 2013 3500 crew cab. "factory" electric brake controller. No malfunction signals. Switch (part no.) 15926102 shows normal display (of increasing power to the "female" trailer connector at base of tail-gate) when I manually over-ride it to turn on electric trailer brakes.

But I am only getting a constant 3.5 volts at the "female" trailer connector regardless of whether the brake pedal is depresed ( lights work normally) . Any ideas ?
 

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When you say female trailer connector, I assume you are talking about the truck-side trailer connector? And where are you getting your ground? The wiring diagram for my 2008 says "

4 ITBC MDL, SSR, AND TRAILER MUST GO TO A COMMON GROUND

And I take it that the common ground is the pin in the trailer connector, not external to the frame, etc. You can see what the ELECTRICAL MANUAL says about your wiring diagram here: GM UPFITTER - Body Builder Manuals
 

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Discussion Starter · #3 ·
When you say female trailer connector, I assume you are talking about the truck-side trailer connector? And where are you getting your ground? The wiring diagram for my 2008 says "

4 ITBC MDL, SSR, AND TRAILER MUST GO TO A COMMON GROUND

And I take it that the common ground is the pin in the trailer connector, not external to the frame, etc. You can see what the ELECTRICAL MANUAL says about your wiring diagram here: GM UPFITTER - Body Builder Manuals
you lost me - thanks for your input....not clear how you think that can help?

What has PRE 2013 wiring issues have to do with my mid production 2013
?

For your info, there was a production change for 2013; idea was to make it mandatory to order the "factory" elect. trailer brake installation - no provision for hooking up an after-market system.

If changing out the #15926102 dosn't resolve this, I may have to go "back-yard" and try & figure out how to defeat their attempt to make it impossible to go aftermarket...
 

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Think that for the normal braking signal you have to be moving for brake application. The slider will set the brakes at any time.
 
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you lost me - thanks for your input....not clear how you think that can help?

What has PRE 2013 wiring issues have to do with my mid production 2013
?
The voltage check needs to be at the same place as the ITBC system or you may get an incorrect reading of how the system actually operates. A wire with voltage may get different readings if tested to ground at two different points, i.e. the voltage at the B+ terminal vs the connector attached to that terminal. The ITBC system is quite sensitive to varying voltages. If you grounded your DVM to the frame or other point, that MIGHT be why you got the reading you did.

While there are some differences between years, many of our GM trucks, even the gassers, share a great deal of commonality in their systems, wiring, etc. I looked at the wiring diagram I have for my truck and saw this and thought it was worth mentioning. Perhaps your truck's wiring diagram will have similar or different information.
 

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Discussion Starter · #6 ·
I appreciate the posts. Trouble is, looks like I either ..... 1) failed made myself clear and/or 2) some of you guys dont understand the system.

Let me see if I can help some of you better understand the electric supply wiring for electric trailer brakes.

Imagine the female recept. below your tail-gate as a clock. At the 7:00 o'clock position is the "common" ground. (as a side note, your trailer's hitch should make a seperate excellent "ground" once resting on your hitch's ball.).

The 5:00 o'clock position supplies power from the controller to your trailer brakes - depending on what the controller is set up to do, it should vary anywhere from negligible current up to a full 12 volts for max. electric brake application.

Positions 3 and 9 o'clock are your signal/stop lights. When activated at the same time, that's what "turns on" your controller telling it to engage power to #5. Position 1:00 o'clock is vehicle power. That should be "hot' ( a full 12 volts) (all the time, or only when your key is on, depending on how your truck is wired).

Hopefully, some of you have experience with "failure modes" and can tell me what to replace. For now, I have ordered the dash-board controller. I am getting no "codes" on my 0SB-2 "reader".

? ? ? ?
 

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I appreciate the posts. Trouble is, looks like I either ..... 1) failed made myself clear and/or 2) some of you guys dont understand the system.

Let me see if I can help some of you better understand the electric supply wiring for electric trailer brakes.

Imagine the female recept. below your tail-gate as a clock. At the 7:00 o'clock position is the "common" ground. (as a side note, your trailer's hitch should make a seperate excellent "ground" once resting on your hitch's ball.).

The 5:00 o'clock position supplies power from the controller to your trailer brakes - depending on what the controller is set up to do, it should vary anywhere from negligible current up to a full 12 volts for max. electric brake application.

Positions 3 and 9 o'clock are your signal/stop lights. When activated at the same time, that's what "turns on" your controller telling it to engage power to #5. Position 1:00 o'clock is vehicle power. That should be "hot' ( a full 12 volts) (all the time, or only when your key is on, depending on how your truck is wired).

Hopefully, some of you have experience with "failure modes" and can tell me what to replace. For now, I have ordered the dash-board controller. I am getting no "codes" on my 0SB-2 "reader".

? ? ? ?
Do you have, or have you seen, a “Service Trailer Brake System” message on in the driver’s information center (dic for short)?

Does the dic show the message “Trailer connected” when you connect a trailer to the truck?

Only high end scan tools will read diagnostic trouble codes (dtc) from the trailer brake control module (tbcm).

Finally, the tbcm will only put approximately 3.5 volts down circuit 47 to “test” the wire. A voltmeter or regular test light will not take place of a trailer with at least four magnets on it therefore, the tbcm doesn’t “see a trailer” so it will not send anything other than the 3.5 volts down that circuit.

Pro tip, a halogen headlamp bulb will simulate a trailer very well. When connected between pin c/circuit 47 and ground, of the truck side connector, the bulb should flash once and go out, with the ignition on of course. The trailer connected message should come up on the dic at that time. Then, you should be able to apply the brake pedal, the bulb will come on and you can adjust the brightness of the bulb by using the gain control on the dash mounted trailer brake control switch. The manual apply slide lever on the trailer brake control switch will also cause the bulb to light.

Again, use of a dvom or regular test light will not accurately diagnose this trailer brake system. Seeing the 3.5 volts at least tells you that the tbcm is at least connected to the truck side trailer connector. That’s all. It is super low amperage though so it doesn’t tell you how good that circuit is though so don’t let that fool you.
 

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First-- the hitch ball makes a crappy ground most of the long-time haulers on here will agree. That one assumption has caused a lot of guys waisted time troubleshooting needlessly. Especially when its greased.
Thats why there is a separate ground pin in the trailer and truck connectors and it is white wire.
Secondly- please don't sound so condescending in your replies. I dare say that those of us that haul quite a bit know how the brake control system works.
There are several posts on here with very good descriptions on the ITBC operation and indications.
 

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Discussion Starter · #9 ·
First-- the hitch ball makes a crappy ground most of the long-time haulers on here will agree. That one assumption has caused a lot of guys waisted time troubleshooting needlessly. Especially when its greased. Thats why there is a separate ground pin in the trailer and truck connectors and it is white wire. Yes - well aware of that - I should have added that I ran a 'load" test on the "ground" pin to confirm I have a good "ground"

Secondly- please don't sound so condescending in your replies. I dare say that those of us that haul quite a bit know how the brake control system works. My apologies - You and I arent the only ones who read the posts on this site...was just trying to be helpful to those who might be interested in this particular problem.

There are several posts on here with very good descriptions on the ITBC operation and indications. My aologies...I am incompetent with the Internet, reserach tricks, etc. Will try and follow up on your suggestion.
..
 

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Discussion Starter · #10 ·
Do you have, or have you seen, a “Service Trailer Brake System” message on in the driver’s information center (dic for short)?

Does the dic show the message “Trailer connected” when you connect a trailer to the truck?

Only high end scan tools will read diagnostic trouble codes (dtc) from the trailer brake control module (tbcm).

Finally, the tbcm will only put approximately 3.5 volts down circuit 47 to “test” the wire. A voltmeter or regular test light will not take place of a trailer with at least four magnets on it therefore, the tbcm doesn’t “see a trailer” so it will not send anything other than the 3.5 volts down that circuit.

Pro tip, a halogen headlamp bulb will simulate a trailer very well. When connected between pin c/circuit 47 and ground, of the truck side connector, the bulb should flash once and go out, with the ignition on of course. The trailer connected message should come up on the dic at that time. Then, you should be able to apply the brake pedal, the bulb will come on and you can adjust the brightness of the bulb by using the gain control on the dash mounted trailer brake control switch. The manual apply slide lever on the trailer brake control switch will also cause the bulb to light.

Again, use of a dvom or regular test light will not accurately diagnose this trailer brake system. Seeing the 3.5 volts at least tells you that the tbcm is at least connected to the truck side trailer connector. That’s all. It is super low amperage though so it doesn’t tell you how good that circuit is though so don’t let that fool you.
 

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Discussion Starter · #11 ·
SPECIAL THANKS to "ChevyTech77". His above post is an outstanding example of the benefit to us "backyard" types, to being a part of this forum....we meet people like "ChevyTech77" who can provide competent technical analysis.

In this particular case..maybe I wasn't clear when I typed "no malfunction signals; switch # 15926102's LED display shows normal display of increasing power" (to the trailer output - meaning the 5:00 o'clock position".

I should have made it clearer - yes, I see "trailer connected" display when I hook up. And, as a side-note, with the switch # 15926102 removed, I get the "service trailer brake system" message, which goes out when I reconnect the switch.

Perhaps I should have explained further that each and every time I hook up, I drive foward a little bit, say 20 mph, run the dash-switch to full application, and see the LED's show "full power"........Up to now, that applies the trailer brakes...hard! This time...I could feel the trailer brakes going on, but with not applying the way they should..little more than a "dragging" brake. To double check my trailer's brakes, I applied 12 vots to the MALE side of the connector...instand brake application "lock up" on all four wheels.

So - what kind of failure modes are typical of this system? When I look under the dash, of course I have NO clue what I am looking at! So I dont know what to replace.

I've ordered a new GM "factory" 15926102......any chance that is where the failure is (or have I thrown out thirty bucks..meaning do I still need someone with a more sophisticated 0BD-2 "reader"? than I have...?
 

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SPECIAL THANKS to "ChevyTech77". His above post is an outstanding example of the benefit to us "backyard" types, to being a part of this forum....we meet people like "ChevyTech77" who can provide competent technical analysis.

In this particular case..maybe I wasn't clear when I typed "no malfunction signals; switch # 15926102's LED display shows normal display of increasing power" (to the trailer output - meaning the 5:00 o'clock position".

I should have made it clearer - yes, I see "trailer connected" display when I hook up. And, as a side-note, with the switch # 15926102 removed, I get the "service trailer brake system" message, which goes out when I reconnect the switch.

Perhaps I should have explained further that each and every time I hook up, I drive foward a little bit, say 20 mph, run the dash-switch to full application, and see the LED's show "full power"........Up to now, that applies the trailer brakes...hard! This time...I could feel the trailer brakes going on, but with not applying the way they should..little more than a "dragging" brake. To double check my trailer's brakes, I applied 12 vots to the MALE side of the connector...instand brake application "lock up" on all four wheels.

So - what kind of failure modes are typical of this system? When I look under the dash, of course I have NO clue what I am looking at! So I dont know what to replace.

I've ordered a new GM "factory" 15926102......any chance that is where the failure is (or have I thrown out thirty bucks..meaning do I still need someone with a more sophisticated 0BD-2 "reader"? than I have...?
Are you 100% certain that your trailer brakes are properly adjusted? Good/clean trailer side ground? How many amps does the trailer brakes draw when applied?

I have no idea what part number that is so sorry if I can’t help there.

The halogen light bulb test is your friend. If you have a trailer connected message and no service message, you most likely don’t have an issue with the truck side system. The bulb test will verify that. This system is very sensitive to most any issue that can cause it to not function properly because it is a safety system.
 

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Discussion Starter · #13 ·
Are you 100% certain that your trailer brakes are properly adjusted? Yes - "not-so-funny" side note - the literature that came with my approx. 28 ft. box trailer claims it has self-adjusting brakes. First time I pulled the wheels for a brake shoe inspection, much to my disappointment, found they are very standard very ordinary automotive type with the typical "star" adjuster system.

Good/clean trailer side ground? Yes excellent question - i put a "load" on the "ground" connection in the female recept.; to confirm....and yes, both my "ball' and my "receiver" are clean, so either way, trailer is well-grounded.

How many amps does the trailer brakes draw when applied? Not sure exactly, not enough to "trigger" the circuit breaker/over-load my battery charger, but enough to "peg" the needle, so best guess is around 8 amps at full application.

I have no idea what part number that is so sorry if I can’t help there. Ha - show some sympathy for amatuers like me.....( "when you can't figure out what is wrong...you throw parts at the problem...."). the above part number is the dash control "head" (with LED display & "over-ride" switch. Of course I understand enough about the system to know that somewhere on the truck is an amplifier of some sort, and some kind of relay. The fact that I get the constant minimal voltage makes me suspect a failure in one of those.



The halogen light bulb test is your friend. If you have a trailer connected message and no service message, you most likely don’t have an issue with the truck side system. The bulb test will verify that. This system is very sensitive to most any issue that can cause it to not function properly because it is a safety system.
CANT THANK YOU ENOUGH FOR YOUR TRYING TO HELP !
 

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Positions 3 and 9 o'clock are your signal/stop lights. When activated at the same time, that's what "turns on" your controller telling it to engage power to #5
That may be how some controllers work, but not those TBCM modules in our chevy trucks. Take a look at pages A65 thru A85 here:Light Duty 2013 - GM Upfitter Also note the locations in the 7-way connector.
 

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Discussion Starter · #15 ·
That may be how some controllers work, but not those TBCM modules in our chevy trucks. Take a look at pages A65 thru A85 here:Light Duty 2013 - GM Upfitter Also note the locations in the 7-way connector.
= = = = = = = =
Thank you for trying to help. Obviously, I am way over my head - I do not see how ANY electric brake system can be "triggered" without some kind of activation of some kind of mechanical switch. or electrical imput. In earlier years...a mechanical switch somewhere in the brake pedal linkage would turn on, activtation tail lights and, if connected, electric brakes, typically thru some kind of controller that could put out a variable and/or pulsating current to the trailer brakes, depending on the command. In later years. electric brake controllers react to current being applied to stop-lights.

I confirmed my trailer's electrical wiring, brake adjustments..etc...are all correct, by the simple act of pulling out and thus activating the "breakaway" switch (that delivers full battery voltage to the brakes). that pretty much locks up my trailer's brakes.

Bottom line - I am way over my head...I am fortunate that here in the Prescott area our GMC dealer is staffed by competent, professional guys eager to help. Their shop rate is about a hundred bucks per hour. My experience with them, on occasion actually watching them, satisfies me they know what they are doing, proud of their work, and most certainly not out to screw their customers. Have appointment 10:30 tomorrow Wed 8-18. I can post what happens if anyone's interested.
 
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Please post your out come after dealer visit
 

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Discussion Starter · #17 ·
Please post your out come after dealer visit
............................................................................................................................
Was a very frustrating demeaning experience.

They took my truck at the exact time of my previously arranged-for hour....had it at least half an hour.

The shop manager then came over to me sitting in the waiting room.... took me back to meet the technician. I have never been so insulted in all my life...!

Both the shop mgr. and the tech (a guy ALMOST as old as me).....tried to be courteous, friendly, and professional...but the bottom line is....they said...and proved with their various tools, that I am CLUELESS about modern technology! Yes...they tried to make it into a joke....but their insinuation was clear.....my knowledge of automotive technology belongs in MUSEUMS! The shame of it all!

They confirmed there is NOTHING wrong with my "factory" electric brake controller system! My primitive method of TRYING to test it out, was throwing me off.... was a waste of my time. Yes, I knew their 0BD II equipment is far more sophisticated than the "consumer grade" stuff I have. That's why I went there!

So they were able to go thru the whole computer-controlled system and confirm it NEVER had any error-codes EXCEPT when I fooled around with the dash controller (replaced it...with a nice new one "genuine GM part" (small print on back of the dash unit says "Made In China"...).

Obviously, during the time I had the unit unplugged for replacement, that triggered a "SERVICE TRAILER BRAKE" code...! Then they led me back out of the shop, told me they'd bring the truck out of the shop themselves - I should see the cashier about my keys.

It gets worse. When I went to the cashier to pay & pick up my keyes, the insult continues....They flatly REFUSED to give me a bill! Can you imagine? All in all, I wasted a full hour of their time....and all I got was a handshake!

Let this be a lesson to you...do not deal with Lindley GMC in Prescott, Arizona. They simply cannot be trusted to behave the way so many of you post in here about GM authorized dealerships.....

Oh...by the way....I think I figured out what was fooling me into thinking there was something wrong with my system. THe "female" trailer "plug-in recept." under the tail-gate......maybe my imagination...i didn't like the looks of one of the pins....specifically, the vital "5 '0'clock" position.....so I changed out that socket with a new one.
 
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Was a very frustrating demeaning experience.

They took my truck at the exact time of my previously arranged-for hour....had it at least half an hour.

The shop manager then came over to me sitting in the waiting room.... took me back to meet the technician. I have never been so insulted in all my life...!

Both the shop mgr. and the tech (a guy ALMOST as old as me).....tried to be courteous, friendly, and professional...but the bottom line is....they said...and proved with their various tools, that I am CLUELESS about modern technology! Yes...they tried to make it into a joke....but their insinuation was clear.....my knowledge of automotive technology belongs in MUSEUMS! The shame of it all!

They confirmed there is NOTHING wrong with my "factory" electric brake controller system! My primitive method of TRYING to test it out, was throwing me off.... was a waste of my time. Yes, I knew their 0BD II equipment is far more sophisticated than the "consumer grade" stuff I have. That's why I went there!

So they were able to go thru the whole computer-controlled system and confirm it NEVER had any error-codes EXCEPT when I fooled around with the dash controller (replaced it...with a nice new one "genuine GM part" (small print on back of the dash unit says "Made In China"...).

Obviously, during the time I had the unit unplugged for replacement, that triggered a "SERVICE TRAILER BRAKE" code...! Then they led me back out of the shop, told me they'd bring the truck out of the shop themselves - I should see the cashier about my keys.

It gets worse. When I went to the cashier to pay & pick up my keyes, the insult continues....They flatly REFUSED to give me a bill! Can you imagine? All in all, I wasted a full hour of their time....and all I got was a handshake!

Let this be a lesson to you...do not deal with Lindley GMC in Prescott, Arizona. They simply cannot be trusted to behave the way so many of you post in here about GM authorized dealerships.....

Oh...by the way....I think I figured out what was fooling me into thinking there was something wrong with my system. THe "female" trailer "plug-in recept." under the tail-gate......maybe my imagination...i didn't like the looks of one of the pins....specifically, the vital "5 '0'clock" position.....so I changed out that socket with a new one.
I wish our dealers around here were like that sounds like you have a great dealer out there I'm jealous. lol
 

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Discussion Starter · #20 ·
But my goodness you were super rude to the people trying to help you here.
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have you thought of using this site for exchanging technical info. and issues regarding Durmax vehicles, rather than limiting yourself to personal attacks?
 
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