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Juan Crespo
Juan Crespo, ASE Certified Technician
Category: Chevy
Satisfied Customers: 1516
Experience:  A.S.E. Master Technician, Advanced Level, Emissions - Asian, Domestic, & European
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My name is ***** *****, Silverado ITBC problem. No codes, no

Customer Question

My name is ***** *****, Silverado ITBC problem. No codes, no trailer recognition on DIC, Snap-on Modis shows TBCM sees trailer and brakes connected. Low SSR command volts(8) and feedback(.9). Supply voltage fine
JA: Thanks. Can you give me any more details about your issue?
Customer: 2007 2500HD, no problems prior with controller. Owner has had it for thee years, towing every year
JA: OK got it. Last thing — Chevy Mechanics generally expect a deposit of about $18 to help with your type of question (you only pay if satisfied). Now I'm going to take you to a page to place a secure deposit with JustAnswer. Don't worry, this chat is saved. After that, we will finish helping you.
Submitted: 9 months ago.
Category: Chevy
Expert:  Juan Crespo replied 9 months ago.

If there have been three years of trouble-free towing up to this point and no modifications have been made lately, then the symptoms you describe are probably being caused by open circuit(s) between the trailer and the truck. If I were you, I'd start by taking a close look at both halves of the trailer connector. If both show no signs of corrosion or debris and there aren't any broken wires, then get a spare battery and start running voltage drop tests on the trailer circuits until the culprit is found.

Please let me know what you find.

Customer: replied 9 months ago.
already did that, connections clean. Continuity checks between SSR. TBCM and trailer connector all good. Power and grounds also check out good. The SSR command and feedback voltage are low for sure. 12+ volts to TBCM but 8-9 for relay command is not right so what next
Expert:  Juan Crespo replied 9 months ago.

99.99% of the cases, additional circuit resistance is what causes this type of fault - GM even came up with TSB 09-05-22-004 to address this issue by reprograming the ITCBM with an updated calibration. In a case like this, where the system was OK one moment and bad the next, I couldn't think of anything other than excessive circuit resistance that could cause this issue.

Please let me know if there is anything else I can do to help.

By the way, I'm curious. You used the acronym "SSR", which I could not find in the GM glossary other than when used in reference to the little truck they no longer make. What does it mean?

Customer: replied 9 months ago.
SSR is solid state relay. TBCM was reprogrammed Friday as were system checks per TSB. Grounds cleaned, all connectors checked. No corrosion found. Disconnected fifth wheel/camper harness to eliminate that. Again, no codes in TBCM and no message of any kind on DIC. Information I have says SSR command voltage should be same as battery. wiring diagram appears to show that should come from TBCM but I'm not sure which one
Customer: replied 9 months ago.
Which circuit I mean. Relay enable(pin 33) or trailer brake relay(pin 14)?
Expert:  Juan Crespo replied 9 months ago.

The only way we would be able to see what's going on is by establishing communication with the DIC and the ITBCM through the bus network. Can your scan tool get on the serial data bus? Can it communicate with those two modules?

Customer: replied 9 months ago.
I have access to the TCBM, it shows trailer and brakes connected but DIC does not. I will check tomorrow to see if I have DIC. Is that instrument panel?
Expert:  Juan Crespo replied 9 months ago.

OK. If I understand you correctly, the TCBM is showing trailer and brakes connected even though those are in fact disconnected. Is that so? Then, DIC is not showing trailer and brakes connected whether those are connected or not. Is that correct?

Regarding the DIC, there are two instrument panel cluster (IPC) options for these trucks; one with and one without Driver Information Center (DIC).

The only way we're going to be able to solve this puzzle is with the help of a Tech-II or equivalent scan tool that can communicate with all of these modules.

Customer: replied 9 months ago.
The TBCM only shows trailer and brakes when connected to trailer. It does not show when disconnected. DIC never indicates anything. I will try to recheck circuits between SSR and TBCM today and get into IPC then contact you again
Customer: replied 9 months ago.
IP has no DIC. all circuits between SSR and TBCM show no voltage drop(both disconnected, using battery+ and ground at SSR connector and checking circuits through harness). Is there a way to test the relay?
Expert:  Juan Crespo replied 9 months ago.

OK. Now I'm really confused; first you posted "No codes, no trailer recognition on DIC", now you post "IP has no DIC". What are we trying to do here?

Customer: replied 9 months ago.
I am confused also. Information I have says trailer info shows up on IP display, however there are no buttons to change display other than odometer trip/reset button. Should I still get trailer brake info(connection,gain+/-) on dash readout?
Expert:  Juan Crespo replied 9 months ago.

Now you're confusing me even more; what do you mean by "Information I have says trailer info shows up on IP display". What information are you talking about? Are you not there with the truck? Can't you just look at it and see if it has a DIC? If you need to, click here to look at a video on how the DIC works.

Expert:  Juan Crespo replied 9 months ago.

OK. I did some digging of my own and this is what I came up with: if it has OEM trailer brakes system, then it has a control panel on the left lower side of the dash. Also, the trailer brakes system will communicate with DIC and show when trailer is connected and disconnected plus rate of brake application.

Now, here is the important part; if the total resistance of the circuit for the electric brakes on the trailer exceeds 3.0 Ohms, the DIC won't show the trailer brakes at all.

Hope that helps, brother. If not, I don't know what else to do that can help you fix this thing.

Customer: replied 8 months ago.
That clears up a lot. My confusion was caused mostly by no buttons to access DIC information. Sorry for the confusion. So my original thinking was correct, no codes and no display on DIC. I have connected a brake magnet to brake/ground terminals on the vehicle at trailer connector and while there is nothing on the DIC, the Snap-on Modis data from the TBCM shows trailer and brakes detected. Also shows SSR enabled but Trailer Brake Control is NO(I assume that means disabled). Battery voltage is 12.5v, trailer brake relay command is 7.7v(supposed to be same as battery?) and relay feedback is 0v(supposed to be 4.8-5.2v?). Thanks for your help, still wondering if there is a way to test relay
Expert:  Juan Crespo replied 8 months ago.

Click here for instructions on how to test SSR

Customer: replied 8 months ago.
Unfortunately that is a generic relay with 4 terminals, the SSR on the trailer brake system has 6. Is there a schematic of the actual relay?
Expert:  Juan Crespo replied 8 months ago.

OK. Before we get confused again, what are we trying to fix; the trailer electric brakes, or why they don't show up on the DIC?

On second thought, the more I learn about the system, the more I see the need for a Tech-II or equivalent scan tool as the only tool that will allow us to conduct an effective diagnosis. Since it seems you don't have access to one of those (Modis doesn't have serial data communication capability), I'll opt out so others may try to help you. Please don not reply until another expert responds or the site will cycle your request back to me.

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