Service trailer brake system and more?

2017 GMC SIERRA
75,000 MILES • 5.3L • V8 • 2WD • AUTOMATIC
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BLAZ85
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The vehicle belongs to a friend, Sierra 1500. The problem began with the "Service Trailer Brake System" message being displayed on instrument panel. Ever since this message first appeared, and it now appears every time the ignition is turned on, the battery keeps losing its charge similar to a large parasitic drain. First thoughts were a bad battery. Had battery tested at Advance Auto Parts. They concluded battery was fine, just low on charge. Charged battery and did not try to start until next day. Would not start, just a click at the starter. Got out jump box and vehicle started readily. We also checked alternator output and found it to be 14.3V.

My question is, "could this fault with the Trailer Brake System be causing what seems like a large parasitic draw?" Just to see what would happen, we removed the 30A fuse powering the Trailer Brake System and ran the vehicle quite some time to charge the battery. Tomorrow we will try starting it again. I realize Advance is not always correct on assessing the condition of a battery. Also, battery failure and appearance of message at same time may just be a coincidence.

Thank you in advance for your reply. You have always been a great help!
Oct 6, 2023 at 9:07 PM
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JACOBANDNICKOLAS
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Hi,

Interestingly, there are multiple technical service bulletins related to the "Service Trailer Brake System." See pic below. Note that there are several other bulletins for this message.

I looked through a couple of them. Just to let you know, one of the things was a weak battery. However, what you are doing is a perfect test. I'm interested in knowing what you find.

If the battery isn't the issue, we need to scan the can-bus to see if there are specific codes related to the issue. You'll notice that there are codes listed in the pic below. Without knowing which one or ones are present, it becomes a guessing game.

CAN stands for controller area network. Basically, the different modules are tied together via a few wires. This type of scan will retrieve codes regardless of the module storing them.

Here is a link that shows how it is done:

https://www.2carpros.com/articles/can-scan-controller-area-network-easy

Let me know if that is something you can do or have done. Also, let me know the outcome of the battery test.

Take care,

joe

See pic below.
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Oct 6, 2023 at 10:23 PM
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BLAZ85
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Sorry so late in getting back with you but my friend has been out of town on business.

Well, it was the battery after all. After pulling the fuse and charging the battery fully, it started for the next two days until he carried it to the dealer concerning the "Trailer Brake" message displayed. Dealer tested battery and said it had low voltage and that was what was causing the message to appear (just like you said you saw in a Factory Service Bulletin). When questioned about why it tested OK at Advance Auto Parts , they stated their equipment was much more sensitive. Anyway, dealer replaced battery, message went away and has not reappeared. Maybe this experience will help others if they get the "Trailer Brake" message. First order of business if to check battery voltage.

As always, thank you for help and have a great day!
Oct 17, 2023 at 6:25 PM
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JACOBANDNICKOLAS
  • AUTOMOTIVE REPAIR CONTRIBUTOR
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Hi,

Thanks for the update. That helped verify the weak battery bulletin I had, and it will now help others in the future. Your reply is appreciated.

Regardless, I'm glad to know the problem has been resolved. At least it didn't require anything major. LOL

Take good care of yourself and feel free to come back anytime in the future.

Joe

Oct 17, 2023 at 6:36 PM