2024 GM Truck Trailer Brakes Randomly Disconnect!

I may know what’s causing this dangerous problem…

Dear Mike,

Why is this happening?

2024 GMC Sierra 2500 (6.6L Duramaxx), mileage: 2657

Trailer: 2023 2109S, mileage: 942

When we picked up the trailer new at dealership (truck was 9 days old) we got message in truck that said “Check Trailer Wiring” and after many “ding ding”🎵s eventually got the message “Service Trailer Brake System”. We turned around and went back to dealership and the service tech tested the 7-pin plug with his tool (multimeter?) and it was fine, he stuck a screwdriver in the trailer plug and bent the pins a little and plugged it back in, magically fixed.

Fast forward 6 weeks and 3 camping trips: we go get it out of storage this morning, load up at home, climb in to leave and the message starts again. We try the pinch the plug trick, try some contact cleaner, nope. Take it to camper dealership, they try dielectric grease, changing the trailer plug end (good-bye nice new molded plug) no go. We run to GM dealer and buy a new plug receptacle for truck, nope not solved. Doesn’t matter if the truck is moving, sitting still or if the plug is being wiggled. It is completely random as it gives us the connected and then disconnects moments later. And worst of all, trailer brakes are not working when disconnect message comes across.

I think this is a truck problem but had to post and see if maybe anyone has any advice. Thanks for reading my sob story!!! - Amy S.

Dear Amy

I had a discussion with the Dexter Axle guy at the FROG Rally last week, and here’s what he discovered about GM brake controllers.

We think this problem could be due to the truck pinging the trailer through the 7-pin connector to see if the lead-acid house battery is fully charged. That’s to confirm there’s enough battery power in the RV house battery for the breakaway brakes to work.

If the trailer battery is within the correct voltage range then the truck’s brake controller is happy. But if it reads a voltage outside of that range then the truck’s controller gives you the “Check Trailer Wiring” and disconnects itself. That’s dangerous because now you have no trailer brakes.

It appears that GM wasn’t aware that many RVs now have Lithium batteries with a different voltage profile. Surprise!

Voltage Curves

Here comes the sun!

In addition to Lithium batteries, many new RV trailers also have solar panels. These are generally connected to your house batteries while driving so you’ll arrive at your camping spot with batteries fully charged. But if the sun is shining then your RV Lithium battery could be seeing up to 14.6 volts while charging from your solar panels.

If you have Lithium batteries and solar panels on your RV plus the sun is shining, then you’ll have a higher voltage than a Lead-acid battery at rest. So my theory is that the truck’s brake controller is seeing too high of a voltage and will give you the “Check Trailer Wiring” message, then shut down the brake controller.

Don’t let the dealership bend your plug contacts!

The plug contact bending fix at GM dealerships is a desperate attempt to fix something they don’t understand. But I’ll bet that your brake controller fails when the sun is shining and works when your Lithium batteries are partially discharged or there’s clouds in the sky. That would reduce the battery’s voltage to a level that the truck’s brake controller will accept.

Quick Fix?

Everyone: Try shutting off your solar charging system and see if that temporarily fixes the problem. And please post in the comments below if that quick fix works! If my hypothesis is correct the real solution is a firmware update from GM and having them issue a Vehicle Safety Recall to their dealerships.

I’ll keep you posted! - Mike