How to Install Roadmaster 690 Diode to Fix 12V Power Issue on 2024 GMC Sierra Denali

Published 08/07/2024

Products Featured in This Question

Roadmaster Single Park Light Diode

(120)

$22.21

Hook Probe Circuit Tester

(60)

$13.33

Question:

Hi, I have a new '24 GMC Sierra 3500 Denali Ultimate DRW that is giving me problems towing my '24 DRV Mobil Suites 41RKDB fifth wheel trailer. The integrated brake controller in my truck is sensing back-charging from my trailer (probably from my inverter that powers the fridge) and is disconnecting my trailer brakes. The message says "Trailer brakes disabled - service required." This is a known problem that GMC is currently working on and has a service bulletin. I've taken my new truck in for the service bulletin where a new integrated trailer brake controller was installed. Unfortunately it didn't fix my problem and the trailer brakes still disconnect. I've read from several forums that a fix for this problem is to install a diode on the 12 volt wire in the 7 way pin of the trailer. Apparently it allows the tow vehicle to power the trailer but NOT allow the trailer to back-charge the truck. I also read from the forums that the Roadmaster 690 single park light diode works great and I have one being delivered today. My question is which wire in the 7-way pin to install it on and if eTrailer has a video to watch this procedure. I'm not real experienced with this kind of electrical fix & would appreciate any help. Thank you!

asked by: Jim M

Expert Reply:

Thanks for reaching out, Jim!

The Roadmaster Single Park Light Diode # RM-690 will work great for this application, and I'm glad to hear others have been successful using it!

You'll want to install the diode on the wire connected to the 1 o'clock position on your 2024 GMC Sierra 3500 7-Way socket (see attached photo). I'm unsure of what the wire color will be, but you can either disassemble the 7-Way to get at that wire, or you can use a Circuit Tester # PTW2993 to probe for the wire that is always hot when your vehicle is running (you may need to plug the trailer in for the truck to send 12V power though).

The installation itself is fairly straightforward since you'll just have to cut the 12V power wire and then insert the stripped wires into either blade terminal and crimp it down. Make sure the diode is facing the correct direction, connect the blade terminals, and wrap the wiring up with electrical tape or something similar just to help make sure those connections are safe.

I'm linking a video of the diode being installed on a 2012 Grand Cherokee for your reference. This is a different install in a different location, but the basics of installing the diode are still present.

expert reply by: Jon G

12V Power Pin Location on 7-Way
(click to enlarge)

Jim M.

8/7/2024

Hello Jon, Thank you very much for your prompt reply with the 7-way pin photo and the attached video. Most if not all the people who have installed the diode have done it on the trailer side versus the tow vehicle or truck side. So do you have any information on going that route? Pretty sure you can find the 7-way wiring in the box under the kingpin of my fifth wheel. I assume it would be on the #3 pin terminal located at the 1 o'clock position like you mentioned. Thank you Jon, Jim McDonough

Jon G.

8/7/2024

@JimM If you're installing it on the trailer side then it will be on the 3 o'clock pin just like you stated. If there is a junction box then that will make things easier because you'll have an easier time locating the correct wire, but if not you'll either have to cut that single wire, or you can install a junction box with the kit # e99011 to make labeling and connecting the different functions much easier. Installing the diode in-line with your current system will look the same as the install video I linked above, you'll just be doing that on the trailer side 12V pin. Hopefully this helps as well!