Wiring A Brake Light Relay Kit In A 2020 Ford F-150 XLT

Updated 06/20/2024 | Published 04/27/2024

Question:

I have a 2020 Ford F-150 XLT and a Blue Ox aftermarket tow light harness. I received a Brake light relay RM-88400 in my Air over Hydraulic kit. How does the RM-88400 connect to that harness? It looks like the Brown wire on the harness connects to the red wire 86. Does this also connect the fuse tap or does the power come from the brown wire in the Blue Ox harness?

asked by: Dan G

Expert Reply:

Hey Dan,

The ground for the Roadmaster Brake-Lite Relay Kit for Towed Vehicles # RM-88400 typically gets tied into the ground for the braking system to intertwine them so that they're active at the same time. When used to prevent turn signals from overriding the brake signal the relay is essentially just a switch that requires the running light signal from the motorhome in order for it to turn on so that it doesn't interfere with the vehicle's signals when not being towed.

We find wiring the relay in Ford's to be easier with less issues if you follow the keyless start installation diagram (attached below) and ensure that your motorhome running lights are on when towing the vehicle. The power will come from the motorhome's running light function in this setup.

expert reply by: Kevin C

Dan G.

6/18/2024

I installed the relay as you suggested and am still having an issue with the left turn signal not working. We are leaving on a 6,000-mile trip in three weeks and probably need to get the diode to fix the issue. brake lights work and the right turn signal work along with the park lights

Kevin C.

6/18/2024

@DanG Does the left turn signal work when parked? If it's not working when parked then it would likely be an issue with diode wiring running to the light and not the relay.

Dan G.

6/18/2024

@KevinC No, it does not work parked, and there is no diode in any of the wiring. This was a wiring harness that came with the Blue Ox kit I ordered. Do I need to add an RM-790 to the brown wire?

Kevin C.

6/19/2024

@DanG If the turn light isn't working when you're not even hitting the brake pedal then the issue is likely with the Blue Ox wiring kit you have. I would check the connection at the tail light to see if the signal is coming through. The brake lite relay kit is to preventing the towed vehicle's brake signal from overriding the motorhome's turn signal. If the brake pedal isn't being depressed there's no way that the brake signal from the vehicle would be allowed through to interfere with the turn signal.

Kevin C.

6/20/2024

@DanG We do offer the Hopkins # HM56009 which is a fit but we don't currently carry the Blue Ox BX88366. That being said, the majority of troubleshooting questions I get for towed vehicle wiring are typically from plug-in wire harnesses and there's nothing you can do to fix them besides replacement. Because of this I avoid recommending any plug-in harnesses and always recommend a splice-in diode kit. With a splice-in diode kit every single component is replaceable without having to replace the entire harness. My honest recommendation would be to switch to the Roadmaster Diode 7-Wire to 6-Wire Flexo-Coil Wiring Kit # RM-15267. Splicing in the diodes and wiring isn't as easy as just plugging in a connector but it's really simple to do. I've attached a video below where we can walk you through the installation process.

Helpful Links

tv-install-roadmaster-diode-wiring-kit-2019-ford-f-150-rm-154.aspx