Brake-Lite Relay Install and Testing For 2018 Jeep Wrangler JK

Updated 08/21/2020 | Published 08/16/2020

Products Featured in This Question

Roadmaster Brake-Lite Relay Kit for Towed Vehicles

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$43.40

Digital Multimeter with 42" Long Test Leads - LED Display

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$23.43

Question:

I have the brake lite relay kit 88400 . I have nothing but problems trying to find the cold side of the brake. heres what I have when the key is off no power to the six lines. key on the blue/white 7.8 with brake on off 0, black brake on 0 off 0, blue brake on 0 off 12.1,red\green on 12.43 off 12.43, green\red on 1 off 0, brown\white on 1 off 1. SO whats the cold side???

asked by: Bob C

Expert Reply:

Whenever you are testing for the cold side of the brake switch you'll want to use a volt meter like # PT89ZR rather than a standard test light, which I assume you are since you gave me the numbers of the read outs.

The cold side of the brake switch should be a white wire with a tan/brown stripe. I suggest retesting that wire and making sure that you have a good connection. It should read no voltage with the brake pedal released. When the pedal is pressed you should get 12v (or very close) on the white wire with a tan stripe. If you are not getting those signals, don't cut any wires just yet.

2018 was a little bit of an odd year for Jeeps, the JK and JL models were both being manufactured and were very different from each other in regards to towing and accessories. The JL model has a push button ignition but does not have a traditional stop light switch that can be tapped in to, and it is possible that the last year for the JK shares some similarities with the new JL.

Some vehicles have a retained power source, but most of the signals goes dead shortly after the key is turned off. To make sure this is not the case for your Jeep, you can test the brake light circuit a little differently.

If you have your tow bar wiring already installed, you can hook up your Jeep to the motor home and put it into tow mode, you can find the procedure in your owners manual. Then turn the lights on inside the motor home and verify that all the lights are working together.The turn on one of the motor homes's turn signals. Next you'll want to use a broom handle or something long to reach into the Jeep from the window and press the brake pedal. Don't open the door because it will reactivate the cars power and won't let you accurately test. If the turn signal on the Jeep is still flashing with the motor home and the brake pedal pressed, the relay won't be required.

If the light goes solid on the Jeep and stops blinking with the turn signal, then the Roadmaster Brake-Lite Relay # RM-88400 is needed to turn the brake signal off from the Jeep, so it doesn't override the turn signal.

I added a few videos for you to check out so you can see the installation process. The first one is a 2017 Jeep JK that I installed # RM-88400 in, and the other video is of a braking system install on a 2018 JK wrangler, but if you skip forward to about 18 min mark it shows how # RM-88400 is installed.