Cruise control does not work when headlights are on

FYE456

MEMBER
2006 GMC SIERRA
5.3L
V8
4WD
AUTOMATIC
85,000 MILES

Hi there, nine months ago I converted all bulbs to LED and put the correct relay in it to fix the hyper flash. Never had an issue, until now.

Now whenever the headlights are on the cruise control will shut off.

I checked all grounds, fuses, brake light switch is fine.

I swapped the third brake lights back to incandescent bulbs because I was told that's where the cruise feeds through. No luck.

Any suggestions on what next?

Thanks for your help.

3 Replies

KASEKENNY

MECHANIC
18,907 POSTS

If the cruise only works when the lights are off, then you are correct that most likely one of the brake lights is back feeding through to the brake pedal switch making it think that the brakes are on.

However, the third brake light does not feed this system. It is the brake light switch that controls the cancel function. This is a common misunderstanding because the circuit is labeled CHMSL supply volt. However, it is fed from the switch. So, whenever you press the pedal, the cruise shuts off.

What I would suggest is unplugging each of the brake light assemblies one at a time until you find which one is causing the issue.

There is no guarantee that this is the issue, but it is the likely place to start.

The next thing we need to do is check the power on the CHMSL circuit when the head lights are on. If you have voltage on this circuit, then we are missing the short circuit somewhere and we will have to dig further.

https://www.2carpros.com/articles/how-to-check-wiring

Take a look at the wiring diagram below and you will see from the brake light switch to the actuator is the circuit we are focused on.

Let me know what questions you have.

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