LED taillights and cruise control inop

toasted311February 6, 2006

OK, electrical gurus out there. I've got an 05 GMC Sierra. I installed a set of LED taillights and now the cruise will not work. GM says that the leds don't send the same voltage/ohms back to the brake switch, thus it reads the switch being pressed when it isn't. Lights work normally.

Any ideas how I can up the voltage back to the OEM level with the led lights so that the cruise will work? Would a incandescant(sp?) brake light in a drop hitch do the trick?

thanks all

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Somehow, I get the impression that volt, amps, and ohms are being confused. But that aside, let's look at what may be causing your problem.

The dealer is saying that the cruise control is detecting a closed brake switch and is disengaging. The exact method of how this detection is accomplished varies in different models. In some, it may look to see if the brake switch is closed, in others it may sense current in this circuit. Discover how your cruise control system detects an energized brake lamp, then examine the new LED installation. Find out exactly where in the wiring the LED is getting its power. If the LED kit includes a tail light as well as the stop lamp, then it draws current when the lights are on (without brakes applpied). The current draw for the tail lamps should not come from the stop light circuit.

There may be another complication: A popular combination LED tail/stop lamp assembly is really one assembly with a number of LEDs. The tail lamp function is done by turning on part of the LEDs and the stop lamp function is accomplished by turning on all of the LEDs.

Examine the circuitry of your installation, especially pay attention to where the LED assembly gets it power source.

I presume that the maker of the LED kit supplied installation instructions and a circuit diagram. If not, contact the LED kit maker. Does the kit maker have a web site? Maybe there's information there.

    Bookmark   February 6, 2006 at 12:28PM
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The cruise control module gets a stop lamp signal from the brake pedal switch, through the 10amp vehicle stop fuse. It is the light blue wire pin G at the module.

On the switch side of the fuse, there is a connection to the exterior lamps, or in otherwords the feed to the brake lamps. If you have any voltage being back fed through that white wire it will go through the vehicle stop fuse and be sensed by the cruise module and disengage the cruise.

Using a scan tool, monitor the brake pedal input, and see if it says "applied". A Tech II is the preferred too for this vehicle, but others may suffice, such as a Snap-On Modis, or 2500MTG, or a Genisys.

Now the cruise module does put some power out of the Lt/Blu wire, through the vehicle stop fuse, and down the circuit to the stop lamps. This ensures that the cruise module does get a stop lamp input. If the LED lamps you purchased, dont have the correct input resistance, as compared to the original lamps, that could allow that voltage to be too high, and the module essentially not being able to confirm that it will get a proper brake lamp input. If you go the vehicle stop fuse, and measure the voltage at the fuse, with the brake not applied, you should see close to 0v.
Now pull the fuse, and you will see 0v on one side, and the diagnostic voltage on the other side. Touching the brake pedal enough to close the brake switch will take the 0v side to system voltage. If you plug your original lamps back in, and remeasure the diagnostic signal at the fuse, you will now have an indication of the difference in resistance between your LED lamps, and the O.E. ones. You can measure this difference directly, by measuring from the brake lamp side of the vehicle stop fuse, to ground. Make sure you dont touch the brake pedal as you do this or you could harm your meter. Repeat this check with the other set of lamps.

The real problem here is if the inputs are a different resistance than the O.E's your solution could be to add a resistor to the lamp side of the vehicle stop fuse to ground. But it will also become part of the stop lamp circuit, and could get pretty warm and potentially cause issues down the road, so the wattage rating of the resistor is going to be critical.

    Bookmark   February 7, 2006 at 8:54AM
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John G,

The dealer showed me on the GM Tech tool that with the truck on and the lights totally off the brake switch showed applied when the pedal wasn't. When he borrowed a set of OEM lights into the harness it showed correct settings.
The LED's are a direct plug in. They have a built in resistor/converter box on the light that let them be plugged directly in. I didn't have to cut any wiring or anything.
How can I figure out what kind of resister I did to use?


    Bookmark   February 8, 2006 at 8:43PM
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I meant how do I figure out what size resister I need, not did. Long night at work...lol

    Bookmark   February 9, 2006 at 1:38AM
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