Cruise Control and Tail Lights, 2003 Silverado

[gears]

Not sure if anybody is aware of this one. So, I got a 2003 Silverado 1500 LT ext. cab loaded. 168k miles. So I thought it's worth to fix all the minor and not so minor (shocks, control arms, tires ect.) things. Among others replaced the dulled out and cracked head and tail lights.

 

Long story (not so) short. My cruise control stopped working. I new it worked when I got the truck. So, I hooked it up on my Tech2 for Win. No DTC's. Just one oddity. In the cruise control diagnostics it keeps showing stop light applied. Easy I thought and replaced the brake light switch even though it checked out fine. Made no difference. For over 2 weeks I wrecked my brain, checked voltages and inputs on the cruise control module, throttle actuator module, PCM and so forth. Checked anything I could think of. It still showed the stop light applied state in Tech2 no matter what. I checked all the electrical diagrams on ALLDATE. It then dawned upon me.

 

ANSWER, CAUSE AND SOLUTION:
​Aftermarket LED bulbs in the tail lights. Even though with a proper resistor installed (to avoid the hyper turn signal) it caused a weird voltage fluctuation in the none active state to the TAC module. instead of approx. 5V it received "wondering" voltages all over 2.66 to 12 V causing the TAC to believe the brake light switch was actuated.

 

REMOVED LED BULBS. PUT OLD FASHION BULBS IN. CRUISE CONTROL WORKING.

In case any one else runs into that I hope they find this post. There is no documentation or post I was able to find anywhere. And the Chevy Dealers just would have swapped out all the modules to then find out it still wouldn't work.