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Symptom: GMC 2019 Sierra when attached to 16-foot Cargo trailer (20 years old), the running lights come on for 1 second then shut off. If you turn the headlights off and on again, the come on again for 1 second and go off. The dealer could not fix this. GMC corporate was no help. It’s taken 2 years to figure this out. Trailer lights work perfectly when attached to 2017 GMC Yukon. 2019 Sierra powers new trailers just fine. This has nothing to do with the software upgrade for the trailer app build into the 2019 Sierra. Dealer did that and it made no difference. Even replaced trailer receptacle on truck….no difference.
Solution: The issue has to do with the total wattage load of the running lights. The trailer app shuts down the lights if the total wattage is too great. Even my medium size trailer has 18 running lights ( 36 bulbs) around the trailer. Found this out by temporarily disconnecting half of them and the then plugged it into the truck. I replaced 10 incandescent bulbs with LED bulbs, and the problem is finally solved! There is no GMC guidance, rating, or warning about this. -
This might answer your query, feel free to browse this thread from another forum if you have time.
https://www.gm-trucks.com/forums/topic/224130-no-power-to-trailer-lights/
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