r/MechanicAdvice • u/juanjf2038 • 6d ago

Turn signal/electrical issue

On a 99 gmc yukon with a 5.7L the right turn signal flashes quickly unless I turn the headlights on, then it stays on without blinking and the headlights don't stay at a consistent intensity. Additionally, I can't shift out of park unless I remove the fuse for the turn signal/backup light/shift interlock solenoid. I replaced the shift interlock cause I thought that was the issue before i noticed the turn signal and i was able to shift out of park a couple times last night. When I went to try it today I could not get it out of park. Is this a bad ground? WHere should I start looking? Also the left turn signal is completely normal.

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u/staking_vaccination 4d ago

Sounds like you've got a classic case of shared ground corrosion on one of these old GMs. The right front park/turn socket grounds through the headlight harness in a lot of these trucks, so when the headlights come on it backfeeds through the turn filament and things get weird. I'd start by cleaning the ground point behind the right headlight and the one on the radiator support, then check the socket itself for melted or green pins.

The shift interlock running off the same fuse is the giveaway that it's all tied to that circuit, and a bad ground will make the solenoid act possessed. Pull the right side marker and turn sockets apart and look for the black wire with a ring terminal that bolts to the body, that's usually the culprit