1990 GMC Sierra vechical electrical problem

CODY14011

MEMBER
1990 GMC SIERRA
V8
2WD
AUTOMATIC
105,000 MILES

I bought this truck and when I turn on the headlights I loose my turning signals and taillights. The blinkers work when the lights are off. I obviously only can drive during the day. I also had an 89 chevy with the same problem. I wouldn't think it would be a coincidence I bought two vehicles with the same problem, other people had to encounter such issues. Do you any ideas as to what if causing this. I changed the entire steering colunm and nothing changed. It is driving me nuts, any help would be greatly appriciated. Thank you
Sam

1 Reply

JDL

MECHANIC
16,098 POSTS

Welcome to the forum, if the headlamp switch is on the dash and the dimmer is on the steering column, there should be a multiwire connector under the dash close to steering column, for circuits going to and from the steering column. You should be able to test voltage circuits there, using a testlite.

Your headlamp switch probably has two circuits, one is a circuit breaker, only for the headlamps, red wire carries voltage for that. The other circuit is a fused circuit for tail lites--dash lites and stuff. orange wire carries voltage for that. You can also see that the yellow wire carries voltage to the dimmer, I believe.

At the headlamps themselves, lite green wire is voltage for high beam. Tan wire is voltage for low beam.