strange electrical problem

[ndh87]

I have a 90 chevy silverado K1500 stepside with a standard cab. This is one of the weirdest electrical problems i've ever seen.

first the tail lights dont work, when you turn on a turn signal, both lights activate like hazard flashers and the digital clock readout flashes in time with the turn signals. The brake lights work as long as the headlights are off, but when you turn the headlights on the brake lights(not the tail lights) come on.

I've replaced both the brakelight switch and the dash switch.

Does anyone have any idea what is going on here?

[brokenbudget]

probably time to change the bulbs in the back. if a filamint (burned out bulb) is crossing over it will cause all types of weird glitches.

[scottie]

have you done any work on it recently? could be a harness piched and signals crossing over circuits.

[crabtruck]

It sounds like you have crossed up wires in rear. If you have a trailer plug, disconnect it from the tap.
You probably have the problems with just park lamps on, not just with headlights.
Clock dimming from overload is likely causing the "flashing" effect.
Visually inspect the wire runs under and in rear for bad looking stuff.

Starting at rear remove the stop/turn/tail bulbs, then the marker/turns from front.
one by one looking for change. Odds are good one's your problem.
Very few places the park and the stop/turn wires are close
If the bulbs don't get it, you need to crawl around and check wires to front and back.
Back most likely, then front, last is under dash.

[ndh87]

All the tail light bulbs have been replaced in the process of trying to figure this problem out, I'll check the grounds next. there is no hook up for a trailer, and the only recent work was replacing some oil cooling lines to the trans. but it did sit for a few months (dont see any evidence of rodent damage)

[crabtruck]

Why would you check the grounds next ?
The only place they would short is to ground and you would have blown fuses IMO.

Still haven't checked the front yet ?
Tail is not the only place where stop/turn and park circuits are close.
Have you looked at the lamps in front to see what they're doing ?

[Hayden]

I've had a similiar problem once and I found it to be a bad turn signal switch. but my switch had all the controls on it for headlights,brights, blinkers etc

[crabtruck]

That's why I don't like the "all-in-one" setups for anything.
Part goes, it's all got to go. And usually is hard to get at too...

[Hayden]

I hear that. The one in my Jimmy is going bad now. Sometimes after a turn and my blinker shuts off the wipers will make a single run across the windshield.

[katherinel]

I'd like to know if you get this fixed... the exact same thing happened to my 96 Chevy yesterday. I replaced all the bulbs, checked all the wires and the ground is good, getting juice, but no tail lights! I guess I'll try the signal switch tomorrow.