94 K1500 taillight issue

Disclaimer: Links on this page pointing to Amazon, eBay and other sites may include affiliate code. If you click them and make a purchase, we may earn a small commission.

2010gmcyukon

Full Access Member
Joined
Feb 20, 2019
Posts
306
Reaction score
220
Location
Midwest
I posted this on a gm truck forum earlier, but haven't gotten any response yet. Hoping maybe someone over here might have an idea.

I have at 94 K1500 Truck with an odd rear light issue...

The front lights (headlights, parking lights, blinkers) and dash lights all work as expected.

When my headlight switch is off...the rear lights work as expected:
  • Brake lights work
  • Blinkers work
However, when the headlight switch is on, nothing works:
  • No tail lights
  • No brake lights
  • No reverse lights
  • No blinkers
Just to note, there is a 3rd light above the back window...I have never seen it come on, so I haven't done anything with it.
Here's what I've do so far:
  • I checked all the fuses on the panel right under the steering wheel. None are blown.
  • I checked all 6 taillight bulbs and they appear good (both filaments).
  • I swapped the headlight/parking light switch assembly with another, same result.
  • The ground wire on the frame near the back bumper was all rusty. I ground the frame and reattached the wire with new hardware.
  • I pulled the plug off the back of the left taillight and hooked a test light to it.
    • Hooked the clip to the black wire.
    • When the headlight switch is OFF:
      • The brake light wire goes positive when hitting the brake.
      • The blinker wire flashes positive as expected.
      • The taillight wire is off.
    • When the headlight switch is ON:
      • All of the wires are positive all the time.
Any ideas? I'm not sure what would cause this. I appreciate any info anyone has to share!
 
OP
OP
2010gmcyukon

2010gmcyukon

Full Access Member
Joined
Feb 20, 2019
Posts
306
Reaction score
220
Location
Midwest
Here is a link to the factory service manuals, it looks like 1994 has been scanned in. One of the volumes should include the wiring diagrams and, if they are anything like the later years, may have the factory diagnostic tables for electrical.

Hope this helps.


https://www.gmt400.com/threads/88-95-service-manuals.43575/
Thanks! I downloaded the manual and found the wiring diagram as well as the troubleshooting steps. I'll give this a try.
 

exp500

Full Access Member
Joined
May 14, 2017
Posts
1,781
Reaction score
1,609
Now you have the manual, look for Combination switch. The turn,flasher,cruise assy in steering wheel. Also look at brake light switch and bushing on brake arm under dash.
 

exp500

Full Access Member
Joined
May 14, 2017
Posts
1,781
Reaction score
1,609
Almost forgot, remove 3rd brakelight and check if it is an led- not compatible with your system. Check power to it while you are there.
 

Forum statistics

Threads
128,802
Posts
1,805,634
Members
91,784
Latest member
navyboy14094
Top