Headlamps and Ignition crossover issue.

[Travis Langendoen]

Here's a fun one for ya!
1997 GMC Sierra 2wd SLE.

When the truck is running, the lowbeam headlamps come on and stay on (not just the running lamps).

However, when you turn the lights on with the truck off, everything comes on. You can hear the fuel pump cycle, all the warning lights that are typical when not running with the key in run.

And the most fun part... when the truck is running with the lights in the on position, you can turn the key to off, remove the key (if in park) and the truck stays running! You lose the tach and the speedometer, and when you turn the lights off, the truck turns off normally.

This is weird! I've scoured all the diagrams in the haynes module but aside from a bad module somewhere I'm drawing a blank!

 

[AK49BWL]

While you have the truck running with the headlights on, pull the key out, then pull your gauges fuse (#4 in the dash fuse block).. See if the engine dies then. Then turn the lights off, turn the key to run and see if the headlights come back on... I'm curious if maybe your DRL module went bad and is backfeeding the headlights into the Ignition Run/Start circuit... Those wires don't really cross anywhere else that I'm aware of.

On the DRL module, Pin C is the Ignition input from the Gauges fuse (pink wire), Pin D is the headlights feed (yellow wire). There is a possibility they've gotten shorted together somehow or another. On my 97, the DRL module is just above the OBDII connector, it's a gray oddly shaped small box. Unfortunately, there's no way to unplug it that I can tell... But you should be able to see the wires going into it to see if something got stuck in them or they burned up.

 

[Travis Langendoen]

While you have the truck running with the headlights on, pull the key out, then pull your gauges fuse (#4 in the dash fuse block).. See if the engine dies then. Then turn the lights off, turn the key to run and see if the headlights come back on... I'm curious if maybe your DRL module went bad and is backfeeding the headlights into the Ignition Run/Start circuit... Those wires don't really cross anywhere else that I'm aware of.

On the DRL module, Pin C is the Ignition input from the Gauges fuse (pink wire), Pin D is the headlights feed (yellow wire). There is a possibility they've gotten shorted together somehow or another. On my 97, the DRL module is just above the OBDII connector, it's a gray oddly shaped small box. Unfortunately, there's no way to unplug it that I can tell... But you should be able to see the wires going into it to see if something got stuck in them or they burned up.

This is exactly the information I was hoping to hear. I had suspected the daytime running lights mod based off the diagrams. I figured it was either that, or the headlamp switches. Thanks for the prompt reply. If you're watching this thread I'll shoot another reply in here with what I figure out troubleshooting. Thanks AK49!

 

[Travis Langendoen]

Alrighty AK49,
So just as you suggested. I did two tests.
With the truck running and the lights on I removed the key, the truck stayed running. I then removed fuse #4 for the gages, and the truck died.
Then with the fuse still removed I turned the key to the run position, and the lights did not turn on, as they did when the fuse was in.
It looks like there is a bit of info out there on the DRL module, ill track it down, replace it and see where that gets me.
Appreciate it.

 

[AK49BWL]

Heh, well that feels like it was almost too easy lol. Interested in what you find!