Electrical 91 Daytime Running Lights Troubleshooting

Hi all,

I have a 91 5.0 coupe with daytime running lights (Canadian model) that I am trying to diagnose and fix. Following some common troubleshooting tips I replaced the DRL module with this part from Rockauto:


The DRLs worked for about a minute. After cycling the headlight switch and park brake, they no longer worked. No combination of ignition, headlight and park brake switches gets them to turn on anymore.

Does anyone have any tips for troubleshooting? Assuming that the brand new module is not defective, perhaps there is a wiring fault or short somewhere?

Thanks!
Ken

That makes sense. I'm still searching for a wiring diagram. Would you leave the battery connected while probing to see the "hot", or switch on wires?

@90sickfox I probed every wire at the DRL module connector to see which ones are hot in what situation. I found one wire that is always hot, 2 that are hot when the key is on, and one that is hot when the headlights are turned on. That seemed ok, because there are different switches that get to "vote" to turn on the DRLs.

As I understand it, lo/hi beam, park brake, and key on are the "votes".

Then I checked continuity of every wire from the DRL connector to the various connectors in the dash such as the multifunction plug, headlight switch and park brake switch. There seems to be continuity among 6 out of 7 wires! The wires coming from the Park Brake Switch, and Multifunction Plug seem to be all connected with different resistance between 2 and 40 ohms. The 7th wire seems to be continuous to the PBS alone, and reads open circuit when tested to the MFP

I don't understand how 6 / 7 wires can be connected between the DRL plug and the dash. A wiring harness would have to be very melted for that to happen. And the car has no other headlight issues at all!

Has anyone seen something like this before?

Thanks!