I'll keep it simple:

1994 Volvo 940 Wagon, cruise control works fine until I turn the headlights on. Then it doesn't work. That's a pretty specific symptom so I'm hoping there's a pretty specific answer (knowing in my heart of hearts fixing it probably entails a complete rewiring of the electrical stuff b/c there are other electrical type issues).

Thanks for any tips.

 

what /other electrical issues/ ?

without digging into the schematics, and running a bunch of electrical tests, my first order guess would be a poor ground somewhere. the two main grounds in the dashboard are behind the side 'kick panels' just forward of either front door, the ABS, cruise control, and speedo are grounded on the left side, and nearly everything else, including the ground bus in the main fuse/relay panel, is grounded on the right side.

the headlights themselves are grounded up in the front corners of the two fenders. the power for them comes through 4 fuses (high,low left, high,low right, on fuses 17-20. the low beam power goes through the bulb failure sensor, while the high beam power doesn't. fuse 2 provides power for the high beam 'flash' function so that works even if the ignition is off.

I note the cruise control switches are on the same turn signal stalk as the highbeam selector, maybe that cable is mangled.

a few questions...

if you flash the highbeams with the turnsignal stalk, does that kill the cruise control?

if you turn on the parking lights but not the headlights, does that kill the cruise control?

if you turn on the highbeams only, does the cruise control work?