My headlights will not work and the ABS warning light comes on when my 1995 850 gets hot either by driving in warm weather or by the car just sitting in the sun in hot weather. As soon as the car cools off, the headlights work again, and the ABS warning light goes out. When the headlight failure happens, if I turn the headlight switch to the parking break setting, I have parking lights and directional signals, but for the drl or headlight settings, I have nothing until the car cools. Obviously, something is lose and is affected by heat expansion, then contraction on cooling. One guess is the headlight switch. Any other guesses?

 

poor headlight ground , volvos are notorious for bad head light wiring

 

Switch was replaced, and the problem seems better, but is not resolved. The failure happens slower. When the weather and/or car is hot and I turn it over, the drl's and headlights don't turn on (but brights do when flashed with the stick), directional signals do not work unless the headlight switch is set to the parking light position, the ABS warning light comes on and stays on, and the warning bell doesn't ping. Once, the car was almost cool enough, and the light switch kept cycling on and off, the ABS warning light would go off and back on, and the warning bell would ping every time. Ground has not be checked, but that doesn't seem logical for this set of problems. Car runs great. Sure would like to be able to drive at night.

 

Which switch did you replace?
Head light switch?
Fog Light Switch?
Turn signal switch?
Horn Switch?
Ignition Switch?

 

Check the DIY sticky for a how-to to fix the headlight issue.

ABS warning light means that you need to clean ABS wheel sensors and tooth gear (sensor reads). Plus check wires at the sensor connectors.

 

If the electrical part of the ignition switch is going bad it will cause both issues.
Next time it does it turn the key slightly forward or back and see if the lights come on and the ABS light goes out. If it does you need an electrical part of the ignition switch.

 

So, today after driving then parking on asphalt for a time, symptoms still there, I turned on the car, jiggled the key in the ignition, the ABS light went off and the headlights came on.

Thanks for all the great input!