Volvo: Dim Headlight and wipers come on, volvo 240 dl, volvo 240

Question
Hello,
  My wife has a 1990 Volvo 240 DL sedan. It has been a great car and has 103,000 miles on it. The other day something strange happened. When she turned on the headlights, the wipers came on and the passenger side headlight was dim. Turned the car off and when she restarted it, the problem went away. It just happened again, and this time I went and tapped on the head light, and it came on to normal brightness and the wipers shut off. Any suggestions?

Thanks,
Lewis

Answer
It sounds like you have a wandering short somewhere in your wiring harness.  A common problem in many 240's was the insulation flaking off the underhood wiring harness, causing random shorts, and problems like you describe.  Officially, this problem was fixed in all models after 1987, but I would suggest checking for frayed or melted wires anyway, just in case.  Another common problem that can affect the new models is corroded grounds.  Volvo wiring uses numerous, sometimes hidden, chasis grounds throughout the car.  This means that rather than routing ground wires to a common orgin, they ground to their location, so the headlights probably have a hidden ground up by the lights themselves, etc.  A thorough examination of all grounds you can find related to the affected systems is probably in order.  A corroded ground will  cause symtoms like you describe, and only do it once in a while.  Because of the nature of this problem, the only way to find the specific issue it to sift through the wiring, a somewhat time consuming process, but most likely the issue.