Volvo Repair: O/D relay, jumper wire, selonoid

Question
workin on a 1990 240DL   over drive relay gets smoked. customer replaced   worked for 10 min than quit again. obvously there is somthing shorting out the relay, and the cost of these things i cant just keep testing. i bypass it and apply the selonoid to test that. my only guess is that the selonoid gets hot and shorts out. in turn shorting the relay? any ideas?

Already Tried:
tested the od relay with a 12v power suply. it operates fine.   the owner clames to have replaced the od switch, witch seems to be in working order. the customer has also replaced the relay, vehicle worked for about 10 min then lost over drive again. when tested with a meter the output of the relay is 12v, but if i put a load inline(an automotive light bulb) the bulb bairley lights, and i only get 1 volt, with proves the relay fails under a load, but i cant test the system with out a new relay, and these things are going for 50 bucks, kinda steep if all i do is blow them trying to fix his problem.


Answer
  Trace the wire from the relay socket to the solenoid. Or use your light bulb test on a jumper wire to that same socket and to ground. I suspect a bad/shorted wire. Also, I would look at that switch very closely as they are notorious for failure/shorting etc.

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