Truck Repair: radiator fan, radiator fan, fan switch

Question
I have a 1994 Volvo with a 350hp detroit 60 engine, my problem is the fan will not come on during the running of the motor only when I turn off the engine it comes on just for a second or so. What is the problem?

                  Thank you,
                      Joe

Answer
that early model may or may not have ecm controlled fan switch. top right of the engine should be several switches, if one of them has a red crown and screw on terminals thats the fan switch, replace it and see if it works then if they are all plug ins your going to have to go to DDA.