91 speedometer issue

I have a 1991 Grand Am with a wierd speedometer problem. Sometimes the speedometer stops working. When this happens the cooling fan comes on.It might stay this way for minutes or hours before it starts working again and the fan goes off. It has nothing to do with the temp. of the engine as it will do it sometimes when its cold. I have replaced the speed sensor and the speedometer. When I turn the A/C on the fan comes on but this does not affect the speedometer. I can't figure the relationship between the fan and the speedometer.:confused:

Fan and speedometer are not related unless you or somebody mixed up wires. Fan comes on for 3 reasons. 1. when AC is on, 2. when engine gets close to 212F 3. your ECT sensor has open and as safety measure, pcm will command fan on to prevent overheating. Did you notice any strange behavior from your temp. gauge? Any problems with starting or significant decrease in MPG. Next time when fan comes on for no reason have omhmeter handy and measure resistance at the ect connector (usually located near alternator, connector is most of the time white, and has eather yellow and green or yellow and black wires) check for intermitent open, or wiggle wires that lead to ect while engine is working but whatch your fingers from fan. This is just to make sure wires do not have open. good luck. Next time speedometer stops just hit the top of the dashborard above the speedometer with your palm.

In despiration I opened the computer and was lightly pushing and tapping components while the ignition was on and found some spots that when pushed the fan would come on. When examining the circuit board under a magnifier light I found several cracked soldier joints. I repaired these and so far, no problems.