Do you mean that the fan never runs at all?? The fan is controlled by a relay and the fan usually starts when the coolant temperature reaches a specifc temperature. When the engine computer sees a certain temperature it energizes the fan relay and the fan starts to run. The fan will run until the coolant temperature drops to a certain point and the ECM will de-energize the fan relay turning the fan off.
The coolant temperature sensor has to be working in order for the ECM to "see" the coolant temperature and energize the fan relay. Does the coolant temperature display properly on the dash??
I don't have a 90 sevice manual so hopefully somebody with a 1990 will jump in and help. One thing you can easily do is to disconnect the wire going to the fan motor and applying 12v to the fan wire. If the fan is good, it will run. If that works, and the fan does not run at all when the engine is running, I would suspect the relay.
Another test you can make is to ground terminal B of the ALDL to terminal A. That will cause the fan to turn of if everything is working correctly. On the driver's side, look up under the dash in the right corner where the carpeted underside of the dash meets the console tunnel. You should see a plastic connector with two rows of 6 holes (it may have a snap-on cover). Not all of the holes will have terminals in it but the top row should have terminals This connector is used to connect a scan tool to "read" trouble code stored in the ECM. For future reference, these terminals are referred to with letters in this fashion as you are looking at the connector:
F E D C B A
G M
Looking at the connector, use a small paper clip to connect the two right-most terminals (A and B) on the upper row. Have the ignition key turned to "Off" before you insert the paper clip. Then turn the key to the "On" position but DO NOT start the engine. The cooling fan should start as soon as you turn the key on. If it does, the relay is good as is the fan motor. Turn the key off and pull the paper clip and replace the cover if you had one there.
If teh fan does not run, but it will run when you apply 12v as mentioned above, the relay is bad. If the fan runs with the paper clip test but never runs when the engine is running, the coolant temperature sensor may be bad.
HTH!!
