Saab: over heating, proper gauge, gauge readings

Question
QUESTION: hi jerry
i was wondering if you could help if possible, i have a 1988 saab 900 8v carb which is over heating, i managed to narrow the problem.
I have checked that the fan works by bridging the points on the radiator and believe the problem is that the fan is not switching on.
I took the wire connected to the thermostate housing and earthed it on the block and now the dash temperature gauge, which has never worked, went crazy and now kind of works.
I drove the car around and let it idle but the fan did not come on and the dash board gauge was saying that it was fine.. when it was very, very hot
Have you ever encountered this before, how can i keep the engine cool? and get the fan to start up?

as a secondary question the top and bottom hoses are very hot after driving, could this be the thermostate?
My thermostate is stuck in the housing!

thanks for any help

Martin

ANSWER: Martin:

 The fan is controlled by a temp sensor screwed into the upper part of the radiator on the left side, as viewed from the passenger compartment. Shorting this out should turn on the fan with the ignition on (the motor need not be running). If this occurs, then you need to replace the temp sensor.
 If this does not turn on the fan, then you will need to check the fan relay, located in the relay box on the left front fender well and so labeled.

 If you are so inclined. you may get back to me for specific instructions as to how to perform the checking; a volt-ohmmeter would be helpful.
 The upper and lower hoses are quite hot under normal conditions.

Earthing the wire from the thermostat housing is NOT a good idea; besides causing false gauge readings (Since it apparently did not cause proper gauge operation prior to earthing, it sounds like the temperature sensor to which it was attached is faulty.

         Hope this helps,
         Jerry

---------- FOLLOW-UP ----------

QUESTION: thanks jerry
the haynes manual advised to earth the sensor and to short the circuit to check for faults.
The sensor that is attached to the thermostat housing, where does this lead to? (the last owner has secured all the wires with duct tape so its hard to see), does it lead to the temp sensor on the radiator(that would make sense)

i will try and get some parts for the weekend but cant seem to get a replacement of the sensor attached to the thermostate housing

thanks again

martin

Answer
Martin:
 Now I am beginning to recall a similar problem that I'd encountered years ago. It was apparently a wiring problem somewhere in the large harness, which I'd decided was not worth pursuing. What I did instead was to bring 12 volts to one terminal of the temp sensor screwed into the top of the radiator, and then connect the other terminal to the control coil of an external relay that I'd purchased,[the other terminal of which was earthed] which in turn controlled the fan. Thus I bypassed the SAAB original wiring using the original temperature sensor.

 I believe the screw-in  sensor on the thermostat housing controls the instrument temperature guage. (at least on the US versions of the SAAB).
         Jerry