Electric fan problems

I have a 1992 Cherokee and my electric fan isnt turning on. When I connected it straight to the battery it turned on. Is there some sort of heat sensor in the fan or is it an electrical problem? Any help is appreciated.
Thanks,
Chad

The electric fan is controlled by the temp sensor and the A/C clutch. Turn on the A/C and see if the fan comes on. If it does, then either the engine is not getting hot enough to trigger it or the temp sensor could be bad. If the fan does not come on, check the fan relay in the fuse box under the hood.