Why don t my radiator fans start on my 1993 toyota corolla?

Physically look in the radiator under the cap(if one exsist's) when cool, for coolant level, and check the valve in the cap for sticking. If air is present in the radiator it can't get out, and more coolant can't get in,(from the overflow). Either clean the cap or replace it and check that it is the CORRECT cap.

If radiator is low on coolant fill it to overflowing, replace cap, and check that coolant overflow is filled to the COLD line or mark. The overflow is normally where you would add coolant. (never overfill the overflow) After refilling as outlined above, check the level every morning for 2 or 3 days until coolant level stops dropping below the cold mark and add coolant to the overflow ONLY. Do not open the radiator cap. Also, if you have even a tiny coolant leak anywhere in the system, air will get drawn in, instead of coolant from the overflow.

For more OVERHEATING PROBLEMS try these...
Radiator fins dirty, clean with a strong stream of water, not high pressure water.
Radiator clogged, try backflushing it, or replace.
Thermostat stuck open or shut, replace it.
Water pump worn out, can no longer move enough coolant, replace it.
Fan shroud broken or missing....
Electric Fan(s) not working, Check the fan, relay, fuse and engine temperature sensor's.
Belt driven fan, belt slipping, fan clutch is bad, fan blades have flattened out.
Air dam under front bumper is gone, loose, or broken. It actually has a purpose other than scraping on the driveway or curbs. It forces air up into and thru the condenser and radiator. If it's loose, airflow can actually push it out of place making it useless or blocking the airflow. Along with that, there may also be a plastic piece attached to the bottom of, and wraps up behind the bumper. If it