Cooling Fan Circuit Diagnosis (Single Fan)

Cooling Fan Circuit Diagnosis (Single Fan)
Circuit Description
The engine cooling fan circuit operates the cooling fan. The cooling fan is controlled by engine control module (ECM) based on input from the engine coolant temperature (ECT) sensor. The ECM controls the low speed cooling fan operation by providing a ground signal to the low speed fan relay. This energizes the low speed cooling fan relay and operates the cooling fan at low speed. The low speed cooling fan operation is achieved by the cooling fan resistor causing a drop in the voltage supplied to the cooling fan. The ECM controls the high speed cooling fan operation by providing a ground signal to the high speed fan relay. This energizes the high speed cooling fan relay, bypassing the radiator fan resistor. This results in high speed cooling fan operation.

Diagnostic Aids
^ If the owner complained of an overheating problem, it must be determined if the complaint was due to an actual boil over, or the engine coolant temperature gage indicated overheating. If the engine is overheating and the cooling fans are ON, the cooling system should be inspected.
^ If the I/P fuse block fuse F18 or the engine fuse block fuse EF3 or main relay fuse EF2 (1.2L only), become open, blown, immediately after installation, inspect for a short to ground in the wiring of the appropriate circuit. If the fuses become open, blown, when the cooling fan is to be turned ON by the ECM, suspect a faulty cooling fan motor.
^ The ECM will turn the cooling fans ON at low speed when the coolant temperature is 93°C (199°F). The ECM will turn the cooling fans OFF when the coolant temperature is 90°C (194°F).
^ The ECM will turn the cooling fans ON at high speed when the coolant temperature is 97°C (207°F). The ECM will change the cooling fans from high speed to low speed when the coolant temperature is 94°C (201°F).
^ The cooling fan circuit can be inspected quickly by disconnecting the ECM connector and grounding the low cooling fan control circuit. This should create low speed cooling fan operation with the ignition ON. By grounding the ECM low cooling fan relay and high cooling fan relay control circuits and turning the ignition ON, high speed cooling fan operation should be achieved.

Test Description
The numbers below refer to the step numbers on the diagnostic table.
4. This step, along with step 5, checks for the ability of the ECM to operate the cooling fans.
22. By directly grounding the ECM low cooling fan relay and high cooling fan relay control circuits, the cooling fan should run at high speed.

Step 1 - Step 10:

Step 11 - Step 22:

Step 23 - Step 31: