Cooling Fan Circuit Diagnosis (Dual Fans)

Cooling Fan Circuit Diagnosis (Dual Fans)
Circuit Description

Caution: Refer to Electric Coolant Fan Caution in Service Precautions.

The engine cooling fan circuit operates the main cooling fan and the auxiliary cooling fan. The cooling fans are controlled by the engine control module (ECM) based on inputs from the engine coolant temperature (ECT) sensor and the air conditioning pressure (ACP) 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 main cooling fan and the auxiliary cooling fan at low speed as the cooling fans are connected in a series circuit. The ECM controls the high speed cooling fan operation by providing a ground signal to the high speed cooling fan relay and the series/parallel relay a the same time. This energizes the low speed cooling fan relay, the high speed cooling fan relay, and the series/parallel cooling fan relay resulting in high speed fan operation as the cooling fans are now connected in a parallel circuit.

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 engine fuse block fuses EF3 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 fans are 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 97°C (207°F). The ECM will turn the cooling fans OFF when the coolant temperature is 94°C (201°F).
^ The ECM will turn the cooling fans ON at high speed when the coolant temperature is 101°C (214°F). The ECM will change the cooling fans from high speed to low speed when the coolant temperature is 98°C (208°F).
^ The ECM will turn the cooling fans ON at low speed when the A/C system is ON. The ECM will change the cooling fans from low speed to high speed when the high side A/C pressure is 1882 kPa (273 psi), then return to low speed when the high side A/C pressure is 1448 kPa (210 psi). When the A/C system is ON, the ECM will change the cooling fans from low to high speed when the coolant temperature reaches 117°C (244°F), then return to low speed when the coolant temperature reaches 114°C (237°F).
^ The cooling fan circuit can be inspected quickly by disconnecting the ECM and ground the low speed relay control circuit. This should create low speed cooling fan operation with the ignition ON. By grounding the ECM low speed relay control circuit and the high speed relay control circuit 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.
8. This step, along with step 9, checks for the ability of the ECM to operate the cooling fans in response to A/C pressure readings.
16. After confirming battery voltage and the ECM supplying a ground to the coil side of the cooling fan relay A, by jumpering connector terminals 30 and 87, it will be determined if the relay is at fault or a wiring problem is present.
31. This step checks for the presence of battery voltage to the main cooling fan when the A/C is ON. If battery voltage is present and the cooling fans are not operating, the problem is in the ground side of the cooling fan circuit.
37. By directly grounding the ECM low relay and high relay control circuits, the main and auxiliary cooling fans should run at high speed.

Step 1 - Step 10:

Step 11 - Step 20:

Step 21 - Step 33:

Step 34 - Step 41:

Step 42 - Step 49: