Engine Cooling Fan Circuit Check

Circuit Description
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 internally grounding the ECM connector terminal C13. 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 internally grounding the ECM connector terminal C13 and the ECM connector terminal C12 at 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 gauge indicated overheating. If the engine is overheating and the cooling fans are ON, the cooling system should be checked.
- If the engine fuse block fuses EF3 or EF8 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 Engine Control Module (ECM), suspect a faulty cooling fan motor.
- The ECM will turn the cooling fans ON at low speed when the coolant temperature is 199 °F (93 °C). The ECM will turn the cooling fans OFF when the coolant temperature is 194 °F (90 °C).
- The ECM will turn the cooling fans ON at high speed when the coolant temperature is 207 °F (97 °C). The ECM will change the cooling fans from high speed to low speed when the coolant temperature is 201 °F�(94 °C).
- 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 269 psi (1,859 kPa) then return to low speed when the high side A/C pressure is 210 psi (1,449 kPa).
- The cooling fan circuit can be checked quickly by disconnecting the ECM white connector and grounding the connector terminal C13. This should create low speed cooling fan operation with the ignition ON. By grounding the ECM connector terminals C13 and C12 and turning the ignition ON, high speed cooling fan operation should be achieved.

Steps 1 - 4:

Steps 5 - 12:

Steps 13 - 23:

Steps 24 - 35:

Steps 36 - 41:

Steps 42 - 49:

Test Description
The number(s) below refer to step(s) on the diagnostic table.
4. This step, along with step 5, checks for the ability of the Engine Control Module (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 low speed cooling fan relay, 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 connector terminals C13 and C12, the main and auxiliary cooling fans should run at high speed.