Chart C-12A, Fan Functional Check: Notes

Battery voltage for the cooling fan motor is supplied through a fusible link to circuit No. 2 to the relay(s). The relays are energized when current flows to ground through the Quad Driver Modules (QDM) in the ECM.

The ECM energizes the low speed relay through circuit No. 535 when coolant temperature reaches 214°F (101°C). The cooling fan will be turned off when coolant temperature falls below 210°F (99°C), or ignition is turned off. The low speed fan will run any time A/C is requested.

The ECM or the A/C high pressure switch will energize the high speed relay. If A/C refrigerant pressure reaches 275 psi (19 kg/cm² ) or coolant temperature exceeds 226°F (108°C), the high speed relay will operate cooling fan at high speed.

1. Grounding the ALDL connector will cause the ECM to ground circuit No. 536. Cooling fan should run at high speed.
2. By ungrounding the ALDL connector and disconnecting the coolant temperature sensor connector will cause the ECM to think coolant is -38°F (-39°C) and cause a rich fuel mixture. "SCAN" should display a coolant temperature of -38°F (-39°C) and coolant fan should run on low speed. "SCAN" should display "FAN 1 ON".
3. When jumpering coolant temperature sensor wires together, "SCAN" should display coolant temperature of 304°F (151°C). Fan No. 1 will be on. Fan No. 2 will be on momentarily.
4. Jumpering A/C pressure switch harness will cause coolant fan(s) to run on high speed.
5. This step will see if the ECM monitors the coolant temperature signal properly. Any time the coolant temperature exceeds 214°F (101°C), unless vehicle speed exceeds 35 MPH, the ECM should turn on the low speed fan by grounding circuit No. 535. If coolant temperature should continue to climb above 226°F (108°C), the high speed fan should come on. A hot soak condition, key on engine off, will not cause cooling fan operation.