Part 1 of 4

Chart C-12A:

Chart C-12A Wiring Diagram:

CIRCUIT DESCRIPTION:


Power for the fan motor comes through the fusible link to terminal "1" on all relays. The relays are energized when current flows to ground through the ECM (quad-drivers).

Low Speed Relay - The ECM energizes the relay through terminal "YC2" when the coolant temperature reaches 101~C (214~F). As long as the ignition remains in the "ON" position the coolant fan will continue to run until the coolant temperature falls below 99~C (210~F). The low speed fan will also run any time the A/C is requested.

High Speed Relay - The high speed relay is energized by the ECM or the A/C pressure switch. If the A/C refrigerant pressure reaches 275 psi (1896 kPa) or the coolant temperature reaches 108~C (226~F), the high speed fan relay is energized.



TEST DESCRIPTION: Numbers below refer to circled numbers on the diagnostic chart.

1. Grounding the diagnostic test terminal should cause the ECM to ground CKT 536 and the fan should run in high speed speed.

2. By ungrounding the "test" terminal, and disconnecting the coolant temperature sensor connector will cause the ECM to think coolant is -39~C and cause a rich fuel mixture. "Scan" should display a coolant temperature of -39~C (-38~F), coolant fan should run at low speed, the "Scan" should display FAN 1 "ON."

3. When jumping the coolant temperature sensor wires together "Scan" should display coolant temperature of 151~C (304~F). Fan 1 will be "ON." FAN 2 will be "ON" momentarily as will the high speed on the fan when the key is cycled from "OFF" to "ON."

4. Jumping the A/C pressure switch harness will cause the coolant fans to run at high speeds.

5. This step will see if the ECM monitors the coolant temperature sensor signal properly. Any time the coolant temperature exceeds 101~C the ECM should turn on the low speed fan by grounding circuit 535. Unless vehicle speed exceeds 35 mph. If engine temperature should continue to climb beyond 108~C the high speed fan should come "ON" as the engine is running. A hot soak condition, key "ON" engine "OFF" will not cause coolant fan operation.