The engine control module (ECM) activates the auxiliary or pusher cooling fan when the engine coolant temperature exceeds a calibrated value. In normal operation, the left and right cooling fans will activate first at low speed, second at high speed, and third the pusher fan will activate if the coolant temperature continues to increase. The ECM applies ground to the coil side of the pusher fan relay. This energizes the coil and applies voltage directly to the fan through the switch side of the relay.

Schematic Reference

Engine Cooling Schematics

Connector End View Reference

Description and Operation

Cooling Fan Description and Operation

Electrical Information Reference

1. Ignition OFF, remove the PUSHER FAN relay.
2. Verify that a test lamp illuminates between the relay coil ignition circuit terminal R6 and ground.

⇒	If the test lamp does not illuminate, test the coil ignition circuit for an open/high resistance.

3. Verify that a test lamp illuminates between the relay switch B+ circuit terminal R7 and ground.

⇒

If the test lamp does not illuminate, test the relay switch ignition circuit for a short to ground or an open/high resistance. If the circuit tests normal and the fan fuse is open, test the relay controlled output circuit terminal R8 for a short to ground. If the circuit tests normal, test or replace the relay.

4. Connect a 30A fused jumper between the relay switch B+ circuit terminal R7 and the relay controlled output circuit terminal R8. Verify the fan activates.

⇒	If the fan does not activate, test the relay controlled output circuit for an open/high resistance. If the circuit tests normal, test the fan ground circuit for an open/high resistance. If the circuit tests normal, replace the fan.

5. Ignition ON, verify that a test lamp does not illuminate between the relay coil control circuit terminal R10 and ground.

⇒	If the test lamp illuminates, test the coil control circuit for a short to voltage. If the circuit tests normal, replace the ECM.

6. Test the relay coil control circuit terminal R10 for an open/high resistance or a short to ground.

⇒	If the circuit tests normal, test or replace the relay. If the relay tests normal, replace the ECM.

1. Ignition OFF, disconnect the cooling fan relay.
2. Test for 70-110 ohms of resistance between terminals 85 and 86.

⇒	If the resistance is not within the specified range, replace the relay.

3. Test for infinite resistance between the following terminals:

•

30 and 86

•

30 and 87

•

30 and 85

•

85 and 87

⇒	If not the specified range, replace the relay.

4. Install a 20A fused jumper wire between relay terminal 85 and 12 volts. Install a jumper wire between relay terminal 86 and ground. Test for less than 2 ohms of resistance between terminals 30 and 87.

⇒	If greater than the specified range, replace the relay.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.