The engine control module (ECM) controls the cooling fan operation depending on cooling requirements. To activate the fan, the ECM applies ground to the coil side of the cooling fan relay. This energizes the coil and applies voltage directly to the cooling fan through the switch side of the relay.

Schematic Reference

Engine Controls Schematics

Connector End View Reference

Electrical Information Reference

Scan Tool Reference

Engine Control Module Scan Tool Information

Ignition ON, command the cooling fan relay ON and OFF with a scan tool. The coolant fan should respond to the scan tool commands. Repeat the commands as necessary in order to verify cooling fan operation.

1. Ignition OFF, disconnect the cooling fan relay.
2. Connect a test lamp between the cooling fan relay control circuit terminal 86 and B+.
3. Ignition ON, command the cooling fan relay ON and OFF with a scan tool. The test lamp should turn ON and OFF as commanded.

⇒	If the test lamp remains OFF all the time, test the control circuit for a short to voltage, or an open/high resistance. If circuit/connections test normal, replace the ECM.

⇒	If the test lamp remains ON all the time, test the control circuit for a short to ground. If circuit/connections test normal, replace the ECM.

4. Test for B+ between the following voltage supply circuit terminals and ground:

•	Relay Coil Supply Terminal 85

•	Relay Switch Supply Terminal 30

⇒	If less than the specified range, repair the voltage supply circuit for a short to ground or an open/high resistance. Replace the fuse if necessary.

5. Connect a 30A fused jumper between the relay switch B+ circuit terminal 30 and the relay output circuit terminal 87. The cooling fan should operate.

⇒	If the cooling fan does not operate, test the ignition supply circuit, and the ground circuit of the cooling fan for a short to ground or an open/high resistance. If the circuits test normal, replace the cooling fan.

6. If all circuits test normal, test or replace the cooling fan relay.

1. Test for 70-110 ohms between terminals 85 and 86 of the cooling fan relay.

⇒	If not within the specified range, replace the cooling fan relay.

2. Test for infinite resistance between the following terminals of the cooling fan relay:

•

30 and 86

•

30 and 87

•

30 and 85

•

85 and 87

⇒	If continuity is detected, replace the cooling fan relay.

3. Connect a 20A fused jumper wire from the positive terminal of the battery to relay terminal 85. Connect a jumper wire from the negative terminal of the battery to relay terminal 86. Test for less than 2 ohms between terminals 30 and 87 of the relay.

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

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.