Electrical Issues:
* Fan Relay: The high-speed relay is a common culprit. It's a small electrical switch that allows power to flow to the fan motor at high speed. It might be faulty, stuck open, or have burned contacts. Testing the relay with a multimeter is crucial.
* Fan Motor Resistor (if equipped): Some vehicles use a resistor to control fan speeds. A faulty resistor can prevent the high speed from engaging. This is less common than relay failure but should be checked.
* Wiring and Connections: Look for broken, corroded, or loose wires in the fan circuit, especially near the fan motor, relay, and the connector to the PCM (Powertrain Control Module). A poor connection can prevent high-speed operation.
* PCM (Powertrain Control Module): While less likely, a faulty PCM can prevent the fan from working at high speed. This usually involves other symptoms as well. Diagnosing a PCM issue requires specialized tools.
* Fan Speed Sensor (if equipped): Some vehicles use a sensor to monitor coolant temperature and signal the PCM to activate high speed. A faulty sensor might only send a low speed signal. Check for DTCs (Diagnostic Trouble Codes) related to the fan or sensor.
* Fuse: Though less likely for the high-speed circuit alone (often a separate relay controls this), check the relevant fuse in the fuse box. A blown fuse could interrupt power to either the fan or the relay.
Mechanical Issues (less common):
* Fan Motor: The fan motor itself could be failing. It might still run at low speed, but the high-speed windings could be damaged.
* Fan Clutch (if equipped): While less common on electric fans, some vehicles use a viscous clutch to control fan speed. A failed clutch might prevent the fan from reaching high speed.
Troubleshooting Steps:
1. Check for Diagnostic Trouble Codes (DTCs): Use an OBD-II scanner to retrieve any stored codes. This can point you in the right direction.
2. Inspect the Wiring: Visually inspect all wiring related to the cooling fan, looking for damage, corrosion, or loose connections.
3. Test the Relay: Use a multimeter to test the high-speed relay. There are many online resources showing how to do this.
4. Test the Fan Motor: With the relay bypassed (carefully!), try applying 12V directly to the fan motor to see if it runs at high speed. (This should only be done with the negative terminal disconnected from the battery to avoid damage.)
5. Check the Fuse: Inspect the appropriate fuse.
Important Note: Working with car electrical systems can be dangerous. If you're not comfortable working with electricity, it's best to take your GMC Jimmy to a qualified mechanic. Incorrectly diagnosing or repairing electrical issues can lead to further damage or injury.
