* Faulty Fan Relay: The relay is an electrical switch that controls power to the fan. A stuck-closed relay will keep power flowing to the fan even when the engine is off. This is the most common cause.
* Bad Fan Control Module (PCM/ECM related): The Powertrain Control Module (PCM) or Engine Control Module (ECM – sometimes these terms are used interchangeably in this context) monitors engine temperature and sends a signal to turn the fan on or off. A malfunctioning PCM/ECM might send the wrong signal, keeping the fan running constantly. This is less common than a relay problem but more serious as it requires a potentially expensive fix.
* Short Circuit in the Wiring: A short circuit in the wiring harness leading to the fan can provide a constant power source, bypassing the normal control system. This is less common but can be tricky to diagnose.
* Faulty Fan Switch (Temperature Sensor): While less likely to cause continuous running (it usually causes *inconsistent* running or failure to run), a malfunctioning temperature sensor that always reads high could trick the system into thinking the engine is overheating, hence keeping the fan on.
* Incorrectly installed aftermarket parts: If you've recently had work done on the cooling system or electrical system, improperly installed parts could cause a short or incorrect connection.
Troubleshooting Steps:
1. Check the Fan Relay: Locate the cooling fan relay (your owner's manual or online resources should help you find its location). Try swapping it with a relay of the same amperage rating from another circuit (if possible and safe) to see if the problem moves with the relay. If it does, the relay is bad.
2. Inspect the Wiring: Carefully inspect the wiring harness leading to the fan for any signs of damage, fraying, or shorts. Look for bare wires touching each other or the chassis.
3. Diagnostic Scan: A professional mechanic can use a scan tool to check the PCM/ECM for any diagnostic trouble codes (DTCs) related to the cooling fan system. This can pinpoint the exact problem.
4. Temperature Sensor Check (Less Likely): A mechanic can test the coolant temperature sensor to confirm it's providing accurate readings.
Important Note: Don't attempt to directly test the wiring or components with a multimeter unless you have experience working with automotive electrical systems. Incorrect testing can lead to damage or injury. If you're not comfortable doing this yourself, take your Jeep to a qualified mechanic. Continuing to run the fan with a dead battery can cause further damage.
