Fan won’t shut off on a C15 cat

All switches are input devices to the ECM which the ECM monitors. The ECM turns the fan on and off via the solenoid based on those inputs.

 

1. Check the Fan Control Fuse & Relay:
   
   • Locate the fan solenoid fuse and relay (typically in the main fuse/relay box or ECM-related panel).
   • Use a multimeter or test light to check for:
     
     • Blown fuse (replace if blown).
     • Relay clicking when activated — swap with another known good relay to test.
2. Inspect ECM Output for Fan Control:
   
   • The ECM controls the ground (or sometimes power) to the fan solenoid. Use CAT ET (Electronic Technician) if available.
   • If not available, check the wiring from the ECM to the solenoid — look for continuity and any shorts to ground or power.
3. Coolant Temperature Sensor (CTS) Fault:
   
   • If the ECM thinks the engine is too hot (due to a bad CTS), it may keep the fan on.
   • Check the sensor values in CAT ET or replace the CTS if readings are clearly wrong or sensor is damaged.
4. AC Pressure Switch or Override Inputs:
   
   • Many systems will keep the fan on if the AC system is demanding cooling or if a manual override switch is stuck or engaged.
   • Unplug the AC pressure switch to test. If the fan shuts off, the switch or wiring may be faulty.
5. Broken or Corroded Wiring:
   
   • Carefully inspect the harness between ECM and fan solenoid, especially near connectors and bends.
   • Look for rubbed-through insulation, corrosion, or pin damage.