* Wiring harness issues: This is the most common problem. Look for:
* Broken wires: Carefully inspect the wiring harness between the engine's coolant temperature sensor (CTS), the fan relay, and the fan motor itself. Look for frayed insulation, broken strands, or corroded connections. Pay close attention to areas that flex or move during engine operation.
* Loose or corroded connections: Check all connectors and terminals for corrosion, loose pins, or poor contact. Clean them with a wire brush and dielectric grease.
* Short to ground: A short circuit in the wiring harness could prevent power from reaching the relay's coil. This requires careful tracing of the wires.
* Coolant Temperature Sensor (CTS): The CTS tells the PCM (Powertrain Control Module) when to activate the cooling fan. A faulty CTS might not send the correct signal, preventing the relay from engaging. Test the CTS's resistance with a multimeter against the vehicle's specifications. A failing CTS can sometimes provide intermittent readings.
* PCM (Powertrain Control Module) issue: Though less likely, a faulty PCM could be preventing the signal to engage the cooling fan relay. This requires more advanced diagnostics and potentially a scan tool.
* Fuse: Check the fuse(s) that supply power to the cooling fan circuit. A blown fuse would prevent power from reaching the relay.
* Ground connection: A poor ground connection can prevent the relay from functioning correctly. Make sure the ground wires connected to the engine block and chassis are clean and securely fastened.
Troubleshooting Steps:
1. Visual inspection: Thoroughly inspect all wiring and connections related to the cooling fan, starting at the CTS and following the wires to the relay and fan motor.
2. Fuse check: Check all relevant fuses.
3. CTS testing: Test the CTS with a multimeter according to your car's specifications (you'll need a repair manual or online resource for those).
4. Wiring continuity test: Use a multimeter to check for continuity in the circuit leading to the relay. This will require disconnecting the relay and carefully checking each wire leading to the relay's coil. You should have power at one terminal when the engine is running at the appropriate temperature, and a ground connection on the other.
5. Ground check: Make sure the engine block ground is good.
6. Advanced diagnostics (optional): If you've ruled out everything else, you might need a scan tool to check for PCM codes related to the cooling fan circuit.
Jumping the relay works because it bypasses the entire circuit leading to the relay. Therefore, the problem is somewhere *upstream* in that circuit. Systematic checking of the wiring and components is essential. A repair manual specific to your 1991 Sunbird will be immensely helpful.
