* Normal Operation: The most common reason is that the engine coolant temperature is high enough to trigger the fan. The fan's purpose is to help cool the engine when the radiator alone can't keep up. This is especially true during idling or slow driving in hot weather.
* Faulty Coolant Temperature Sensor (CTS): This sensor tells the engine control module (ECM) the temperature of the coolant. If it's faulty, it might send a signal indicating the coolant is hotter than it actually is, causing the fan to turn on unnecessarily. This is a common culprit for fans running constantly.
* Faulty Radiator Fan Relay: The relay acts as a switch, activating the fan based on the signal from the CTS or other sensors. A malfunctioning relay might cause the fan to run continuously, even when it shouldn't.
* Faulty Fan Control Module (if equipped): Some vehicles have a dedicated module to control the fan speed and operation. A problem with this module can lead to incorrect fan operation. Less likely on a 1991 model, but possible.
* Low Coolant Level: If the coolant level is low, the engine may overheat more easily, causing the fan to run frequently or constantly to compensate. Check your coolant reservoir.
* Stuck Thermostat: A thermostat that's stuck closed prevents coolant from circulating properly, leading to overheating and fan activation.
* Clogged Radiator or Radiator Hoses: Restricted coolant flow will cause the engine to overheat, resulting in the fan running more often.
* Failing Water Pump: If the water pump isn't circulating coolant effectively, the engine will overheat, and the fan will run more.
* Wiring Issues: Short circuits or damaged wiring in the fan circuit can also cause the fan to run continuously or intermittently.
Troubleshooting Steps:
1. Check Coolant Level: The easiest first step. Low coolant is a serious issue.
2. Check Engine Temperature: Does the engine feel excessively hot to the touch (carefully)?
3. Listen to the Fan: Is it running constantly, even after the engine has cooled down significantly? This points to a sensor or relay problem.
4. Visual Inspection: Check the radiator and hoses for any obvious leaks or blockages.
If you're not comfortable working on your car yourself, it's best to take it to a qualified mechanic for diagnosis and repair. They have the tools and expertise to pinpoint the exact cause of the problem. Ignoring a constantly running fan could lead to engine damage.
