Owner's Problem Description
The vehicle owner reports that the engine is overheating due to the cooling fan not activating. They have previously replaced the water pump and thermostat and performed a coolant flush. Despite these actions, the fan remains inactive, leading to concerns about engine temperature. The owner is seeking a method to bypass the sensors to run the fan continuously as a temporary solution.
Professional Technical Analysis
The 1994 Dodge Intrepid is equipped with a 3.3L V6 engine and has approximately 150,000 miles. The owner has noted that the cooling fan does not activate, leading to overheating issues. A systematic diagnosis of the cooling fan system is necessary, as it involves multiple components. The fan operation is controlled by the Powertrain Control Module (PCM) based on inputs from the engine temperature sensor and the air conditioning controller. Two relays in the under-hood power distribution box manage the fan circuits, and two fuses (F18, 10A in the junction block and a 20A fuse under the hood) protect these circuits. The lack of fan activation is likely due to a failure in one of these components, or possibly a wiring issue. Initial checks should include verifying the condition of the fuses and relays, inspecting the temperature sensor in the thermostat housing, and ensuring the PCM is functioning correctly. If the temperature sensor is unplugged and the fan still does not activate, this points to potential relay or wiring faults. A voltage test at the fan connector should reveal if power is reaching the fan under the appropriate conditions. It's critical to address this issue promptly to prevent engine damage due to overheating.
Possible Causes
Most common causes (ordered by frequency):
- Cooling Fan Relay Failure: The relay controlling the cooling fan may be defective. If the relay does not switch on when the PCM sends the signal, the fan will not operate. Testing the relay with a multimeter for continuity and proper operation under load is essential. OEM part number for relay is 56041476AB. This is a common failure, especially in vehicles with high mileage, and should be tested first. - No fan operation despite power; relay clicks when tested but fan does not engage.
- Faulty Engine Temperature Sensor: If the engine temperature sensor is malfunctioning, it may not send the correct temperature reading to the PCM, preventing the fan from activating. The sensor should provide a resistance value of approximately 2.5k ohms at room temperature. Testing the sensor for proper voltage range (typically around 5V reference) and resistance is crucial. OEM part number for the temperature sensor is 04891821AA. - Engine temperature gauge reads incorrectly; possible stored fault codes related to temperature sensor.
- Wiring Issues: Damaged or corroded wiring between the PCM, relays, and cooling fan can prevent the circuit from completing. Inspect wiring harness for signs of wear, corrosion, or physical damage. Repair or replace any damaged wires to restore proper function. This is a frequent problem in older vehicles due to age and environmental factors. - Visual inspection reveals frayed wires; continuity tests fail at damaged sections.
- Cooling Fan Assembly Failure: The fan motor itself may be defective. If the fan motor has failed, it will not operate even when power is supplied. Testing the fan motor directly for voltage at the connector while the PCM commands it to activate is necessary. The typical replacement cost for the fan assembly is around $150-$200 with OEM part number 5010186AA. - Fan does not spin when powered directly; audible noise indicates motor failure.
Diagnostic Steps
Professional Diagnosis Process
Follow these systematic steps to accurately diagnose the issue. Each step builds on the previous one to ensure accurate diagnosis.
- STEP 1 - Initial Visual Inspection: Begin with a thorough visual inspection of the fuses, relays, and wiring related to the cooling fan system. Check the condition of fuse F18 (10A) and D (20A) for continuity. Inspect the relays in the under-hood power distribution box for signs of damage or corrosion. This initial step is crucial to rule out obvious issues before deeper diagnostics.
- STEP 2 - Testing the Cooling Fan Relay: Use a multimeter to check the continuity and operation of the cooling fan relay. Activate the relay manually to see if the fan responds. If the relay fails to function properly, replace it with OEM part number 56041476AB. This step is necessary to confirm if the relay is the source of the problem.
- STEP 3 - Check Engine Temperature Sensor: Disconnect the engine temperature sensor and measure the resistance across its terminals. It should read approximately 2.5k ohms at room temperature. If the reading is significantly off, replace the sensor with OEM part number 04891821AA. Ensure the wiring harness to the sensor is intact and free of corrosion.
- STEP 4 - Test the Fan Motor: With the PCM commanding the fan to activate, measure the voltage at the fan connector. It should be around 12V when activated. If there is no voltage present, trace back to the relay and PCM. If voltage is present but the fan does not operate, the fan motor may need replacing. Directly power the fan to confirm its function.
