Owner's Problem Description
The cooling fans on my 1994 Acura Legend start running at high speed as soon as I turn the key to the ON position, even before starting the engine. They continue to run at high speed while the engine is running and only shut off when the car is turned off. This is not normal behavior, and I am concerned that constant operation will lead to premature failure of the fan motors.
Professional Technical Analysis
The 1994 Acura Legend is equipped with a cooling fan control system designed to operate the fans based on engine temperature and air conditioning status. With the fans running continuously upon key ON, despite the engine not being started, it indicates a potential malfunction in the fan control system or wiring. The cooling fans are expected to activate based on PCM (Powertrain Control Module) input, which uses engine temperature data from the coolant temperature sensor. If the fans run constantly, it could be due to a stuck relay, a short in the control circuit, or a faulty PCM. The vehicle's environmental conditions, such as temperature fluctuations and humidity, may exacerbate electrical issues, especially in older vehicles like the Legend. A thorough inspection of the fan relay, wiring harness, and the coolant temperature sensor is essential. Additionally, if the A/C switch is activated, the fans should operate, but the fans running continuously without A/C usage indicates a fault. The vehicle may also exhibit other symptoms such as poor fuel economy due to constant fan operation. Ensuring the fans operate only when necessary is critical to prevent premature wear on the fan motors and maintain operational efficiency.
Possible Causes
Most common causes (ordered by frequency):
- Stuck Cooling Fan Relay: The cooling fan relay may be stuck in the closed position, causing the fans to run continuously. This can occur due to internal relay failure or corrosion at the relay terminals, leading to constant power supply to the fans. Testing the relay with a multimeter for continuity while applying power can confirm this issue. Typical relay specifications involve a switch rating of 30 amps at 12V. - Fans operate continuously even when the engine is off, relay terminals show signs of corrosion, multimeter testing indicates continuity in the relay when it shouldn't be closed.
- Faulty Coolant Temperature Sensor: A malfunctioning coolant temperature sensor may send incorrect signals to the PCM, causing it to erroneously activate the cooling fans. The sensor should typically read around 0.5V at 20°C and 4.5V at 100°C. A major deviation from these readings could indicate sensor failure. - Inconsistent temperature readings on the scan tool, check engine light may be illuminated, fans running with no temperature increase in the engine.
- Wiring Issues: Damaged or shorted wiring in the fan control circuit can lead to continuous fan operation. Inspect the wiring harness for signs of wear, fraying, or damage that may cause a short circuit. The resistance in the circuit should ideally be less than 5 ohms; anything significantly higher may indicate a problem. - Visual inspection reveals damaged insulation on wiring, continuity test shows short circuit between power and ground in the fan control circuit.
- Faulty Powertrain Control Module (PCM): If the PCM itself is malfunctioning, it may incorrectly control the fan operation based on false inputs. This is a less common issue but can occur, especially in older models. Testing the PCM for fault codes and input readings is essential for diagnosis. - Stored fault codes related to fan control, abnormal behavior of other engine components may also be present, indicating PCM evaluation is necessary.
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 OBD-II Scan: Begin with a comprehensive OBD-II scan using a professional scanner or GeekOBD APP to retrieve any existing trouble codes. Pay close attention to codes related to the cooling system or fan control. Analyze freeze frame data to understand the vehicle's operating conditions when the fault occurred. This step is crucial to identify potential issues quickly.
- STEP 2 - Wiring Inspection: Visually inspect the wiring harness associated with the cooling fan relay and coolant temperature sensor. Look for signs of wear, fraying, or corrosion. Use a multimeter to check for continuity in the relay circuit, ensuring the relay is not stuck closed. Measure resistance in the control circuit; it should be less than 5 ohms.
- STEP 3 - Sensor Testing: Test the coolant temperature sensor using a multimeter to verify voltage outputs at different temperatures. The sensor should provide approximately 0.5V at 20°C and around 4.5V at 100°C. If readings are outside these parameters, replace the sensor. This can often resolve fan operation issues.
- STEP 4 - Relay Function Test: If the initial tests do not reveal any issues, perform a relay function test. Swap the fan relay with a known good relay or test the existing relay by applying voltage directly to the coil terminals. If the fans run continuously regardless, the relay is faulty. Replace as necessary and retest system operation.
