Owner's Problem Description
The owner reports that the radiator cooling fan is continuously running, regardless of whether the air conditioning or heater is in use. The fan remains operational even after shutting off the engine. They have replaced the relay switch but the problem persists. Recently, they swapped the engine from an H-23 to an H-22, but the dealer mistakenly provided a computer for an automatic transmission instead of the manual one. They are seeking guidance on whether the issue could be related to the incorrect computer or if there might be another underlying problem.
Professional Technical Analysis
This 1993 Honda Prelude, equipped with an H-22 engine and approximately 150,000 miles, exhibits a persistent issue where the radiator cooling fan runs continuously, even when the vehicle is turned off. Initially, the owner replaced the relay switch, which typically controls the fan operation based on coolant temperature, but this did not resolve the issue. Given the recent engine swap, it’s crucial to assess the compatibility of the engine control unit (ECU) as the dealer provided a unit meant for an automatic transmission. The fan operation could be influenced by the ECU programming or faulty components like the coolant temperature sensor or fan switch. The vehicle’s current condition raises safety concerns, as an overheating engine could result from continuous fan operation affecting other systems. It’s essential to check for diagnostic trouble codes (DTCs) related to the fan control circuit or coolant temperature sensor which may not have been accurately addressed. A comprehensive diagnostic using the GeekOBD APP will provide insights into live data from the cooling system, allowing for precise troubleshooting of the fan control logic and sensor performance.
Possible Causes
Most common causes (ordered by frequency):
- Faulty Coolant Temperature Sensor: If the coolant temperature sensor provides incorrect readings (usually around 0.5V at cold and 4.5V at operating temperature), the PCM may not receive the necessary signal to turn off the fan. This sensor is critical for regulating fan operation and is often located near the thermostat housing. - Check engine light may be illuminated, and DTCs related to the coolant temperature sensor could be stored. Testing the sensor with a multimeter should show expected voltage readings under varying temperatures.
- Incorrect ECU Programming: The ECU supplied for an automatic transmission may not properly manage the fan operation logic for a manual transmission vehicle, leading to continuous fan engagement. This mismatch can cause persistent fan operation regardless of engine temperature. - Diagnosis using GeekOBD APP can reveal communication issues or abnormal parameters that indicate misalignment of the ECU's logic with the vehicle's configuration.
- Malfunctioning Fan Relay: Even though the relay switch has been replaced, a faulty connection or corrosion in the relay socket could still cause the fan to remain powered. Testing the relay operation with a multimeter and ensuring proper voltage at the relay socket is essential. - Symptoms such as a clicking sound from the relay area or overheating of the relay itself can indicate issues. Continuity testing can confirm proper operation.
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 thorough diagnostic scan using GeekOBD APP to retrieve any stored DTCs related to the cooling fan operation or coolant temperature sensor. This will provide insight into any potential electrical faults and help prioritize which systems to check.
- STEP 2 - Live Data Monitoring: Utilize GeekOBD APP to monitor the coolant temperature readings and fan operation parameters in real-time. Ensure the coolant temperature sensor is providing accurate voltage signals under varying engine temperatures (0.5V at cold, around 4.5V at operating temp).
- STEP 3 - Relay and Wiring Inspection: Inspect the fan relay and its wiring for any signs of corrosion or loose connections. Perform a continuity test on the relay and check for voltage at the relay socket when the engine is running and when it is off.
- STEP 4 - ECU Verification: If the previous steps do not yield results, verify the ECU compatibility and programming. Check for software updates or reprogramming options that may align the control logic with the manual transmission configuration.
