Owner's Problem Description
The owner reports that the cooling fan is running constantly from the moment the engine starts, even when cold. Additionally, a check engine light has illuminated, which the owner believes is related to the fan issue. Previous attempts to diagnose the problem involved checking for trouble codes, and it appears that codes related to the coolant temperature sensor were retrieved. The vehicle is currently operational but with these persistent issues.
Professional Technical Analysis
The 1989 Toyota Corolla, equipped with a carbureted engine, is experiencing a condition where the cooling fan operates continuously upon startup, coupled with an illuminated check engine light. This vehicle's cooling system is designed to activate the fan based on coolant temperature readings from the coolant temperature sensor (CTS), located near the engine block. The persistent activation of the fan likely indicates a malfunctioning CTS, which may be sending erroneous signals to the engine control module (ECM). The retrieved trouble codes include 22 for the coolant temperature sensor, 31 for a vacuum sensor, and 41 for a throttle position sensor, despite the vehicle being carbureted, which raises questions about the validity of the TPS code. The continuous operation of the fan can lead to increased electrical load and potential overheating of the fan motor itself if not addressed. Additionally, the check engine light suggests that the ECM has detected a fault in the system. The fan relay, located in the relay box, should also be tested for proper operation, as a stuck relay could cause the fan to remain on. As the vehicle has an estimated mileage of around 150,000, components like the CTS and relays are prone to wear and failure due to age. Proper diagnostic steps should include verifying the operation of the CTS by measuring resistance values at different temperatures, ensuring they correspond to the specifications (approximately 2.5k ohms at 25°C and decreasing as temperature rises). Also, inspecting the fan relay for continuity and functionality under load is crucial. This vehicle's condition raises safety concerns, as a malfunctioning cooling system could lead to engine overheating.
Possible Causes
Most common causes (ordered by frequency):
- Coolant Temperature Sensor (CTS) Failure: The coolant temperature sensor, which should provide accurate temperature readings to the engine control module (ECM), may be faulty. If the sensor outputs a constant low voltage, the ECM interprets this as a cold engine and keeps the cooling fan running continuously. The CTS for this vehicle typically operates within a resistance range of 2.5k ohms at 25°C, decreasing as the temperature rises. A failure in this component can lead to incorrect fan operation and should be tested for resistance values to confirm its status. - Check engine light illuminated, retrieved code 22 indicating a fault in the coolant temperature sensor, abnormal fan operation even when the engine is cold.
- Fan Relay Malfunction: A stuck or malfunctioning fan relay could cause the fan to remain on regardless of the coolant temperature readings. If the relay contacts are welded together or the relay is faulty, the fan will receive a constant signal to operate. Testing the relay for continuity and proper operation under load is necessary. This relay typically operates with a 12V signal and should be replaced if found defective. - Continuous operation of the fan despite cold engine, relay contacts tested for continuity show signs of being stuck.
- Wiring Issues: Damaged or corroded wiring can cause false signals to the ECM, leading to incorrect fan operation. Inspecting the wiring harness for damage, especially near the CTS and fan relay connections, is crucial. Any shorts or breaks in the wiring could lead to continuous fan activation. Ensuring proper connections and repairing any damaged wiring is essential in resolving this issue. - Visual inspection reveals signs of corrosion at the CTS connector and frayed wiring near the fan relay.
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 Code Retrieval: Use a professional OBD-II scanner or the GeekOBD APP to pull diagnostic trouble codes from the ECM. Pay attention to the specific codes retrieved, particularly code 22, which relates to the coolant temperature sensor. Document any additional codes, as these can provide insight into other potential issues affecting the cooling system.
- STEP 2 - Coolant Temperature Sensor Testing: Disconnect the CTS connector and measure the resistance across the sensor terminals using a multimeter. Compare the readings to the expected values (approximately 2.5k ohms at 25°C). If the readings are outside the normal range, the sensor should be replaced. If the sensor appears functional, check for proper voltage supply from the ECM.
- STEP 3 - Fan Relay Inspection: Locate the fan relay in the relay box and perform a continuity test to ensure it operates correctly. If the relay is stuck or faulty, replace it with an OEM or quality aftermarket relay. Ensure proper installation and verify operation after replacement using the GeekOBD APP to monitor the fan's performance.
- STEP 4 - Wiring Inspection: Examine the wiring harness for any signs of damage or corrosion, particularly around the CTS and fan relay. Repair any damaged wires and ensure all connections are secure. After completing the wiring inspection, recheck the fan operation and monitor for any abnormal behavior using the GeekOBD APP.
