I. Safety First:
* Disconnect the negative battery terminal before starting any electrical testing. This prevents accidental short circuits and shocks.
II. Visual Inspection:
1. Inspect the fan blades: Check for obvious damage, such as bent blades or debris obstructing rotation.
2. Examine the fan shroud: Ensure it's properly installed and not interfering with fan operation.
3. Check the wiring harness: Look for any visible damage, fraying, or corrosion to the wires leading to the fan and its relay. Pay particular attention to the connectors.
4. Inspect the fan motor: Look for any signs of physical damage to the motor itself.
III. Testing the Cooling Fan Circuit:
This involves checking the fan motor, the relay, the temperature sensor, and the wiring. You'll need a multimeter capable of measuring voltage and resistance.
1. Check the Fan Motor (Resistance):
* Locate the fan motor connector: Disconnect it.
* Set your multimeter to ohms (Ω): Measure the resistance across the fan motor's terminals. You should get a reading – a very low resistance indicates a good motor; an open circuit (infinite resistance) indicates a bad motor. Consult your repair manual for the expected resistance range for your specific fan motor.
2. Check the Fan Relay:
* Locate the relay: This is usually found in the underhood fuse box. Your repair manual will show its location. Many have diagrams showing relay locations and functions.
* Visual Inspection: Check for any signs of damage or burning.
* Testing (if possible): Some relays can be tested with a multimeter by checking continuity between appropriate terminals. Your repair manual will give specifics. Often the easiest way is to swap with a known good relay of the same amperage rating (a relay from another circuit).
3. Check the Cooling Fan Relay Circuit:
* Check for power at the relay: With the ignition ON and the engine at operating temperature, use your multimeter to check for power at the relay's power input terminal. You should see battery voltage (12V). Lack of power indicates a problem in the power supply circuit to the relay.
* Check for ground at the relay: With the ignition ON and the engine at operating temperature, use your multimeter to check for a good ground at the relay's ground terminal. A poor or missing ground will prevent the relay from working. This is a common point of failure.
4. Check the Engine Coolant Temperature (ECT) Sensor:
* Locate the ECT sensor: Usually located in the engine block or cylinder head.
* Test the sensor: This requires either comparing its resistance to specifications at various temperatures (using a thermometer and multimeter) or using a scan tool to read the sensor's signal. A faulty sensor may not send the correct signal to activate the cooling fan.
5. Check the Wiring:
* Continuity test: Use your multimeter to test for continuity in all wires leading to the fan, the relay, and the ECT sensor. Look for breaks or shorts in the wiring. Pay attention to grounds. A broken wire can be anywhere along the wire run, not just at the connectors.
* Voltage drop test: If you find continuity, you can test for voltage drop along the wires. Excessive voltage drop indicates a problem with the wiring itself (high resistance).
IV. Ground Problem Diagnosis:
Ground problems are notoriously difficult to diagnose.
* Check the fan's ground connection: Trace the ground wire from the fan motor to its connection point. Clean the connection points thoroughly and ensure a solid connection. You may need to use a wire brush or sandpaper. If you suspect a ground issue here, try running a temporary ground wire directly from the fan motor case to a known good ground point on the chassis. If the fan works, you've identified a ground problem.
* Check the relay's ground connection: Repeat the above for the relay's ground connection.
* Check the general chassis ground: Sometimes poor grounds elsewhere on the chassis can affect other electrical systems, including the cooling fan. Clean all ground points you find.
V. Using a Scan Tool (Optional but Highly Recommended):
A scan tool can help monitor the ECT sensor signal and confirm whether the PCM (Powertrain Control Module) is commanding the fan to operate. This will pinpoint the problem much quicker than just using a multimeter.
Important Notes:
* Consult a repair manual: A repair manual specific to your 1996 Mercury Mystique is essential for accurate wiring diagrams, component locations, and specifications.
* Safety: Remember to disconnect the negative battery terminal before starting any electrical work.
* Professional help: If you're uncomfortable performing these tests, it's best to seek professional assistance from a qualified mechanic.
By systematically working through these steps, you should be able to isolate the cause of your electric cooling fan malfunction. Remember to always refer to your repair manual for specific details and diagrams for your vehicle.
