Understanding the TSU:
The TSU is a thermistor; its resistance changes with temperature. As the engine coolant gets hotter, the resistance decreases. The gauge in your dash interprets this changing resistance to show the engine temperature.
Tools You'll Need:
* Multimeter: Essential for measuring resistance. Make sure it's set to the ohms (Ω) function.
* Wire Diagram: A wiring diagram for your specific 89 Ford Ranger will help you locate the TSU connector. You can find these online (often on forums like Ford-trucks.com) or in a repair manual.
* Screwdrivers: To access the TSU (usually located on the cylinder head or intake manifold).
* Container to catch coolant: Some coolant might spill when you disconnect the sensor.
Testing Procedure:
1. Locate the TSU: Consult your wiring diagram to find the exact location of the temperature sending unit. It's usually a single-wire sensor connected to the engine.
2. Disconnect the TSU: Carefully disconnect the electrical connector from the TSU. Be mindful of spilled coolant.
3. Prepare Your Multimeter: Set your multimeter to ohms (Ω).
4. Measure Resistance at Ambient Temperature: Touch the multimeter probes to the sensor terminals. Note the resistance reading. This is the resistance at the ambient (room) temperature. The resistance should be relatively high (several thousand ohms). Your wiring diagram or a repair manual might give you a specific resistance chart for different temperatures.
5. Measure Resistance in Hot Water (Optional but Helpful): This provides a more comprehensive test. Carefully place the TSU (with the connector still disconnected) into a container of hot water (but not boiling water). Allow it to sit for a few minutes to reach thermal equilibrium. Measure the resistance again. The resistance should be significantly lower than the room temperature reading.
6. Compare Results: Compare your readings to the specifications found in a repair manual or online resources specific to your vehicle's TSU. A significant deviation from the expected values at different temperatures indicates a faulty TSU.
7. Check the Wiring (if the sensor seems okay): If the TSU readings seem within the acceptable range, there may be a problem in the wiring between the TSU and the instrument cluster gauge. Inspect the wiring for any breaks, corrosion, or loose connections. You could use your multimeter to check for continuity along the wire.
Important Considerations:
* Safety First: Always disconnect the negative battery cable before working on any electrical components in your vehicle.
* Coolant Temperature: Be cautious when working with hot coolant.
* Accuracy: The accuracy of this test depends on the accuracy of your multimeter.
* Online Resources: Forums dedicated to Ford Rangers can be invaluable for finding specific resistance values and troubleshooting tips for your year and model.
If your readings are outside the expected ranges, it's highly likely that the TSU needs replacing. Remember to always use a replacement TSU that matches the specifications of your original unit.
