1. The Obvious First Steps:
* Check the Fuse: Start with the simplest fix. Consult your owner's manual to find the fuse for the gauge cluster and check if it's blown. Replace it if necessary.
* Check the Gauge Itself: Is the gauge completely dead, or is it stuck at a certain point (like zero)? A stuck needle might indicate a problem within the gauge itself, while a completely dead gauge points to a problem elsewhere in the circuit. Try tapping gently on the gauge cluster – a loose connection inside could be causing intermittent issues. This is a long shot, but sometimes it works.
* Check the Wiring: Visually inspect the wiring harness connected to the gauge cluster. Look for any obvious damage, corrosion, or loose connections. Pay close attention to the wires leading to the temperature sensor.
2. Temperature Sensor:
The most likely culprit is the Cylinder Head Temperature (CHT) sensor. This sensor is located in the cylinder head and sends a signal to the gauge.
* Locate the Sensor: Find the sensor (it's usually a single-wire sensor screwed into the cylinder head). Consult your repair manual or a parts diagram for the exact location. It's often near the thermostat housing.
* Test the Sensor (Requires a Multimeter): Disconnect the sensor's wire. With the engine at operating temperature (or even just warm), use your multimeter to check the sensor's resistance. You'll need to compare your reading to the specifications in your repair manual or look up the resistance chart for your sensor online. A faulty sensor will show incorrect resistance values. Caution: The engine will be hot; be careful when working near it.
* Replace the Sensor: If the sensor tests bad, replace it with a new one from a reputable parts supplier. Make sure you get the correct sensor for your year and engine.
3. Gauge Cluster:
If the sensor is good, the problem might lie within the gauge cluster itself.
* Check the Wiring (Again): More thoroughly check the wiring harness connecting the sensor to the gauge cluster. Look for broken wires, bad connections, or corrosion.
* Test the Gauge (Difficult without specialized equipment): Testing the gauge itself is more involved and typically requires a specialized multimeter or a back-probing method to test the circuit's signals. This is best left to someone with experience. A shop could do this.
* Replace the Gauge Cluster (Expensive): As a last resort, you could replace the entire gauge cluster. This is the most expensive option and should only be considered after ruling out all other possibilities.
4. Grounding Issues:
A poor ground connection can affect the entire electrical system, including the gauge cluster. Check for clean, tight ground connections in the engine bay and the dash area.
Important Considerations:
* Repair Manual: A repair manual specific to your '86 Ford Ranger 2.9L is invaluable for this troubleshooting. It provides diagrams, specifications, and step-by-step instructions.
* Safety First: Always disconnect the negative battery terminal before working on any electrical components. Be cautious when working around hot engine parts.
If you're not comfortable working with automotive electrical systems, it's best to take your truck to a qualified mechanic. Replacing the sensor is usually the most straightforward fix, but the others require more expertise and testing equipment.
