Coolant Temperature Gauge and Sensor – Information, Troubleshooting, and Testing – 2.5L NA

Overview

This procedure outlines how to troubleshoot and test the coolant temperature gauge and sensor on all Porsche 944 variants with an 8-valve engine. The coolant temperature gauge monitors engine coolant temperature, critical for preventing overheating and ensuring optimal performance. The Porsche 944 uses two different gauge styles: one for pre-1985.5 models (1982–85) and another for 1985.5 onwards models (1985.5–88). This guide helps diagnose inaccurate gauge readings, which may stem from a faulty gauge or sensor, and includes model-specific resistance values for testing. Normal coolant temperatures typically range from 80–90 °C (176–194 °F) in cool to moderate climates (up to 25 °C or 77 °F) during highway driving, and up to 100 °C (212 °F) in warmer climates (25–35 °C or 77–95 °F) or heavy traffic, with cooling fans activating around 92 °C (198 °F).

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Tools Required

• Multimeter
• Variable Resistance Potentiometer (0–500 ohms)
• Test Leads
• 19mm (3/4 inch) wrench

Parts Required

• Coolant temperature sensor (if replacement is needed): 944 606 125 00

Step-by-Step Procedure

Step 1: Prepare for Testing

1. Park the vehicle on a flat surface and engage the parking brake.
2. Turn the ignition switch to the OFF position to ensure safety.
3. Locate the coolant temperature sensor on the engine block, near the cylinder head, typically on the driver’s side.

Step 2: Disconnect the Coolant Temperature Sensor

4. Disconnect the two spade connector leads from the coolant temperature sensor. Note the larger lead for testing.

For Pre-85.5 models:

• The sensor uses a two-pin connector; carefully disconnect both leads to avoid damaging the wiring.

For 85.5 Onwards models:

• The sensor may have a different connector style; ensure proper identification of the main signal lead.

Step 3: Set Up the Testing Equipment

5. Using a multimeter and a variable resistance potentiometer, set the potentiometer to the resistance values listed below for your model year.

For Pre-85.5 models:

• 40 °C (104 °F): 287.4 ohms

• 105 °C (221 °F): 33.6 ohms

• 120 °C (248 °F): 22.7 ohms

For 85.5 Onwards models:

• 40 °C (104 °F): 287.4 ohms

• 60 °C (140 °F): 134.0 ohms

• 80 °C (176 °F): 69.1 ohms

• 100 °C (212 °F): 38.5 ohms

• 115 °C (239 °F): 25.8 ohms

Step 4: Connect the Potentiometer

6. Connect the larger of the disconnected sensor leads to one terminal of the variable resistance potentiometer, ensuring the potentiometer setting remains unchanged.
7. Connect the other terminal of the potentiometer to a grounding point in the engine compartment, such as a clean metal surface on the engine block.

Step 5: Test the Coolant Temperature Gauge

8. Turn the ignition switch to the ON position without starting the engine.
9. Observe the coolant temperature gauge reading and compare it to the expected indication for the resistance set on the potentiometer (refer to the resistance tables in Step 3).
10. Repeat the test for each resistance value listed for your model year to verify the gauge responds correctly.

Step 6: Evaluate Results and Next Steps

11. If the gauge readings match the expected values for each resistance setting, the gauge is functioning correctly. If readings are abnormal during engine operation, proceed to test the coolant temperature sensor (see Coolant Temperature Sensor Testing in the Porsche 944 Workshop Manual).
12. If the gauge does not respond correctly to the resistance settings, it may be defective. Replace the gauge or send it to an authorized VDO repair facility for service.
13. If replacing the sensor, use a 19mm (3/4 inch) wrench to remove the old sensor and install a new one (944 606 125 00). Torque to 30 Nm (22 ft-lbs).

Post Procedure Checklist

• Verify all electrical connections to the coolant temperature sensor are secure.
• Check for any loose tools or debris in the engine compartment.
• Start the engine and monitor the coolant temperature gauge. It should read 80–90 °C (176–194 °F) during highway driving in cool to moderate climates (up to 25 °C or 77 °F), or up to 100 °C (212 °F) in warmer climates or heavy traffic.
• Ensure cooling fans activate around 92 °C (198 °F) if the temperature approaches this level.
• Check for coolant leaks around the sensor after reconnection or replacement.

Sources

• Clark’s Garage
• Pelican Parts
• Porsche 944 Workshop Manuals
• YouTube