Sensors and Gauges — Information, Troubleshooting, and Testing

Contents

• Coolant Temperature Gauge Testing
• Oil Pressure Gauge Testing
• Fuel Level Gauge Testing
• Air Flow Sensor Testing
• DME Temperature Sensor Testing
• Throttle Position Switch Testing
• Speed and Reference Sensor Testing
• Oxygen Sensor Testing

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Coolant Temperature Gauge

Two styles of coolant temperature gauges were used on 924s and 944s. The early style was used on all 924s and on 944s up to the 1985 model year. The later gauge was installed on 1985.5 and newer 944s and the 968.

The coolant temperature gauge faces do not display numerical temperature markings, making it difficult to judge absolute temperatures. The diagrams below show the relationship between gauge position and temperature (degrees Celsius).

Normal coolant temperature guidance:

In cool to moderate climates (up to about 25 °C) with highway driving, coolant temperatures typically run between 80–90 °C and the radiator cooling fans will normally not run. In warmer climates (25–35 °C) and in stop-and-go traffic, temperatures may reach 100 °C; the cooling fans should activate at approximately 92 °C. Coolant temperatures consistently above 100 °C warrant investigation of the cooling system, temperature sensor, or gauge.

Note: Increased engine power output raises heat rejection to the cooling system, which will cause higher steady-state coolant and oil temperatures. This is expected behavior after performance modifications — it does not necessarily indicate a cooling system fault.

The temperatures in the early gauge diagram were determined by FR Wilk through interpolation of Bosch sender curves.

Tools

• Multimeter
• Variable resistance potentiometer (minimum 0–500 Ω)
• Test leads

Procedure

1. Disconnect the two spade connector leads from the coolant temperature gauge sensor.

2. Set the variable potentiometer to one of the resistances in the table for your model year.

   Early 944s (pre-1985.5), 924:

   Temperature (°C)	Resistance (Ω)
   40	287.4
   105	33.6
   120	22.7

   Late 944s (1985.5 and newer):

   Temperature (°C)	Resistance (Ω)
   40	287.4
   60	134.0
   80	69.1
   100	38.5
   115	25.8

3. Connect the larger of the disconnected sensor leads to one terminal of the variable potentiometer (do not change the setting). Connect the other terminal to a ground point in the engine compartment.

4. Turn the ignition switch to ON.

5. Check the temperature gauge reading against the expected indication for the resistance set on the potentiometer.

6. Repeat for all temperature/resistance combinations in the table.

7. If the gauge reads normally in testing but shows abnormal temperatures during engine operation, proceed to temperature sensor testing.

8. If the gauge is confirmed bad, replace it or send the cluster to an authorized VDO repair facility.

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Oil Pressure Gauge Testing

At startup, with the ignition on but engine not running, the oil pressure indication should read 5 bar. Once the engine is started it should indicate 4.5–5 bar. As the engine reaches normal operating temperature, pressure typically drops to around 2.5–3 bar. On very hot days it may indicate as low as 2 bar. Consistent readings below 2 bar may indicate a bearing wear problem.

Problems with oil pressure indication are commonly caused by a failing oil pressure sending unit.

Tools

• Multimeter
• Variable resistance potentiometer (minimum 0–500 Ω)
• Test leads

Procedure

1. Disconnect the leads from the oil pressure sending unit.

2. Set the variable potentiometer to a resistance from the appropriate table.

   1982–1985 944:

   Oil Pressure (Bar)	Resistance (Ω)
   0	10.0
   3	116.0
   5	184.0

   1985.5 and newer 944:

   Oil Pressure (Bar)	Resistance (Ω)
   1	29.6
   2	65.3
   3	98.9
   4	133.6
   5	184.0

3. Connect the lead removed from terminal "G" on the sending unit to one leg of the variable potentiometer. Connect the other leg to a ground point in the engine compartment.

4. Turn the ignition switch to ON.

5. Check the oil pressure gauge reading against the expected indication for the resistance set on the potentiometer.

6. Repeat for all pressure/resistance combinations.

7. If the gauge reads correctly in testing but abnormally during operation, the oil pressure sending unit is likely faulty. If test connections to the sending unit cannot be fabricated, replace the sending unit without testing.

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Fuel Level Gauge Testing

See ELECT-13, Fuel Level Indication Problems / Repairs.

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Air Flow Sensor (AFM) Testing

See ELECT-22, Air Flow Meter (AFM) Operation and Testing.

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DME Temperature Sensor Testing

Tools

• Multimeter
• Test leads

Procedure

1. Turn the ignition switch OFF.
2. Disconnect the DME computer electrical connector.
3. Connect an ohmmeter between terminal 13 on the disconnected DME plug and ground.
4. At ambient temperature (15–30 °C / 59–86 °F), resistance should read:
   • At 15 °C (59 °F): approximately 3.3 kΩ
   • At 30 °C (86 °F): approximately 1.46 kΩ

Note: The next portion of this test assumes the dash temperature gauge is functioning correctly. If the dash gauge is suspect, check the surface temperature of the block near the DME temperature sensor directly.

5. Reconnect the DME plug connector.
6. Start the car and run until the temperature gauge indicates approximately 80 °C.
7. Turn the engine OFF.
8. Disconnect the DME computer plug and reconnect the ohmmeter as in step 3.
9. Turn the ignition switch ON (do not attempt to start). Confirm the dash gauge still reads ~80 °C or verify block surface temperature. At 80 °C, resistance should be 280–360 Ω.
10. If resistance is out of tolerance, replace the DME temperature sensor. A higher-than-spec resistance causes a richer mixture; lower-than-spec causes a lean mixture.

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Throttle Position Switch Testing

See FUEL-06, Throttle Position Switch Information, Troubleshooting, Replacement, and Adjustment.

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Speed and Reference Sensor Testing

See IGN-02, Checking, Replacement, and Adjustment.

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Oxygen Sensor Testing

Tools

• Multimeter
• Test leads
• Propane source

Procedure

1. Start the car and run until the engine reaches normal operating temperature.
2. Disconnect the O₂ sensor and connect a voltmeter to the sensor plug.
3. Run the engine at approximately 2500 rpm.
4. Introduce propane into the intake to enrich the mixture until engine rpm drops by approximately 200 rpm. Alternatively, disconnect the vacuum line to the fuel pressure regulator (plug the vacuum line to prevent an intake leak).
5. If the voltmeter rapidly rises above 0.9 VDC, the O₂ sensor is correctly indicating a rich mixture. If the response is sluggish or the voltage remains below 0.8 VDC, replace the sensor.
6. Secure the propane addition.
7. While continuing to run the engine at 2500 rpm, disconnect a vacuum line from the intake to create a lean mixture.
8. If the voltmeter rapidly drops below 0.2 VDC, the O₂ sensor is correctly indicating a lean mixture. If the response is sluggish or voltage remains above 0.2 VDC, replace the sensor.