Speed and Reference Sensors – Checking, Replacement, and Adjustment – 2.5L NA

Overview

This procedure outlines how to check, replace, and adjust the speed and reference sensors on the Porsche 944 (8-valve engine) for model years 1983–1985.5 (Pre-85.5) and 1985.5–1988 (85.5 Onwards). These sensors, located on the clutch housing at the back of the engine, provide critical data to the Digital Motor Electronics (DME) system for determining engine position and speed. The reference sensor identifies the #1 piston’s position relative to Top Dead Center (TDC), while the speed sensor counts flywheel teeth to calculate engine RPM. Proper sensor function is essential for correct fuel injection and ignition timing. Faulty sensors can cause starting or running issues, and this guide ensures they operate correctly or are replaced and adjusted as needed.

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Tools Required

• 10mm (3/8 inch) socket or 5mm Allen head socket (depending on sensor retaining bolt)
• 6mm Allen head socket
• 0.8mm thick washer
• Multi-meter (for checking sensor resistances)
• Oscilloscope (optional, for checking sensor output)

Parts Required

• Speed sensor (if replacement is needed): 944 606 115 00
• Reference sensor (if replacement is needed): 944 606 115 00

Step-by-Step Procedure

Step 1: Check Sensor Resistance

1. Locate the speed and reference sensor connectors at the back of the engine compartment, near the intake manifold.

2. Disconnect the sensor plug connectors.
3. Using a multi-meter set to resistance (ohms), measure the resistance for the speed sensor:

   Speed Sensor:

   Terminals 8 and 27: Should read 600–1600 ohms.

   Terminals 8 and 23: Should read greater than 1 M-ohm.

4. Measure the resistance for the reference sensor:

   Reference Sensor:

   Terminals 25 and 26: Should read 600–1600 ohms.

   Terminals 25 and 78: Should read greater than 1 M-ohm.

5. If any resistance reading is outside the specified range, replace the faulty sensor (see Step 3).

Step 2: Perform Sensor Operational Check

6. Remove the fuel pump fuse to prevent the engine from starting during cranking.
7. Disconnect the DME computer plug, located in the engine compartment.
8. To check the speed sensor output:

   Connect an oscilloscope to terminals 8 and 27 on the DME plug.

   Crank the engine and verify a sawtooth wave output with a peak-to-peak voltage greater than 2.5V.

9. To check the reference sensor output:

   Connect an oscilloscope to terminals 25 and 26 on the DME plug.

   Crank the engine and verify a periodic pulse with a peak-to-peak voltage greater than 2.0V. The pulse frequency depends on cranking speed.

10. If an oscilloscope is unavailable, use a multi-meter to check for an output signal (note: this will not confirm signal magnitude, only presence).
11. Alternatively, crank the engine and observe the tachometer. If it jumps during cranking, the sensors are likely functioning. If it does not, the sensors or DME may be faulty, or the sensor clearance may need adjustment (see Step 4).
12. If either sensor’s output voltage is below the specified value, proceed to adjust the sensor clearance (see Step 4) or replace the sensor (see Step 3).

Step 3: Remove the Sensor

13. Disconnect the negative battery lead to ensure safety.
14. Disconnect the speed and/or reference sensor connectors at the back of the intake manifold.
15. Remove the sensor retaining bolt using a 5mm Allen head socket or 10mm socket, depending on the bolt type.

16. Twist the sensor side-to-side while pulling upward to remove it. If stuck, gentle twisting helps free it.

Step 4: Install the Sensor

17. If replacing both sensors, verify correct installation by checking the wiring harness labels near the connector bracket:

    DG = Speed Sensor (944 606 115 00)

    BG = Reference Sensor (944 606 115 00)

    For Pre-85.5 models:

    Check the mounting bracket labels: D = Speed Sensor, B = Reference Sensor. If labels are unreadable, the sensor closest to the firewall is the speed sensor.

    For 85.5 Onwards models:

    Check the mounting bracket labels: D = Speed Sensor, B = Reference Sensor. If labels are unreadable, the sensor closest to the firewall is the speed sensor.

18. If labels are missing, use a multi-meter to measure resistance from the DME plug to the sensor connectors to identify them (refer to Porsche 944 Workshop Manual for pin assignments).

19. Install the new or reused sensor and secure it with the retaining bolt. Torque the bolt to 8 Nm (6 ft-lbs).
20. If the sensor mounting bracket was moved or loosened, adjust the sensor clearance (see Step 5).

Step 5: Adjust Sensor Clearance

21. Remove the speed sensor if not already removed.
22. Loosen the mounting bracket locking bolt and pivot bolt using a 6mm Allen head socket, then re-tighten them finger-tight.

23. Glue a 0.8mm thick washer to the bottom of the speed sensor (or use a dedicated clearance adjustment sensor with a permanently attached washer).

24. Install the sensor with the washer and tighten the retaining bolt.
25. Move the mounting bracket downward until it contacts the flywheel’s ring gear, then tighten the locking and pivot bolts.
26. Remove the sensor and, if reusing, carefully pry off the washer.
27. Reinstall the sensor and torque the retaining bolt to 8 Nm (6 ft-lbs).
28. Reconnect the battery negative lead.

Post Procedure Checklist

• Verify that all sensor connectors are securely reconnected.
• Confirm that the DME plug and fuel pump fuse are reinstalled.
• Start the engine and check for a steady idle and responsive tachometer.
• Ensure no warning lights appear on the dashboard related to engine management.
• Check for any unusual noises from the sensor area, indicating improper clearance or loose bolts.

Sources

• Clark’s Garage
• Pelican Parts Technical Articles
• Porsche 944 Workshop Manual