Speed and Reference Sensors – Checking, Replacement, and Adjustment

Overview

This guide explains how to check, replace, and adjust the speed and reference sensors on your Porsche 944 S / S2 with its 16v engine. These sensors are critical for the Engine Management Computer (DME) to determine engine position and speed. The reference sensor identifies the #1 piston’s position relative to Top Dead Center (TDC) during the compression stroke, while the speed sensor counts flywheel teeth to calculate engine RPM. Together, they ensure proper fuel injection and ignition timing. The speed sensor also acts as a safety feature, shutting off the fuel pump if engine speed drops below 300 RPM after cranking. Faulty sensors can cause starting or running issues, so this procedure helps you diagnose and fix them.

Pink and blue text represents links to additional resources, tools, parts, and partner websites.

Tools Required

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

Parts Required

• Speed sensor (if replacement needed) – 944 606 115 00
• Reference sensor (if replacement needed) – 944 606 114 00

Step-by-Step Procedure

Step 1: Check Sensor Resistance

1. Locate the sensor plug connector at the back of the engine compartment, near the firewall on the driver’s side (left-hand drive).
2. Disconnect the sensor plug connector.
3. Use a multi-meter to measure resistance across the following terminals:
4. For the speed sensor: Between terminals 8 and 27, expect 600–1600 ohms. Between terminals 8 and 23, expect over 1 M-ohm.
5. For the reference sensor: Between terminals 25 and 26, expect 600–1600 ohms. Between terminals 25 and 78, expect over 1 M-ohm.
6. If readings are outside these ranges, the sensor may be faulty and should be replaced.

Step 2: Perform Sensor Operational Checks

7. Remove the fuel pump fuse to prevent the engine from starting during testing.
8. Disconnect the DME computer plug, located in the engine compartment.
9. For the speed sensor: Connect an oscilloscope between terminals 8 and 27 on the DME plug. Crank the engine and look for a sawtooth wave with a peak-to-peak voltage of at least 2.5V.
10. For the reference sensor: Connect an oscilloscope between terminals 25 and 26 on the DME plug. Crank the engine and look for periodic pulses with a peak-to-peak voltage of at least 2.0V (pulse frequency depends on cranking speed).
11. If you don’t have an oscilloscope, use a multi-meter to check for any output signal during cranking. Note that this won’t confirm voltage magnitude, only signal presence.
12. Alternatively, crank the engine and watch the tachometer. If it jumps, the sensors are likely working. If not, proceed to check sensor clearance or replace them.
13. If voltages are below specs, adjust the sensor clearance (see Step 4).

Step 3: Remove the Sensors

14. Disconnect the negative battery lead to ensure safety.
15. Locate the speed and reference sensors on top of the clutch housing at the back of the engine, near the firewall.
16. Disconnect the sensor connectors at the back of the intake manifold.
17. Remove the sensor retaining bolt using a 10mm (3/8 inch) socket or 5mm Allen head socket, depending on the bolt type, and a ratchet.
18. Twist the sensor side-to-side while pulling upward to remove it. If it’s stuck, gentle twisting helps free it.

Step 4: Install New Sensors

19. If replacing both sensors, check the wiring harness labels near the connector bracket: “DG” for speed sensor, “BG” for reference sensor.
20. Check the mounting bracket labels: “D” for speed sensor (closest to the firewall), “B” for reference sensor. If labels are unreadable, the speed sensor is the one nearer the firewall.
21. Install the new speed sensor (944 606 115 00) or reference sensor (944 606 114 00) into the correct position.
22. Secure each sensor with its retaining bolt and torque to 8 Nm (6 ft-lbs) using a torque wrench.

Step 5: Adjust Sensor Clearance (If Needed)

23. Only adjust clearance if the mounting bracket was moved or loosened, or if operational checks show low voltage.
24. Remove the speed sensor.
25. Loosen the mounting bracket’s locking bolt and pivot bolt with a 6mm Allen head socket, then retighten them finger-tight.
26. Glue a 0.8mm thick washer to the bottom of an old speed sensor (or the one being adjusted) to use as a spacer.
27. Install this sensor with the washer and tighten the retaining bolt.
28. Push the bracket downward until it stops against the flywheel’s ring gear.
29. Tighten the mounting bracket’s locking bolt and pivot bolt securely.
30. Remove the sensor with the washer. If reusing the sensor, pry off the washer.
31. Install the final sensor and torque the retaining bolt to 8 Nm (6 ft-lbs).
32. Reconnect the battery negative lead.

Post Procedure Checklist

• Reconnect all sensor connectors and the DME plug.
• Replace the fuel pump fuse.
• Crank the engine and check if the tachometer jumps, indicating proper sensor operation.
• Start the engine and listen for smooth running, confirming the DME is receiving correct signals.
• Inspect the sensor area for any loose bolts or connections.

Sources

• Clark’s Garage
• Pelican Parts
• Porsche 944 Workshop Manual, Volume 1
• Youtube