944 Speed and Reference Sensor Testing -- No Tach Bounce?
If there is no tach bounce while cranking, these tests are a quick way to determine whether the speed and reference sensor circuits may be the cause. Testing from the 35-pin DME connector in the cabin also checks the harness, which is where many real-world faults occur.
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Quick answer: Each sensor should measure roughly 600 to 1600 ohms at the DME connector, with about 1000 ohms being typical. Also confirm there is no connection (no continuity) between the sensor wires and chassis ground.
Assessing Results Normal resistance: If the speed sensor measures 600 to 1600 ohms between pins 8 and 27, and the reference sensor measures 600 to 1600 ohms between pins 25 and 26, the circuits are probably intact. This test will find the great majority of faults, which are often harness-related. Open or shorted circuit: If the meter shows an open circuit, no continuity, zero ohms, or very low resistance, a broken wire, bad connector, harness problem, or failed sensor is likely.
Wiggle the harness under the hood while watching the meter. Erratic changes usually point to frayed wires or poor connections. Tach bounce while cranking: If the tach bounces while cranking, the DME is likely receiving usable speed and reference signals. If there is no tach movement, the sensors may not be producing a usable signal, or the signal may not be reaching the DME.
Tach bounce is a useful quick check, but it is not a complete test. Use the resistance and wiring checks on this page to confirm circuit integrity. Important limitation: Even if resistance is in range and no obvious harness fault is found, the signals can still be bad if the sensors are improperly gapped or installed in the wrong positions in the bracket. This test checks circuit integrity, but does not fully prove signal quality while cranking.
Assessing Results Normal resistance: If the speed sensor measures 600 to 1600 ohms between pins 8 and 27, and the reference sensor measures 600 to 1600 ohms between pins 25 and 26, the circuits are probably intact. This test will find the great majority of faults, which are often harness-related. Open or shorted circuit: If the meter shows an open circuit, no continuity, zero ohms, or very low resistance, a broken wire, bad connector, harness problem, or failed sensor is likely.
Wiggle the harness under the hood while watching the meter. Erratic changes usually point to frayed wires or poor connections. Tach bounce while cranking: If the tach bounces while cranking, the DME is likely receiving usable speed and reference signals. If there is no tach movement, the sensors may not be producing a usable signal, or the signal may not be reaching the DME.
Tach bounce is a useful quick check, but it is not a complete test. Use the resistance and wiring checks on this page to confirm circuit integrity. Important limitation: Even if resistance is in range and no obvious harness fault is found, the signals can still be bad if the sensors are improperly gapped or installed in the wrong positions in the bracket. This test checks circuit integrity, but does not fully prove signal quality while cranking.