Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Engine Speed (Rpm) Sensor
The engine speed (RPM) sensor provides the engine control module (ECM) with information about the speed and position of the crankshaft. The engine control module (ECM) is able to use the signal from the engine speed (RPM) sensor to determine when a piston is approaching top dead center (TDC).
The signal from the engine speed (RPM) sensor is also used to check the engine for misfires. For further information, see: MISFIRE DIAGNOSTIC
The engine speed (RPM) sensor is at the rear of the engine above the flywheel.
The sensor is inductive with a permanent magnet. An alternating current is induced in the sensor when the flywheel/carrier plate passes the engine speed (RPM) sensor. The generated voltage and frequency increases with the engine speed (rpm).
The signal varies between 0.1-100 V depending on the engine speed (RPM).
The Engine Control Module (ECM) is able to determine the engine speed (RPM) by counting the number of holes per time unit. When the reference position passes the engine speed (RPM) sensor, the voltage and frequency drop momentarily to zero, even though the engine is still running. This allows the engine control module (ECM) to determine the position of the crankshaft.
If the signal from the engine speed (RPM) sensor is incorrect or missing, the control module will use signals from the camshaft position (CMP) sensor.
The engine speed (RPM) sensor can be diagnosed by the engine control module (ECM) and the sensor signal (engine speed (RPM)) can be read off.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.
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