S70 L5-2.4L VIN 61 B5244S (2000)
Engine Speed Sensor: Description and Operation
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). However it is unable to use the signal from the engine speed (RPM) sensor to determine whether the piston is in the combustion stroke
or whether the exhaust valve is open (exhaust stroke). The signal from the camshaft position (CMP) sensor is also required to determine the operating
cycle of the engine. The signal from the engine speed (RPM) sensor is also used to check the engine for misfires (misfire diagnostics).
Cars with manual transmissions have a series of holes drilled in the periphery of the flywheel. Cars with automatic transmissions have a steel ring with
punched holes. This steel ring is welded to the edge of the carrier plate. In both cases, there is ~ between each hole. This arrangement creates a hole
for each tooth. There are 3600 in one revolution. 60 between each hole means that there are 60 holes. However one hole is not drilled/punched, to
create a reference position (tooth) for the crankshaft. This reference position is 720 before the top dead center (TDC) of cylinder I on a 5 cylinder
engine.
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 (AC) 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
tooth 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 the signals from the camshaft position (CMP)
sensor, on the condition that the position of the camshaft has been adapted and the car can be driven if there is no signal.
The engine control module (ECM) can diagnose the engine speed (RPM) sensor. The sensor value (engine speed (rpm)) can be read off using VADIS.
