Neon L4-2.0L SOHC (1995)
Crankshaft Position Sensor: Description and Operation
Crankshaft Position Sensor-PCM Input
Fig. 8 Timing Reference Notches
PURPOSE
The PCM determines what cylinder to fire from the crankshaft position sensor input and the camshaft position sensor input.
OPERATION
The second crankshaft counterweight has two sets of four timing reference notches including a 60 degree signature notch (Fig. 8).
From the crankshaft position sensor input the PCM determines engine speed and crankshaft angle (position).
The notches generate pulses from high to low in the crankshaft position sensor output voltage.
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When a metal portion of the counterweight aligns with the crankshaft position sensor, the sensor output voltage goes low (less than 0.5 volts).
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When a notch aligns with the sensor, voltage goes high (5.0 volts).
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As a group of notches pass under the sensor, the output voltage switches from low (metal) to high (notch) then back to low.
From the width of the output voltage pulses, the PCM calculates engine speed. The width of the pulses represent the amount of time the output
voltage stays high before switching back to low. The period of time the sensor output voltage stays high before switching back to low is referred to
as pulse width. The faster the engine is operating, the smaller the pulse width on the oscilloscope.
By counting the pulses and referencing the pulse from the 60 degree signature notch, the PCM calculates crankshaft angle (position). In each group
of timing reference notches:
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The first notch represents 69 degrees Before Top Dead Center (BTDC).
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The second notch represents 49 degrees BTDC.
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The third notch represents 29 degrees.
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The last notch in each set represents 9 degrees before top dead center (TDC).
The timing reference notches are machined at 200 increments. From the voltage pulse width the PCM tells the difference between the timing
reference notches and the 60 degree signature notch. The 60 degree signature notch produces a longer pulse width than the smaller timing
reference notches. If the camshaft position sensor input switches from high to low when the 60 degree signature notch passes under the crankshaft
position sensor, the PCM knows cylinder number one is the next cylinder at TDC.
If available, an oscilloscope can display the square wave patterns of each voltage pulse.
