Crankshaft Position Sensor: Description and Operation
Information on the position and speed of the crankshaft is supplied to the engine control module via pin 78. The sensor is grounded via pin 20 of the control module.
Mounted on the crankshaft is a slotted ring with 58 ribs. Of inductive type, the sensor is mounted on the wall of the crankcase. The distance between the slotted ring and the sensor is 1.0±0.7 mm. This distance is not adjustable.
The sensor acts as a small alternator and generates sine-wave a.c. By measuring the frequency of this current, the control module can determine engine rpm.
Two ribs are missing after the 58th rib of the slotted ring. When rib 1 then passes the sensor the control module knows that the crankshaft is 84° before top dead centre (BTDC) for cylinder No. 1.
The voltage and frequency signals from the crankshaft position sensor vary with engine speed. At idling speed the sensor input is 5-10 V AC, 725 Hz, and at 2500 rpm it is 15-20 V AC, 2400 Hz. The frequency is of interest to the control module since it is directly proportional to engine speed.
The control module mainly uses engine speed and the crankshaft position as a basis for calculating ignition timing, injection time, injection duration and for idle speed control.
Fuel injection is cut off when engine speed exceeds 6600 rpm.
As soon as the control module receives signals from the crankshaft position sensor it grounds the fuel pump relay, causing it to operate.
Should the sensor fail to work or in the event of a break in the circuit (no continuity), the engine will not start.
The resistance of the sensor is 860 ± 90 ohm.
Engine Speed Signal
The control module sends out an engine speed signal on pin 80.
This signal is a pulse train which varies between 0 V and battery positive voltage (B+),with a frequency of about 37 Hz at idling speed and about 125 Hz at 2500 rpm. This corresponds to 3 pulses per crankshaft revolution.
The engine speed signal is used by the main instrument display panel and the TCS control module.