Crankshaft Position Sensor: Operation

The Crankshaft Position (CKP) sensor is a Hall-effect sensor. The PCM sends approximately 8 volts to the Hall-effect sensor for the SBEC vehicles and 5 volts for the NGC vehicles. This voltage is required to operate the Hall-effect chip and the electronics inside the sensor. A ground for the sensor is provided through the sensor return circuit of the PCM. The input to the PCM occurs on a 5 volt output reference circuit.

The notches generate pulses from high to low in the crankshaft position sensor output voltage. See Fig 1. When a metal portion of the counterweight aligns with the crankshaft position sensor, the sensor output voltage goes low (less than 0.5 volts). When a notch aligns with the sensor, voltage goes high (5.0 volts). As a group of notches pass under the sensor, the output voltage switches from low (metal) to high (notch) then back to low.

If available, an oscilloscope can display the square wave patterns of each voltage pulses. 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.

For SBEC vehicles, each group of timing reference notches, the first notch represents 69 degrees before top dead center (BTDC). The second notch represents 49 degrees BTDC. The third notch represents 29 degrees. The last notch in each set represents 9 degrees before top dead center (TDC). There is also a notch at 11 degrees before top dead center (TDC).

Fig 1: Timing Reference Notches (NGC)

Courtesy of DAIMLERCHRYSLER CORP.

The crankshaft position sensor detects slots cut into the transmission drive plate extension. See Fig 2. There are 3 sets of slots. Two sets contains 4 slots, and 1 set contains 5 slots for a total of 13 slots. See Fig 3. Basic timing is set by the position of the last slot in each group. Once the Powertrain Control Module (PCM) senses the last slot, it determines crankshaft position (which piston will next be at TDC) from the camshaft position sensor input. The 4 pulses generated by the crankshaft position sensor represent the 69°, 49°, 29°, and 9° BTDC marks. It may take the PCM one engine revolution to determine crankshaft position.

Fig 2: Crankshaft Position Sensor - 2.7L

Courtesy of DAIMLERCHRYSLER CORP.

Fig 3: Timing Slots - 2.7L

Courtesy of DAIMLERCHRYSLER CORP.

The PCM uses crankshaft position reference to determine injector sequence, ignition timing and the presence of misfire. Once the PCM determines crankshaft position, it begins energizing the injectors in sequence.