Description & Operation
The Camshaft Position (CMP) sensor provides cylinder identification to the PCM. The sensor generates pulses as groups of notches on the camshaft sprocket pass underneath it. The PCM keeps track of the crankshaft rotation and identifies each cylinder by the pulses generated by the notches on the camshaft sprocket. Four crankshaft pulses follow each group of camshaft pulses.
When the PCM receives 2 camshaft pulses followed by the long flat spot on the camshaft sprocket, it knows that the crankshaft timing marks for cylinder one are next (on the drive plate). When the PCM receives one camshaft pulse after the long flat spot on the sprocket, cylinder No. 2 crankshaft timing marks are next. After 3 camshaft pulses, the PCM knows cylinder 4 crankshaft timing marks follow. One camshaft pulse after the 3 pulses indicates cylinder No. 5. Two camshaft pulses after cylinder No. 5, signals cylinder No. 6. The PCM can synchronize on cylinders Nos. 1 or 4.
When metal aligns with the sensor, voltage goes low (less than 0.3 volts). When a notch aligns with the sensor, voltage spikes high (5.0 volts). As a group of notches pass under the sensor, the voltage switches from low to high then back to low again. The number of notches determines the amount of pulses. If available, an oscilloscope can display the square wave patterns of each timing event.
Top Dead Center (TDC) does not occur when notches on the camshaft sprocket pass below the sensor. TDC occurs after the camshaft pulse (or pulses) and after the 4 crankshaft pulses associated with the particular cylinder. The arrows and cylinder call in the illustration represent which cylinder the flat spot and notches identify, they do not indicate the TDC position.
The camshaft position sensor is mounted to the top of the timing case cover. The bottom of the sensor is positioned above the camshaft sprocket. The distance between the bottom of sensor and the camshaft sprocket is critical to the operation of the system.
Before testing any electrical component, inspect the wiring and connectors for damage. Also wiggle the connectors to ensure that they are firmly engaged. For this procedure a dwell meter, or the equivalent, will be needed.
- Unplug the CMP sensor connector.
- Turn the ignition ON.
- Using a voltmeter, measure the voltage from the wiring harness connector 8-volt supply circuit (orange wire) to ground.
- If the voltage is 8-9.5 volts, skip to the next step.
- If the voltage measured is lower than 8 volts, or greater than 9.5 volts, the CMP sensor is not receiving the correct current to function properly. There is a problem in the wiring or related components.
- Turn the ignition OFF.
- Attach the CMP sensor wiring and engine wiring harness connectors back together.
- Attach a dwell meter to the battery. Attach the lead probe of the dwell meter to the sensor signal wire (light blue wire with dark blue tracer) by back-probing the connector or by using jumper cables between the terminals.
- Place the dwell meter out of the way of any moving components of the engine, and in a position in which it can be seen once the engine is started.
- Turn the engine ON.
- Watch the dwell meter for one or two minutes while the engine is idling. The dwell time shown should be a steady 49-51 percent. If there is any fluctuation, or the dwell time is not 49-51 percent the CMP sensor is defective and must be replaced with a new one.