The crankshaft position sensor can be located near the harmonic balancer or the engine's flywheel. On some models, the ECM has to learn normal signal variations caused by differences in manufacturing tolerances of the sensor's reluctor wheel.

When either signal is lost, the ECM may shut down ignition, injection or both. Sensor failures are extremely common on certain makes. In the past, a failed sensor frequently meant a no-start or an engine that died "just like you turned off the key" going down the road. Today, many engines can limp home despite failure of a single sensor.

However, codes typically set and extended cranking may be required. The relationship between the two signals is as important as the signals themselves. Some DTCs are related to the loss of cam/crank signal correlation, or synch. This is often an indication of an underlying mechanical problem that causes cam and crank pulses, while still arriving, to arrive in an unexpected order.

QUICK CHECKUPS

Using a digital multimeter (DMM), a quick check of sensor resistance can be made against manufacturers' specs. Visual inspection can reveal a physically damaged or oil-saturated sensor. Using the frequency setting on a DMM, it may be possible to detect the presence of an AC signal output while cranking. But a dual-trace oscilloscope remains the preferred tool for verifying signal integrity, amplitude and CMP/CKP synchronization.

Sensors are constructed using a variety of technologies, including Variable Reluctance and Hall effect. Hall effect sensors typically produce a square wave, digital pattern, while Magnetic or Variable Reluctance sensors produce more of a sine wave pattern. Shielded wiring is frequently used to maximize signal quality. Electrical interference from failed alternator diodes, noisy secondary ignition leads and even bad engine/transmission grounding have been known to interfere with CMP and CKP signal integrity.

Other problems that can cause issues with CMP/CKP signals include accumulation of magnetized debris on the sensor tip, stretched timing belts and chains, cracked flexplates and wiring/connector problems. Excessive end play on crankshafts/camshafts can also cause variations in the signal.

This Chrysler flywheel is the reluctor for the CKP (see the windows around the circumference). A cracked reluctor will affect the stability of the sensor's signal to the ECM.