Crankshaft and Camshaft Position Sensors

These two sensors work together to tell the PCM exactly where every piston and valve is at every instant. Without this information, the PCM cannot fire the spark plugs at the right time or inject fuel into the right cylinder at the right moment. These sensors are the heartbeat of the engine management system.

The crank sensor reads a toothed reluctor ring on the crankshaft or harmonic balancer. As each tooth passes the sensor, it generates a signal pulse. The PCM counts the pulses to determine crankshaft speed and position. A missing tooth or specially shaped tooth on the ring tells the PCM where top dead center is for cylinder number one. Without a crank sensor signal — no spark, no fuel injection, no start. The engine cranks normally but will not fire at all.

The cam sensor reads a reluctor ring or target on the camshaft. It tells the PCM which stroke each cylinder is on — because the crankshaft rotates twice per four-stroke cycle, the PCM needs the cam signal to distinguish between the compression stroke and the exhaust stroke on each cylinder. Without the cam signal, some engines default to batch fire mode — firing all injectors and coils in a less efficient pattern. Others will not start at all without the cam signal.

Both crank and cam sensors are notorious for heat-related intermittent failure. The sensor works fine when cold. The engine heats up. The sensor fails — engine dies. The engine sits for an hour, cools down, the sensor recovers, and the engine starts again. The customer says the car randomly stalls and then starts fine later. If you cannot duplicate the fault, monitor the sensor signal with a scope while heating the sensor area with a heat gun to simulate operating temperature. The signal drops out — sensor confirmed bad.