Crankshaft Position and Camshaft Position Sensor Operation
14-04 Describe crankshaft position and camshaft position sensor operation.
Since the control component of a waste spark system, ICM, igniter, or PCM controls more than one coil, there is a separate coil control circuit for each coil (cylinder pairs). Also, a sensor or sensors are needed to identify which cylinder to fire since there is no distributor. Most vehicles use two sensors: a CKP sensor and a CMP sensor. On most systems, the CKP sensor controls the synchronization of the ignition system, and the CMP sensor, which turns at one-half the crankshaft speed, controls fuel injector timing. On some systems, the CMP sensor is also responsible for coil control and ignition timing.
CKP and CMP sensors are used in DISs to calculate engine speed, determine engine position, and control the ignition and fuel systems. For the system to operate correctly, these two signals must be synchronized (also called being in sync). On most systems, the CKP sensor controls the synchronization of the ignition system, and the CMP sensor commands the timing of the fuel injectors.
The CMP sensor may also be referred to as the cylinder identification (CID) sensor. The CKP sensor can be mounted at the front, in the middle, or at the back of the engine and functions like a pulse generator in a distributor. The reluctor or tone ring can be found at the front of the engine, directly behind the crankshaft pulley and harmonic balancer; in the middle of the engine, bolted to or part of the crankshaft; or at back of the engine, as part of the flywheel or flexplate. Similarly, the CMP sensor and tone ring can attach at various locations on the engine, including many of the same places as the CKP. Additionally, some CMP sensors mount in a modified distributor housing, minus the distributor cap and rotor.
By monitoring the number of slots or teeth that pass the CKP sensor over a set time, the PCM can determine engine rpm. Besides the obvious use of engine rpm for tachometer control, the PCM uses rpm for engine load calculations, rev limiter control, fuel pump control, torque converter application, shift strategy, and monitoring transmission slippage. Engine load and rpm are crucial inputs for fuel control strategy on either mass airflow (MAF) or speed density/manifold absolute pressure (MAP) systems. Many PCMs disable fuel if the engine overspeeds, exceeding preset limits, or if engine rpm drops to zero, such as during a vehicle accident or stall concern. The PCM uses CKP to control the timing of the fuel injectors and ignition coils, including dwell.
The PCM uses the CMP sensor signal to synchronize the crankshaft and camshafts, determining which cylinder is on which stroke. The CMP sensor monitors the position of the camshaft to let the PCM know which cylinder is approaching its power stroke. The two sensors work together to control the firing of the spark plugs, including advance and retard functions. Both sensors are also crucial for synchronizing fuel injector actuation on sequential fuel injection (SEFI) engines. Plus, CKP and CMP sensors provide more accurate readings than contact breaker points since there are fewer mechanical parts to wear, which affects accuracy.
As an example, in one system, the identification of each pair of cylinders and a signal for their triggering is provided by a dual crank sensor affixed to the engine timing cover. This contains two Hall-effect switches sharing a central magnet that forms two air gaps between them. Two concentric interrupter rings, which look like one shallow cup placed inside a larger shallow cup with a number of blades and windows on their outer edges, are mounted on the rear of the crankshaft balancer and rotate through the air gaps (FIGURE 14-37). The voltage signals provided allow the ICM to identify which pair of companion cylinders must be provided with ignition.
FIGURE 14-37 The interrupter rings on a six-cylinder engine’s crankshaft pulley allow the CKP sensor to create a waveform sourced from the rotation of the crankshaft.
When scope testing CKP and CMP sensors on a waste spark system, the pattern may be different from that of a COP system. Since the module fires cylinders in pairs, the number of pulses from the CMP sensor may range from a single pulse that identifies Cylinder 1 to half the number of oscillations as there are cylinders (one for each paired group) and to an individual pulse signal for each cylinder. Other manufacturers may use unique patterns of notches, including multiple groups of slots. Increasing the speed and accuracy that the PCM/module can determine engine rpm and cylinder position (FIGURE 14-38). Manufactures may choose these unique patterns to provide starting capabilities even if either the CMP sensor or the CKP sensor fails. In the event of a sensor failure on these systems, the customer may notice extended cranking times before the engine starts. Other engines may fail to start if either sensor fails. Depending on the manufacturer, the output signal from these sensors can be in either analog (sine wave) or digital form (square wave).
FIGURE 14-38 Example from a Chrysler engine using a unique or signature CKP and CMP signal to determine engine rpm as well as CKP and CMP. Manufacturers may choose to use several slots in groupings to determine cylinder location.
CKP and CMP sensors are typically located around the front of the engine, near the belts and other moving parts. Always remove the key from the ignition switch before working around the moving parts of the engine.
To improve control over ignition timing, fuel injector timing, emissions, and misfire detection and to improve vehicle drivability, OEMs are often now increasing the number of slots or teeth in the tone ring/reluctor. Each slot or tooth on the tone ring supplies the PCM with the engine’s position. This increases the accuracy of the sensor in reporting the CKP and CMP and engine speed to the PCM or module, allowing for more precise control over engine subsystems.