Automotive Engine Performance

Speed Sensors

18-04 Describe how speed sensors generate a signal for the powertrain control module.

A speed sensor is used to monitor the speed and or position of a spinning shaft in the engine or the transmission (FIGURE 18-31). This type of sensor is usually either Hall effect or permanent magnet (PM) because of the nature of the measurement. The PCM uses these speed sensors to adjust the fuel, ignition, and shift settings of the engine and transmission based on how fast the vehicle is operating. The engine needs different things at different rpm, and the transmission needs to know how fast the vehicle is going so that the PCM knows when to shift it. All of these sensors work in conjunction with the PCM to efficiently operate the vehicle.

FIGURE 18-31 The speed sensors on the engine and transmission feed into the PCM to effectively manage the operation of the vehicle drivetrain.

Crankshaft Position Sensor

The crankshaft position (CKP) sensor is used by the PCM to indicate where the crankshaft is in its rotation (FIGURE 18-32). This sensor helps the PCM correlate the ignition and fuel events with the proper stroke in the combustion process. There are two main types of CKP sensors: PM and Hall effect. Like other sensors, the PM sensor uses a magnet and a reluctor wheel as a part of either the flywheel or the crankshaft to generate a voltage that can be used by the PCM to locate where the pistons are in the engine. These types of systems are very reliable because they have few moving parts (either the flywheel or the crankshaft) that can fail. The sensor itself is sealed within plastic, so oil and other contaminants have no access to the electrical components. The Hall-effect sensor is usually mounted on the front of the engine and takes its pick off a windowed wheel attached to the crankshaft. This type of sensor must have a reference voltage supplied to it from the PCM to generate a signal. If the windowed wheel gets damaged or the reference voltage is reduced because of a wiring issue, the sensor can give an erratic reading that will cause the PCM to think the pistons are in a different location. These types of sensors are very precise when they produce a square wave signal, but if something is slightly off, the sensor will output a signal that may not be readable.

FIGURE 18-32 The CKP sensor uses a pickup component attached to the crankshaft so that it is getting a true reading of the location of the pistons in the engine. The PCM needs to know which part of the four-stroke cycle the crankshaft is on to control when spark and fuel injection is supposed to happen.

Camshaft Position Sensor

Locating the camshaft position within the combustion events helps with providing the necessary information to the PCM to time the fuel injector’s on time. To do this, a camshaft position (CMP) sensor must be employed (FIGURE 18-33). These types of sensors are similar to the CKP sensors in that they are either PM or Hall effect. A reluctor that is built into the camshaft is used to make a PM sensor excite to create a voltage that the PCM can use to calculate CMP. The Hall-effect sensor uses a windowed type of pickup that pulls the reference voltage to ground, creating a square wave signal (FIGURE 18-34). In an engine management system, the position of one camshaft is monitored, and then it is assumed that if there are other camshafts in the engine connected by a timing chain or belt, they are also in the same position. If when in a V or multiple-camshaft situation one camshaft jumps and the other doesn’t, the CMP sensor may not pick up the non-monitored camshaft location. This could cause a running issue without check engine light illumination. Visual verification of the timing positions on the timing gears is sometimes required to diagnose a timing issue.

FIGURE 18-33 The CMP sensor is used to obtain a camshaft location in its revolutions. This reading assumes that the timing components are operating correctly and that if more than one camshaft is installed in the engine, it is timed correctly.

FIGURE 18-34 Using an oscilloscope to verify the cam/crank correlation will allow the technician to assess the performance of the sensors and whether the timing components are in specification. This is just an investigative tool to direct the technician to a more thorough exploratory disassembly.

Vehicle Speed Sensor

Vehicle speed sensors (VSSs) are used to create a signal that can be interpreted as a speed reference. VSSs are usually one of two different types: PM or Hall effect. The PM sensor has a magnet integrated into it, and there is a reluctor located on a tail shaft or in the rear differential that it generates a signal voltage off of (FIGURE 18-35). As the reluctor spins, it generates a voltage in the PM sensor that is sent to the PCM and the IPC. The VSS can also be located in the rear end housing. This type of application is usually used on a rear-wheel-drive (RWD) vehicles, and in RWD vehicles, the reluctor is located on the differential. The Hall-effect speed sensor uses a reference voltage that is applied to the sensor so that it can generate a square wave. As the speed increases, the square wave gets closer together to indicate vehicle speed. The PCM needs to know this so that it can adjust the timing and fuel mixture to the specific application.