Making Sense of it All

In today’s modern engines, sensors play a critical role in maintaining peak performance and engine efficiency.

Engine sensors play a critical role in today’s advanced automotive engines. The information provided by these sensors enable the Powertrain Control Module (PCM) to accurately set the parameters of operation. While you might be somewhat familiar with many of these sensors, Manifold Absolute Pressure (MAP) sensor, camshaft position sensor, crankshaft position sensor, throttle position sensor, etc), let’s take a closer look at how these sensors work and the data each sensor supplies to the PCM.

Camshaft Position Sensor

The camshaft position sensor is an important sensor because it provides cylinder position information to the powertrain control module. The PCM uses this information to keep track of crankshaft rotation and to identify each cylinder.

This sensor generates pulses as groups of notches on the camshaft sprocket (see Figure 1) pass underneath it. When metal on the sprocket aligns with the sensor, the voltage switches low, or decreases (typically, less than 0.3 volts). When a notch aligns with the sensor, the voltage switches high, or increases (about 5.0 volts). As the sprocket rotates and a group of notches pass under the sensor, the voltage switch from low (metal, or no notch) to high (notch), then back to low (metal). The number of notches determines the number of pulses.

For a six-cylinder engine, when the PCM receives 2 cam pulses (2 notches on the sprocket for #6) followed by the long flat spot on the camshaft sprocket, it knows that the crankshaft timing marks for cylinder number 1 are next. Furthermore, when the PCM receives one camshaft pulse (1 notch for #2) after the long flat spot on the sprocket, the crankshaft timing marks for cylinder number 2 are next. After 3 camshaft pulses (3 notches for #4), the PCM knows cylinder number 4 crankshaft timing marks are next. One camshaft pulse (1 notch for #5) after the 3 pulses indicate cylinder number 5 and 2 camshaft pulses (2 notches for #6) after cylinder 5 indicates cylinder number 6.

Top Dead Center (TDC) occurs after the camshaft pulse (or pulses) and after the 4 crankshaft pulses associated with the particular cylinder. The notches on the camshaft sprocket in Figure 1 do not indicate TDC position.

Crankshaft Position Sensor

In order to determine crankshaft position (in other words, which piston will be next at TDC) from the input received from the camshaft position sensor, the PCM must sense the last slot in a group of 4 slots on the torque converter drive plate extension. This is done by the crankshaft position sensor.

There are 3 sets of 4 slots (12 slots total) cut into the torque converter drive plate extension. As the drive plate rotates, the crankshaft position sensor generates a pulse for each slot (the 4 slots represent 69°, 49°, 29° and 9° BTDC marks). Basic timing is set by the position of the last slot in each group.

The PCM uses crankshaft position information to determine injector sequence, ignition timing and the presence of a misfire. Once the PCM determines crankshaft position, it begins to energize the injectors in sequence.

Throttle Position Sensor

Another sensor that provides information to the PCM that is used to determine engine operating conditions is the Throttle Position Sensor (TPS). The TPS is a variable resistor that is connected to the throttle blade shaft (see Figure 2). As the position of the throttle blade changes, the resistance of the TPS changes, as does the input signal (voltage) that is sent to the PCM.

The PCM supplies about 5 volts to the TPS. The TPS output voltage (the input signal to the PCM) represents the position of the throttle blade. The TPS output voltage varies from 0.6 volt at idle to a maximum of 4.5 volts at wide open throttle (WOT).

Along with inputs from other sensors, the PCM uses the TPS input to determine the required engine operating conditions. The PCM, based on these inputs, will also adjust fuel injector pulse width and ignition timing.

MAP Sensor

The MAP Sensor, mounted to the intake manifold and utilizing a silicon-based sensing unit (see Figure 3), provides data to the PCM on the manifold vacuum that draws the air/fuel mixture into the combustion chamber. This information is needed in order to determine the correct injector pulse width and spark advance. When the manifold pressure equals the barometric pressure, the pulse width is at a maximum.

The most important function of the MAP sensor is to determine barometric pressure. Barometric pressure and altitude have a direct inverse correlation; as altitude increases, barometric pressure decreases. And the PCM needs to know if the vehicle is at sea level or up in the mountains at 5,000 feet above sea level.

The first thing that occurs when the ignition key is turned to the ON position is a check of the MAP voltage. Based on this voltage, the PCM knows the current barometric pressure relative to altitude. Once the engine starts, the PCM looks at voltage continuously every 12 milliseconds and compares the current voltage to what the voltage was when the key was ON. The difference between current voltage and the value at key ON is the manifold vacuum.

Like the cam and crank position sensors, a 5-volt reference voltage is supplied to the MAP sensor by the PCM. The MAP sensor is a linear sensor; in other words, as pressure changes, voltage changes proportionately. A return voltage signal reflects intake manifold pressure. The MAP sensor input is the most important input used to determine injector pulse width.

O2 (Oxygen) Sensor

The O2 sensor is used to monitor oxygen content in the exhaust stream to determine if the air/fuel mixture is too rich or too lean. In other words, this sensor tells the PCM the oxygen content of the exhaust. In many vehicles, two O2 sensors are used. One is located upstream, or before the catalytic converter (see Figure 4) and the other is located downstream, or after the converter. The upstream sensor is the one that determines oxygen content.

O2 sensors produce voltages ranging from 0 to 1 volt, depending on the oxygen content of the exhaust gas. When a large amount of oxygen is present, the sensor produces a low voltage. When the oxygen level is much lower, the output voltage is higher. By monitoring the oxygen content and converting it to electrical voltage, the sensors act a rich-lean switch. As a result, the PCM fine tunes the air/fuel ratio by adjusting the injector pulse width.

The downstream O2 sensor is used to detect catalytic converter deterioration. As the converter deteriorates, the input from the downstream O2 sensor begins to match that of the upstream O2 sensor (except for a slight time delay). By comparing the downstream O2 sensor input to the input from the upstream O2 sensor, the PCM calculates the catalytic converter efficiency.

Knock Sensor

The knock sensor detects engine vibration that is caused by detonation. When this sensor detects a knock in one of the cylinders, it sends an input signal to the PCM. In response, the PCM retards the ignition timing for all the cylinders by a specified amount.

Knock sensors contain a piezoelectric material which constantly vibrates, sending an input voltage signal to the PCM while the engine is operating. As the intensity of the vibration increases, the output voltage from the sensor will also increase. Detonation increases the intensity of the vibration to the point where the output voltage is above a pre-determined level. At this point, ignition timing is retarded.