Sensors on Engine Control Systems
Sensor devices are a critical and integral part of the engine control system. Before starting a detailed discussion of all the variety of sensors and diagnostics techniques for them, we need to introduce several fundamental concepts.
WHAT IS A SENSOR, WHY IS IT REQUIRED, WHAT FUNCTION DOES IT PERFORM?
The main element of the engine control system is an electronic control unit (ECU). It is only capable of perceiving information in the form of electrical signals characterized by one or another value of voltage, frequency, duty cycle, etc. But engine operation parameters are purely physical characteristics. In order to make them known to the control unit, a physical magnitude must be transformed into an electric magnitude suitable for processing in the control unit according to the embedded program. Thus,
The sensor is an element of the engine control system that serves to transform the physical magnitudes that characterize engine operation into electric magnitudes suitable for processing by the electronic control unit.
Let us list the physical magnitudes and phenomena about which the control unit requires information:
- air amount
- spatial position
All of the information above is converted by sensors into the following electrical parameters:
HOW TO DIAGNOSE EQUIPMENT’S SENSORS
Diagnostics of any electronic engine control system sensor comes down to checking the validity of the conversion of physical parameters to electric parameters.
A pre-known value of the parameter should be set at the sensor input, while the sensor output signal must be monitored using a motor tester or a scanner.
A simple example: and absolute pressure sensor in the intake manifold. As a reference, you can use atmospheric pressure present in the intake manifold of a shut-off engine. Scanner-based monitoring of the pressure displayed by the sensor in this condition enables you to make a conclusion about the reliability of its readings.
The above example is quite primitive and intended only to demonstrate the general principle of performing diagnostics on equipment’s sensors. The Petrol Engine Diagnostics: Three Steps to Success training video describes in great detail techniques to test each sensor type.
Let’s assume there is a sensor connected to the ECU, and you need to assess its performance (please see the figure). Let us study a classical sensor-to-unit connection diagram.
The 5V supply voltage and the ground are fed from the control unit into the sensor. A signal from the sensor goes to and is processed by the unit.
The two main diagnostic instruments are applied to check the sensor’s performance: a scanner and a motor tester.
By connecting the scanner, the diagnostician gets an opportunity to “see” the sensor’s signal “through the eyes” of the control unit. To assess the sensor’s output signal using the motor tester, its probes should be connected to the sensor circuit, as shown in the figure: one to the ground and the other to the signal wire.
Working with the scanner is easier and more convenient, but remember that ECU-and-scanner information exchange is not at all instantaneous and some interesting aspects of the signal can simply remain undetected. Additionally, the scanner cannot be used on very old cars, approximately those made before the mid-1990s, due to the low intelligence and response levels of control units from that time.
Conversely, a motor tester allows the sensor signal to be estimated well and in detail, without missing even the smallest nuances, though using it takes more work than the scanner. Please note that it is proper to connect the motor tester’s probes directly to the sensor connector. This is especially true for the ground probe: it should not be connected to the first place found on the engine.
A sensor converts a physical parameter into an electric parameter that can be processed in the ECU. Physical parameters include temperature, pressure, concentration, spatial position, air amount, and vibration. The electric parameters the sensor handles are voltage, current and frequency. The sensor can be tested using two instruments: the scanner, by connecting it to the ECU, and the motor tester, by connecting its probes directly to the sensor’s signal and ground outputs.
This type of sensor is most uncomplicated in terms of understanding how it works and how to perform diagnostics.
What is potentiometer?
The idea is encoded in the name: this is a meter of electric potential. A potentiometer is represented as follows on electrical schematics: a standard resistor symbol, but with an arrow that represents a moving contact.
If we supply a voltage to the potentiometer’s upper output, let’s say, 12 V, and connect the lower one to the ground, the voltage between the ground and the signal output while moving the potentiometer’s slider will vary between zero and 12 V. This is the ideal, but the actual voltage will not reach zero and 12 V. In terms of design, the sensor is an arch- or horseshoe-shaped resistance track, along which the slider moves. One end of the resistance track must be connected to the ground, while the supply voltage is fed to the other. The output signal should be read from the slider.
This sort of potentiometer was used a long time ago on electronic radios to control volume: an audio-frequency voltage was supplied to it, taken from the slider, and then sent to an amplifier. As a result, you could set the desired audio volume by turning a control knob.
Where in the car can be this sensor be applied?
It is quite evident that it can be used where the spatial position of some assembly should be measured. It doesn’t matter which one: a mobile assembly is mobile, an assembly that moves and takes different positions. If we need to determine the position, potentiometer-type sensors are used for this almost everywhere.
A classic example of the position sensor is a fuel level indicator in the tank. This is an arm-equipped float installed on a joint and able to move in one plane. The arm is connected to a potentiometer sensor’s slider. The voltage from the slider is supplied to the dashboard and deflects the indicator arrow. Note that such an operating scheme for the fuel level indicator is already outdated and not used in most modern cars that have an electronic dashboard.
Where on the engine can sensors of this type be used? Let’s list the major fields of application:
- Throttle position sensor (TPS)
- Accelerator pedal position sensor (APPS)
- Exhaust gas recirculation valve position sensor
- Vane-type volume airflow sensor
- Intake manifold damper position sensor
This list is by no means comprehensive. In a word, potentiometer-type sensors are applied everywhere we need information on the spatial position of an assembly.
We will consider diagnostic methods for such sensors based on a specific example of a throttle position sensor. It is installed on the throttle assembly and converts the current throttle position into voltage. A voltage of 5V is supplied from the ECU to the sensor, but the sensor is designed in such a way that the voltage on it would never be equal to 0 or 5 V. This was done to enable the ECU to control the sensor circuit and to distinguish the zero position and signal circuit fault to ground or, the opposite, the position of maximum throttle opening and a short-circuit to the 5V supply voltage. Therefore, the actual voltage on the sensor varies between 0.3...0.5 V and 4.5...4.7 V rather than between 0 and 5 V.
The performance of a sensor can be checked in two ways:
- With a scanner. To perform the check, connect the scanner, enter Data Stream mode, and find the voltage on the sensor in the list. Then, monitor the numeric voltage value, slowly turning the throttle from the closed state to the fully open one. It should increase smoothly, with no drops down to zero or surges up to the maximum. Alternatively, you can estimate the throttle position calculated by the unit in percentages rather than voltage. Once again, the percentage should rise smoothly, without any chaotic appearances of 0% and 100%. It should be noted that, because of the finite exchange rate between the ECU and the scanner, with this method it is possible to skip over a defective spot on the sensor’s resistance track.
- With a motor tester. Measurement is carried out in recording mode. The motor tester’s probes should be connected to the ground and the sensor signal output. Switch on the ignition. Smoothly moving the throttle, watch the oscillogram. Checking with a motor tester is most reliable and allows you to detect the smallest problems in the resistive layer; it should be preferred for full-scale sensor diagnostics.
Let us review several oscillogram examples of both fault-free and faulty potentiometer-type sensors.
It is impossible to discuss diagnostics of all sensor types within one article. All the fine points and nuances in diagnostics of thermoanemometric, thermal resistive, piezoelectric and other sensor types are dealt with in detail in the Petrol Engines Diagnositcs training video.