- MAP sensor
- Features of the MPX4250AP
- Signal voltage of a naturally aspirated engine
- charging pressure sensor
- Combination with temperature sensor
- Diagnosing the boost pressure sensor
An engine's intake manifold may be equipped with a Manifold Air Pressure sensor, abbreviated as MAP sensor. This pressure sensor measures the absolute pressure in the intake manifold. The sensor can be mounted on the intake manifold, or connected externally by means of a hose. The under or overpressure is converted by the sensor into a signal voltage that is created from the supply voltage. This makes the MAP sensor an active sensor. The measuring range is often from 20 – 300 kPa (0,5 to 3 bar). We distinguish between the MAP sensor for an atmospheric engine and a boost pressure sensor for an engine with boost pressure control.
MAP sensors are used to measure engine load. The manifold (under) pressure is a measure for the degree of filling. The fuel injection is determined, among other things, from the value registered by the MAP sensor.
In the MAP sensor, two air chambers are separated from each other by a membrane. The pressure in the MAP sensor causes the membrane in the sensor to bend. In the illustration, the outside air pressure prevails in the upper part and the negative pressure in the lower part. Several strain gauges are applied to this membrane, which register the deflection of the membrane. A greater pressure difference causes the membrane to bend further.
The MAP sensor consists of – usually – four piezo-resistive strain gauges mounted on a diaphragm in a Wheatstone arrangement. When compressing or stretching the material, the resistance value of the strain gauges changes. In the Wheatstone bridge the resistance change is converted into a voltage change. This forms the signal voltage, which is sent to the ECU. Inside the ECU there is a A/D converter which digitizes the voltage signal before it enters the microprocessor.
Features of the MPX4250AP:
The level of the output voltage therefore depends on the pressure in the intake manifold and is between 0,1 and 4,9 volts. The figure below shows the characteristic of a commonly used MAP sensor of type: MPX4250AP. The line is linear. At an outside air pressure of 100KPa (which is equal to 1 Bar), the sensor delivers a voltage of approximately 1,8 volts at an average operating temperature (TYP).
It can be seen in the characteristic that the sensor registers nothing at p ≥0, ≤20. This means that with fully open throttle and high load, the engine no longer starts from the value of the MAP sensor, but switches to a replacement value via software. The registered opening angle of the gas valve offers a solution here.
The component properties of the MPX4250AP are shown in the table.
Signal voltage of a naturally aspirated engine:
The signal voltage of the MPX4250AP sensor can look like this with a naturally aspirated engine. In this graph, throttle is alternated, released, accelerated and decelerated.
Charging pressure sensor:
Supercharged combustion engines are equipped with a boost pressure sensor to measure the pressure in the intake path. This sensor is located in the air hose (or pipe) between the intercooler and the engine throttle. The supercharging can be realized in the following way:
- diesel engines: exhaust gas turbo;
- petrol engines: exhaust gas turbo or mechanical compressor, or a combination thereof.
The boost pressure sensor (also called turbo pressure sensor or boost sensor) is actually a MAP sensor with a larger measuring range than that of a naturally aspirated engine:
- atmospheric engine: up to 1,5 bar;
- supercharged motor: up to 2,5 bar;
- supercharged motor: up to 3,5 bar.
The engine management system translates the voltage signal from the pressure sensor into a pressure and thus regulates the wastegate of the turbo. When a turbo with VGT is equipped, the position of the blades is adjusted.
- When accelerating, the turbo has to deliver more pressure. The wastegate remains closed until the desired inlet air pressure or. loading pressure is reached.
- When the desired charge pressure is reached, the ECU controls the wastegate, which will partially open. The pressure is kept constant or reduced by opening the wastegate more.
Combination with temperature sensor:
MAP sensors can be housed in one housing with the intake air temperature sensor. This can be recognized by four connections. Temperature is also an important factor in determining the injection rate.
We can recognize the following from the air temperature:
- Intake air temperature should not differ more than 5 degrees from the coolant temperature when the engine is cold;
- Intake air temperature higher than coolant temperature: EGR valve remains open.
In case of deviations from the two points above, the ECU may generate an error code.
Diagnosing the boost pressure sensor:
We can recognize malfunctions at the boost pressure sensor by the following symptoms:
- Reduced engine power;
- Not constant pulling force during acceleration;
- Excessive fuel consumption and emissions;
- Malfunction Indicator Lamp (MIL) with related trouble codes (DTCs).
Of course, in the case of the above complaints, it is self-evident to read the fault memory of the engine electronics. In the event that the engine management system stores a trouble code regarding an erroneous signal from the boost pressure sensor, we can expect the following codes: P0105, P0106, P0107, P0235, P0236, P0238.
Causes of a false signal can be:
- Internal wear, contamination or even clogging of the sensor element;
- Excessive contamination in the intake tract, e.g. due to carbon deposits in the intake manifold or cylinder head inlet channels;
- exhaust blocks;
- Leakage on the air hoses;
- Wiring problem between the sensor and ECU.
Contamination in the intake path can be determined by dismantling components such as the throttle / throttle valve and the intake manifold, or by checking the inside of the manifold with an endoscope. Exhaust blockages can be caused by a defective catalytic converter interior or a clogged particulate filter.
Problems with the sensor electronics or the wiring between the ECU and the sensor can be investigated by studying and measuring the schematic.
The image below shows the schematic of a boost pressure sensor. Click here for an explanation of schematic reading.
The charge pressure sensor and air temperature sensor are integrated in one housing. The sensors have a common positive (pin 3) and ground (pin 1). This shows that it is an active sensor. The signal wire from the boost pressure sensor (pin 4 on the sensor) is grey/black in color and is connected to pin 56 of the engine control unit. In this diagram we cannot determine whether the signal is an analog voltage (AM) or digital (PWM). We will find out by measuring.
The charging pressure sensor shown transmits an AM (Amplitude Modulation) signal, which can be seen from the scope image. The height of the voltage translates the pressure profile with respect to time. The following screenshot shows the voltage profile of a boost pressure sensor. The scope settings are: 1 volt per division and 200 ms per division.
When the engine is idling, the turbo does not yet deliver boost pressure. The absolute pressure in the intake manifold is approximately 100 kPa. The sensor translates this pressure into a voltage of approximately 1,6 volts.
When accelerating, the engine speed and thus also the turbo pressure is increased. The pressure gradually rises to 1,4 bar. The voltage in the scope image reaches almost 3 volts at that pressure. After that point, the accelerator pedal is released and the charge pressure drops.
In the event of a defect in the charge pressure sensor or wiring, irregularities in the signal will be visible. The voltage signal should be between 0,5 and 4,5 volts at a supply voltage of 5 volts. The two images below show a signal with interference (left) and without interference (right).