Manifold absolute pressure sensor (turbo diesel)

The purpose of this test is to investigate the operation of an analog type Manifold Absolute Pressure (MAP) sensor on a turbo diesel engine during idle, free revving and overrun conditions. 

How to perform the test

View connection guidance notes.

1. Use manufacturer’s data to identify the MAP sensor signal circuit.
2. Connect PicoScope Channel A to the MAP sensor signal circuit.
3. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
4. Run the engine and allow to idle.
5. Start the scope to see live data.
6. Depress the accelerator pedal fully until the engine approaches peak RPM, then release the pedal.
7. With your waveform on screen stop the scope.
8. Turn off the engine.
9. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.

Example waveform

Waveform notes

This known good waveform has the following characteristics:

• With the engine running at idle the waveform shows a stable output around 2.4 V.
• After the accelerator pedal is depressed (around 1.5 s) the voltage increases as engine speed increases.
• At 2.5 s the voltage peaks around 4.1 V, at peak engine speed.
• There is a gradual voltage decrease over 2.5 s as the engine overruns and returns towards the original 2.4 V idle condition.

Waveform Library

Go to the drop-down menu bar at the lower left corner of the Waveform Library window and select Manifold Absolute Pressure MAP sensor (analogue).

Further guidance

MAP sensors respond to the air pressure within the intake manifold and allow the Engine Control Module (ECM) to estimate two important parameters:

• Atmospheric pressure at key on.
• Engine load.

The sensor can be mounted on the manifold housing or remotely, with pipework connecting the sensing element to the manifold volume.

The sensing element is usually a piezoelectric strain gauge having a voltage output proportional to the manifold air pressure. They require three electrical circuit connections:

• A reference (supply) voltage.
• An earth.
• A voltage output signal to the ECM.

MAP sensors measure absolute pressures. i.e. they are zero referenced against a perfect vacuum. The ECM will use known MAP sensor calibration values to convert the sensor signal voltage to an estimate of absolute pressure. At sea level, atmospheric pressure averages about 1013 mbar or 101.3 kPa. Therefore, when the ignition is on and the engine is off, the sensor output will be a positive, non-zero, voltage, which the ECM will interpret as a pressure around 1013 mbar or 101.3 kPa (dependent on the exact atmospheric conditions at the time).

On turbocharged engines, manufacturers use sensors capable of measuring pressure both above and below atmospheric pressures. Therefore, they can provide a measure of the boost (extra air mass) provided by the turbocharger.

Generally, MAP sensors fall into three pressure measurement categories, ranging from atmospheric, up to:

1. around 1.5 bar (usually used on normally aspirated engines)
2. around 2.5 bar boost
3. around 3.5 bar boost

Modern diesel engine turbochargers can produce around 2.7 bar boost. In this scenario, having the wrong type of sensor in the engine will send false data to the ECM, which can cause erratic/poor performance and possible engine damage.

Symptoms of a faulty MAP sensor:

• Malfunction Indicator Lamp (MIL) illumination.
• Diagnostic Trouble Codes (DTCs).
• Erratic or rough idle.
• Delay in acceleration.
• Excessive fuel consumption and emissions (sensor reads too high, engine over fuels).
• Lack of power (sensor reads too low, insufficient fuel - engine may run hot increasing N0x emissions).

Possible failures that can cause erroneous MAP sensor signals are:

• Electrical circuit issues, such as a short, open or high resistance.
• Internal wear or damage within the sensor unit (from heat, vibration or pollutants).
• Blocked sensor inlet from excessive pollutants/deposits within the intake manifold.
• Engine intake or exhaust blockages or leaks.
• Other engine mechanical issues affecting the engine intake or exhaust.

GT010-EN