Manifold absolute pressure sensor (digital)

The purpose of this test is to investigate the operation of a digital Manifold Air Pressure (MAP) sensor during idle, free revving, and overrun engine operating 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. Start the engine and allow it to idle.
5. Start the scope to see live data.
6. Fully depress the accelerator pedal until the engine approaches peak RPM, then release the pedal.
7. With your waveforms 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:

• A cyclic square waveform switching from just above 0 V to around 5 V.
• The signal frequency increases with increasing manifold pressure but the amplitudes remain constant.
• With the engine at idle, the waveform frequency is around 100 Hz.
• Although some noise is to be expected, it is not excessive and there are no signal drop outs.

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 (digital)

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 internal circuitry requires three electrical circuit connections to operate:

• A reference (supply) voltage (typically 5 V).
• An earth.
• A voltage output signal to the ECM.

The output frequency of a digital MAP sensor increases with increasing absolute pressure (or decreasing vacuum if referenced relative to atmospheric pressure). Therefore, on a normally aspirated gasoline engine, the sensor output is normally highest (around 160 Hz) at atmospheric pressure and lowest (around 80 – 100 Hz) when there is a significant depression within the intake manifold, such as at idle or on overrun.

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.

GT025