Manifold absolute pressure sensor (gasoline)

The purpose of this test is to investigate the operation of an analog Manifold Absolute Pressure (MAP) sensor on a gasoline 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. 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 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:

• A voltage from around 1.25 to 1.5 V with the engine running at idle.
• A sharp increase to just over 4.5 V when the throttle is snapped open.
• Around 4.5 V, which is maintained whilst the throttle is fully open.
• A sharp voltage drop as the throttle is closed.
• The voltage continues to fall to below 0.5 V before gradually returning to the idle value during overrun.
• The hash on the waveform is due to engine intake pulses and is not detrimental to sensor operation.

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 (typically 5 V).
• 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 a normally aspirated gasoline engine, the sensor output is normally highest (around 4.5 V) at atmospheric pressure and lowest when there is a significant depression within the intake manifold, such as at idle (around 1.5 V) or on overrun (below 0.5 V).

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.

GT024