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.
This known good waveform has the following characteristics:
MAP sensors respond to the air pressure within the intake manifold and allow the Engine Control Module (ECM) to estimate two important parameters:
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:
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:
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:
Possible failures that can cause erroneous MAP sensor signals are:
Selection of component-related Diagnostic Trouble Codes (DTCs):
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