Manifold absolute pressure and temperature sensor (turbo diesel)

The purpose of this test is to investigate the operation of a combined Temperature sensor and Manifold Absolute Pressure (TMAP) sensor unit 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 TMAP sensor signal circuits.
2. Connect PicoScope Channel A to the TMAP pressure signal circuit.
3. Connect PicoScope Channel B to the TMAP temperature signal circuit.
4. Start the engine and allow it to idle.
5. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
6. Start the scope to see live data.
7. Depress the accelerator pedal fully until the engine approaches peak RPM, then release the pedal.
8. With your waveforms on screen stop the scope.
9. Turn off the engine.
10. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.

Example waveform

Waveform notes

These known good waveforms have the following characteristics:

Channel A illustrates the air pressure waveform:

• The engine idle voltage is 1.5 V.
• This rises quickly to 1.6 V the instant the accelerator pedal is pressed at 15 s.
• The voltage linearly increases up to a peak around 2.6 V, whilst the pedal is fully depressed.
• When the pedal is released, there is a symmetrical decrease back to the engine idle value, 1.5 V.
• There is some hash on the waveform due to the intake pressure pulsations from the action of the inlet valves.

Channel B illustrates the air temperature waveform:

• The engine idle voltage is 2.4 V.
• Within 1 s of the accelerator starting to move the voltage falls to 2.2 V.
• At peak engine speed, the voltage waveform curves up to 2.5 V and back down to 2.1 V.
• When the engine is back at idle speed the waveform gradually falls to 1.1 V.

There are no anomalies or drop-outs in either signal.

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

TMAP sensors fulfil the functions of standard MAP sensors. That is, they 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.

However, they also have an additional intake temperature sensor to help the ECM better estimate the air density within the intake manifold. On turbocharged vehicles, this is necessary as the air density and temperature are altered by the actions of both the turbocharger and the intercooler.

The sensor can be mounted on the manifold housing itself, on pipework between the manifold and the intercooler, or on the outlet of the intercooler.

The pressure sensing element is usually a piezoelectric strain gauge having a voltage output proportional to the manifold air pressure. Temperature monitoring uses a Negative Temperature Coefficient (NTC) thermistor (the sensor resistance decreases as temperature increases). Both sensors reference their output relative to a 5V supply voltage.

All TMAP sensors will have four electrical circuit connections:

• A reference (supply) voltage.
• An earth.
• A pressure sensor output voltage to the ECM.
• A temperature sensor output voltage to the ECM.

Care should be taken during TMAP sensor testing, as the pressure and temperature signals are both analog and can be confused.

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.

GT136