The purpose of this test is to verify the correct operation of the TMAP sensors on turbocharged common rail diesel engines.
Connection for diagnostic work will vary dependent on application.
Technicians should whenever possible gain access to the test circuit without damage to seals and insulation. If this is not possible then make sure appropriate repairs are completed.
General connection advice
PicoScope offers a range of options within the test kits.
Dependent on difficulty of access, choose from:
Testing sensors and actuators (to include relevant circuit/connectors):
Operate the accelerator pedal fully, allow the engine to rev to maximum then release the pedal.
The waveform above shows the variable output voltages from both sensors with the Manifold Absolute Pressure (MAP) sensor in blue and the temperature in red.
The voltage from the MAP sensor (blue trace) indicates the boost pressure from the turbocharger. At idle it is about 1.5 volts and, as the throttle is opened and the turbo speed increases, it rises sharply to 2.6 volts. The voltage at idle tends to be manufacturer-specific, and the maximum voltage depends on how hard the engine is accelerated and the boost pressure generated by the turbocharger.
The voltage from the temperature sensor (red trace) fluctuates with the incoming air temperature change, due to the increased air speed through the intercooler.
It may take several attempts at capturing the TMAP sensor signal in order to position the boost pulse in the middle of the screen.
The combined Temperature and Manifold Absolute Pressure (TMAP) sensor measures the system air temperature and pressure after the turbocharger. The component is found in the inlet manifold or the turbo hose. It is used in conjunction with a conventional air flow meter (AFM).
All TMAP sensors have four electrical connections: a supply voltage, an earth, a variable voltage output from the manifold absolute pressure (MAP) sensor and another from a temperature sensor. There is also a direct vacuum connection, as the TMAP is mounted directly in the inlet manifold or on the turbo hose.
As the TMAP consists of two sensors located together, we will explain the two outputs separately.
Manifold Absolute Pressure (MAP) sensor
This output is sent back to the engine management system and determines the fuelling, the ignition position or the turbo boost control.
The output voltage from the sensor rises and falls depending on the pressure. When the engine is at idle and the manifold pressure is low, the sensor output is around 1.5 volts. As pressure is applied, the voltage increases in proportion to the pressure. As the turbo reaches maximum boost pressure, the voltage levels off. The 'hash' on the waveform is due to pressure changes from the induction pulses as the engine is running.
Inlet air temperature sensor
The air temperature sensor is a thermistor whose resistance decreases with increasing inlet air temperature. This type of thermistor is said to have a negative temperature coefficient. The temperature of the air flow is proportional to the air density and therefore provides a crucial input to the vehicle's fuelling. As the air is compressed and forced through the intercooler, it is likely to cool as shown in our example waveform.
The TMAP sensor contributes 20% to the temperature compensation of the engine. The other 80% comes from the coolant temperature sensor.
The voltages from different manufacturers' sensors are similar. Too low a voltage causes a loss of power due to fuel starvation, while an excessive voltage causes overfuelling and, if left uncorrected for too long, failure of the catalytic converter. A high voltage could result from any number of problems but the cause may be as simple as an inlet manifold air leak or incorrectly adjusted tappet clearances.
The voltage on a turbocharged engine rises as the incoming air temperature rises. If the voltage fails to rise, the sensor can be tested by monitoring the voltage and carefully heating the component with either a hairdryer or a heat gun.
This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.
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