The purpose of this test is to verify the correct operation of the TMAP sensors on turbocharged common rail diesel engines.
Plug a BNC test lead into Channel A on the scope. Place a black clip on the black (negative) test plug and a Back-pinning Probe on the colored (positive) test plug. Clip the black clip onto the battery negative terminal or a suitable earth connection. Back-probe the TMAP's manifold pressure output terminal with the acupuncture probe as illustrated in Figure 1.
Plug a second BNC test lead into Channel B on the scope. Place a Back-pinning Probe on the colored (positive) test plug. Back-probe the air temperature sensor's output with the acupuncture probe, also illustrated in Figure 1.
Figure 2 shows an alternative arrangement where the TMAP is mounted in the turbo air hose.
With the example waveform displayed on the screen, you can now hit the space bar to start looking at live readings. Open the throttle quickly from idle to full throttle and observe the voltages on the waveforms.
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). Figure 3 shows the airflow system and location of the AFM (1) and the TMAP (2).
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.
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.
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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