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How do I check the MAP sensor with the QST-5?

07.02.2023

MAP sensors are among the basic sensors that have been present in vehicles for years. In the following text, we will introduce a little about how they work and show how to check such a sensor with the QST-5 tester also outside the vehicle.

 

What is MAP sensor

MAP is short for Manifold Absolute Pressure and stands for absolute pressure in the intake manifold. The MAP sensor transmits a signal corresponding to the absolute pressure to the engine controller (ECU). When we talk about pressure, we mean several different terms. Simplifying, electronic and mechanical pressure gauges measure pressure as a force exerted on a certain surface inside the instrument. From what is on the other side of the surface we get different types of measurement. The relative measurement will be if there is ambient (atmospheric) pressure on the other side. This is how we measure, for example, the value of tire pressure. For absolute measurement, a vacuum is necessary. This way we will measure the atmospheric pressure. And if we seal the second chamber of the pressure gauge and add an additional input we will get a differential measurement (we use such a measurement for DPF/FAP filters, for example). In practice, the absolute value of pressure will be higher than the relative value by the value of atmospheric pressure, which, simplifying, is about. 1 bar. In naturally aspirated engines (without a turbo) we measure vacuum, and the value never exceeds atmospheric pressure.

What is the MAP sensor needed for?

In non-turbine engines, the pressure information is used to determine the density of the incoming air and the approximate air mass flow. In such an engine, you can interchangeably use the MAF (Mass Air Flow) sensor, which measures this parameter directly. Older solutions relied solely on the MAP sensor, newer ones may include both for added assurance. Accurate knowledge of the amount of air in the cylinder is necessary to select the fuel dose of specific engine conditions (the ECU relies on other signals such as the throttle position sensor, accelerator pedal and many others).

Turbocharged engines usually have a MAF flow meter at the cold air inlet (after the filter) in addition to the MAP sensor. In such engines we can deal with both vacuum and overpressure values (thanks to the turbocharger). In the case of supercharged engines, the presence of both sensors is necessary to monitor the operation of the turbocharger. Used for turbo engines, the term boost pressure refers to relative pressure (above atmospheric) so to get it you only need to subtract 1 bar from the pressure measured by the MAP sensor.

MAP sensors in practice

Sensors are most often located on or near the intake manifold housing (connected by a hose). These are components that typically have 3 or 4 leads. The 4-pin sensors additionally have a built-in thermistor that measures the temperature of the air in the manifold. The most common sensors are powered by 5V from the controller and provide a voltage proportional to the measured pressure. Few sensors provide an output signal in the form of a variable frequency rectangular waveform (as do many MAF sensors).

Sensor diagnostics

Symptoms of MAP sensor problems are related to improper fuel dosage and will be similar to problems with injectors or the MAF sensor. Symptoms may include:

  • Excessive fuel consumption, lack of power , the smell of gasoline in the exhaust (even after heating), uneven idling – symptoms for too rich a mixture.
  • Misfiring, speed rippling, engine stalling, excessively high exhaust temperature, engine jerking and detonation combustion – symptoms for a too-poor mixture.

In the situation of the above symptoms, make sure that the culprit is not the fuel system or the ignition system. If the MAP sensor gives a significantly different signal than expected, the controller may generate an error code. This will almost certainly happen if the sensor output is completely out of range – as in the case of a broken sensor circuit. Worse if the signal is only skewed, then the controller can use, for example, information from the flow meter to detect inconsistencies. Remember that the controller does not know which signal is reliable. It may happen that the mechanic reads a code indicating a MAP sensor, while the MAF flow meter is defective.

MAP sensor diagnostics is best started with a thorough assessment of manifold leaks – leaks are a typical problem with sensor readings (so-called “left air”). It is worth noting the contamination around the sensor – they are among the typical causes of the malfunction. Next, we verify the wiring – the continuity of the connection between the pressure sensor and the engine controller.

Independent sensor test

When testing a pressure sensor outside the vehicle, we need to provide it with power and an output voltage reading. We also need a pump with a pressure gauge that will produce different pressures for the test. Also useful will be any data on the available sensor, such as the measurement range or an example of voltage under given conditions (for example, voltage when the engine is turned off).

Drut - Okablowanie elektryczne

Sensor from a naturally aspirated engine

We will test a sample MAP sensor number 0 261 230 235 from a Skoda Fabia III 1.0 MPI (CHYB) model. Without knowing the layout of the leads, we can use the automatic sensor lead detection mode with the tester. However, an additional problem arises – the sensor under test has 4 leads. As we mentioned earlier, an additional lead is connected to the temperature sensor. It is attached between this additional pin and ground.

If we use the electrical measurement mode of the QST-5 and switch between leads, we will detect a resistor of approx. 2.3 kΩ between the temperature sensor terminals. Ground will be one of the leads of this resistor (we can independently test it by connecting the QST-5 leads to these pins only – it will measure the resistance and optionally convert it to temperature). This leaves us with two connection options for the MAP sensor (1,3,4 or 2,3,4). We can confidently test both by comparing current draw and performance. The correct variant will be associated with higher current consumption (connecting the sensor in the incorrect variant will connect its ground to the negative of the power supply through the thermistor). A working sensor will provide a voltage proportional to the pressure.

If we do not know the range of pressure measured by the sensor, it is important whether we are dealing with an engine equipped with a turbocharger. The 1.0 MPI (CHYB) engine is a small naturally aspirated gasoline engine. The absence of a turbine means that a given MAP sensor will measure values only slightly above 1 bar of absolute pressure. The maximum signal value is usually 4.5 or 4.75V, we expect a value slightly lower. Measuring the signal at atmospheric pressure, we obtain approx. 4V, which is in line with expectations (we typically expect a voltage of about 3.5 – 4.2V). If we measure the signal for different pressures then we get an approximate characteristic.

 

Sensor from supercharged engine

The second sensor we will test will be the 0 281 002 401 sensor from a VW Crafter 2.5 2.5 TDI (BJM) vehicle. After checking with the QST-5 device, it turns out that the pinout is consistent with the previously tested sensor. The built-in temperature sensor has a similar resistance, approx. 2.3 kΩ at room temperature. After powering the sensor, we read a voltage of 1.64V. This voltage is significantly lower than that of the previous sensor. The range of the supercharged engine sensor must cover the required range of boost pressure.

Czujnik mapy - Czujnik

Sensor replacement

Fixing a map sensor failure involves replacing it with a working component. Since MAP sensor replacement is simple and the sensor is relatively inexpensive, the whole process is seamless. Diagnosis and skillful inspection of the component by a mechanic is key. In this way, it does not waste time replacing operable components. Having mastered the technique of testing sensors outside the vehicle, we gain additional flexibility in handling parts.

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