MANIFOLD ABSOLUTE PRESSURE SENSOR (MAP SENSOR)

General description 
      MAP sensor (MAP) measures dilution in the intake manifold and its sensitive element converts the signal to electrical that can be returned to the onboard controller. MAP sensor is used mostly as a cheap alternative to sensors for engine load. Its relatively low cost is the reason for its wide distribution, though its measurements are not as accurate as the various types of sensors for the amount of air. MAP can be located in the engine compartment as a separate component or integrated in the onboard controller. MAP is used in both types of systems - MPi and SPi, but is more common in SPi.

Appearance 
Fig. 1 shows a typical MAP sensor.


Fig. 1

Types of sensors
According to the principle of operation are:

  • With analog output - Widely used. Its voltage is proportional to the engine load.
  • With a digital output - it is used in systems such as the Ford EEC IV. Digital MAP sends signals in rectangular shape with certain frequency. When the load rises the frequency also increases, and measured in milliseconds time between pulses decreases. The onboard controller responds much quickly to a digital signal, because it is not necessary to convert it to analog.

Principle of operation of the MAP sensor
      MAP is connected to the intake manifold through a vacuum hose. The vacuum in the intake manifold actuate the MAP sensor’s diaphragm. Converter transforms the measured pressure into an electrical signal which is fed into the onboard controller. ECU estimates the data from the MAP sensor values as: "Absolute Pressure" = "Atmospheric pressure" - "manifold pressure”.
By using the speed/density method, the onboard controller calculates the fuel mixture composition depending on the signal of MAP and engine speed. This method is based on the theory that with every turn the engine sucks fixed volume of air. Accuracy of this method can not be compared with that of the air quantity sensor, which after accurate measure of the airflow calculates the fuel mixture ratio based on mass or volume of air sucked in from the engine.
      When there is a high level of vacuum in the intake manifold (e.g. idling), MAP output signal is relatively low and the onboard controller provides less fuel.
      In systems with "wet" type intake manifold (e.g. SPi) changes in manifold pressure can cause the fuel entering the vacuum hose to reach the MAP. To avoid this, a special trap is used and accordingly traced vacuum hose. If the fuel reaches the MAP sensor, its diaphragm may be damaged. 
In MPi systems the manifold is "dry" type and fuel can’t enter as it being sprayed over the intake valves. Therefore there is no risk of MAP sensor fuel penetration and contamination of the diaphragm, and therefore a special trap is not used.
When the MAP sensor is used as a separate component, inexpensive maintenance could be achieved. When MAP sensor is built into the onboard controller, possible replacement of the MAP will require the replacement of the whole controller.

Order to verify functionality of the MAP sensor

NOTE: If the MAP sensor is located inside the onboard controller, verification of the output signal is impossible. 
1.) MAP SENSOR IN THE INTAKE MANIFOLD – ANALOG TYPE 
-- Initial general inspection

  • Connect a vacuum gauge between the intake manifold and the MAP sensor, by using a tee-joint.
  • Left the engine idling. If the vacuum in the engine is small (less than 570mbar¸700mbar), check for the presence of the following failures:
    • depressurization;
    • damaged or cracked vacuum tube;
    • blocked vacuum hose;
    • mechanical engine problem, such as incorrectly adjusted timing belt which drives the camshaft;
    • Leaks in the MAP sensor’s membrane (if the sensor is built in the onboard controller).
  • Disconnect the vacuum gauge and connect in its place a vacuum pump.
  • Using the pump, create vacuum around 750mbar (75kPa) in the MAP sensor.
  • Turn off the vacuum pump. The sensor membrane must keep the same vacuum value at least 30 seconds.

-- Check the accuracy of the external MAP sensor
Terms under which checks are carried out - the engine not started and vacuum is provided with a vacuum pump.

  • Connect the negative terminal a DC voltmeter to the chassis ground.
  • Identify the supply voltage, signal and ground terminals.
  • Join the positive terminal of the voltmeter to the signal wire of the MAP sensor.
  • Disconnect the vacuum hose from the sensor.
  • Connect the MAP sensor to a vacuum pump.
  • Turn on the ignition (but do not start the engine).
  • Compare the voltage with the regulatory value for this type of vehicle and engine.
  • Create vacuum with the referred in Table 1 value and monitor the smoothness of voltage change.
  • The results of turbo engines (Table 3) are different from the results of "atmospheric" engines (Table 2).

Applied vacuum, mBar

Voltage, V

Value of МАР, Bar

0

4.3 – 4.9

1.0 ± 0.1

200

3.2

0.8

400

2.2

0.6

500

1.2 – 2.0

0.5

600

1.0

0.4

Table 1

Condition

Voltage, V

Value of МАР, Bar

Vacuum, Bar

Fully open throttle

4.35

1.0 ± 0.1

0

Turn on ignition

4.35

1.0 ± 0.1

0

Idling speed

1.5

0.28 – 0.55

0.72 – 0.45

Stop engine

1.0

0.20 – 0.25

0.80 – 0.75

Table 2
Condition

Voltage, V

Value of МАР, Bar

Vacuum, Bar

Fully open throttle

2.2

1.0 ± 0.1

0

Turn on ignition

2.2

1.0 ± 0.1

0

Idling speed

0.2 – 0.6

0.28 – 0.55

0.72 – 0.45

Applied voltage

Voltage, V

0.9 Bar (checking the pressure of the turbocharger

4.75

Table 3

-- Fast verification of analog MAP sensor using an oscilloscope

  • Restore all connections to the MAP sensor as during normal engine work.
  • Attach the ground probe of the oscilloscope to the chassis ground.
  • Connect the active end of the oscilloscope probe to the signal terminal of the MAP sensor.
  • Start the engine and left it idling.
  • Sharply press the throttle and then immediately release it. You must watch the signal as in fig. 2.


Fig. 2

      If voltage sharply rises to its maximum value when the accelerator is pressed, and quickly falls to the minimum when accelerator is released - MAP sensor is working properly.

-- Possible failures in the analog sensor:
      Chaotic output signal

  • Chaotic output signal is, when the voltage signal changes randomly, drops to zero and disappears. This usually happens when an inefficient MAP sensor is present. In this case the sensor must be replaced.

      Missing of signal voltage

  • Check whether the reference voltage is applied (+5.0V).
  • Check the grounding for problems.
  • If the reference voltage and the grounding are correct, check the signal wire between the MAP sensor and the onboard controller.
  • If the reference voltage and/or grounding are not correct, check the integrity of the wires between the sensor and the ECU.
  • If all wires of the sensor are correct, check all connections for the reference voltage and ground of the onboard controller. If they are correct under suspicion falls the controller.

      The MAP sensor power supply or signal is equal to the car battery voltage.

  • Check for a short circuit to the positive terminal of the car battery.

-- Other checks:

  • Check for excessive fuel in the vacuum hose or trap.
  • Check the vacuum hose for leaks and/or other damages.
  • Check for mechanical damaged parts of the engine, ignition system or in the fuel system, causing low vacuum.

2.) MAP SENSOR IN THE INTAKE MANIFOLD – DIGITAL TYPE 
NOTE: Real signal from the output of this type of MAP sensor can only be seen with the oscilloscope.

  • Determine the voltage supply, signal and ground terminals.
  • Connect the ground probe of the oscilloscope to the chassis ground and the active end - to the output signal wire of the sensor.
  • Start the engine. You must watch waveform similar to that in fig. 3.


Fig. 3

  • If you have a fault reader and you can read the change of engine speed, perform the process described below.
  • Increase the engine speed to 4500 - 4900 rpm.
  • Connect a vacuum pump to the MAP sensor’s vacuum hose. Vacuum must be maintained at the same level for all voltage values. Dependence of change of pressure and speed is shown in Table 4.

200 mBar

The speed must be reduced to 525 ± 120 rpm

400 mBar

The speed must be reduced to 1008 ± 120 rpm

600 mBar

The speed must be reduced to 1460 ± 120 rpm

800 mBar

The speed must be reduced to 1880 ± 120 rpm

Table 4
  • When the pressure is cut off, measured value of the number of cycles must be equal to the starting position - 4500 - 4900 rpm.
  • Replace the MAP sensor, if it works differently than described above.

-- Possible failures in a digital sensor: 
      Lack of signal voltage

  • Check whether the +5.0V reference voltage is present.
  • Check the grounding for problems.
  • If the reference voltage and the grounding are correct, check the signal wire between the MAP sensor and the onboard controller.
  • If the reference voltage and/or grounding are not correct, check the integrity of the wires between the sensor and the ECU.
  • If all wires of the sensor are correct, check all connections for the reference voltage and ground of the onboard controller. If they are correct under suspicion falls the controller.

      The MAP sensor reference voltage or signal is equal to the car battery voltage.

  • Check for short circuit in the wire, connected to the positive terminal of the car battery, or in the wire which turns on and off the power supply.

-- Other checks:

  • Check for excessive fuel in the vacuum hose or trap.
  • Check the vacuum hose for leaks and/or other damages.
  • Check for mechanical damaged parts of the engine, ignition system or in the fuel system, causing low vacuum.
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