• The Manifold Absolute Pressure Sensor contains a diaphragm and a piezo-electric circuit which converts the measurement into a signal voltage. The Map sensor plays a vital roll for the Engine control unit.

• The Map sensor can be mounted inside the Ecu, directly into the manifold or remote mounted with a vacuum pipe connected to the manifold.

• A crude but effective way of checking the map sensor function is to slowly remove the pipe if one is fitted, to slowly let air into the sensor. You should be able to hear a noticeable change in the note of the engine. The mixture will run rich and the ignition timing will retard.The engine may even stall.  Reducing the vacuum simulates a load on the engine.

 

• The correct way to test the Map sensor is either with a voltmeter or for detailed analysis use an oscilloscope.

You can test it with the engine running and by varying the throttlen note the changes in the reading or apply vacuum with a vacuum pump and check the reading as you change the amount of vacuum acting on the Map sensor.

Testing with a Volt meter:
First check the power supply and the earth connection. The take a reading from the signal terminal with everything connected.

Analogue Type - Set your volt meter to Dc volts. The sensor will generally have 3 terminals, some have 4 when a air temperature sensor is  combined.  It has a reference voltage or supply voltage of 5 volts an earth connection and a Signal voltage which will vary with vacuum / pressure changes.


Digital Type - This is more difficult to get an accurate reading with a voltmeter as the signal is very different. The digital map sensor produces a square wave. If your Voltmeter has a hertz or RPM function then a reading can be taken. If you have a minimum and maximum record function as on a Fluke you can use this to see changes in the reading. If you let the engine idle your average reading will be low. If you open the throttle and keep the engine under load then the average reading will be higher.The frequency increases as the vacuum increases.

Testing with an Oscilloscope: This is by far the most accurate way to test the Map sensor.

Analogue Type:
 

As the engine idles, the low reading of approx 1 volt can be seen. As the throttle is open the vacuum is reduced in the inlet manifold and the voltage in ceases to a reading of 5 volts.  This is a healthy sensor. 

Digital Type:
 

  Readings taken with a Pico oscilloscope

The difference between the analogue and digital sensors can be clearly seen. The Frequency of the signal voltage will again be low at idle and will increase when the throttle is opened and the vacuum is reduced.

When a Engine running issue is investigated and the Map sensor is suspected a hand and eye check should be carried out. Check for split pipes and leaking o rings. The vacuum connection to the brake servo should also be checked.

If you have a Vacuum pump ensure that the sensor itself can hold vacuum.