Digital MAP sensor

The purpose of this test is to evaluate the correct operation of a digital MAP sensor based on the output voltage and frequency during engine idle, WOT and over-run conditions.

Connection guidance

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,

  1. Breakout leads.
  2. Back-pinning probes.

Testing sensors and actuators (to include relevant circuit/connectors):

  • When testing a sensor, it is desirable to gain access at the control module.
  • When testing an actuator, it is desirable to gain access at the actuator.

How to perform the test

Plug a BNC test lead into Channel A on the PicoScope, place a black clip on the test lead with the black moulding (negative) and a Back-pinning Probe or multimeter probe onto the test lead with the coloured moulding (positive).

Place the black clip onto the battery negative terminal and probe the MAP sensors output connection with the Back-pinning Probe or multimeter probe as illustrated in Figure 1. If you cannot reach the terminal or plug with a probe, then you may be able to use a breakout box or lead if you have one available

The three electrical connections will comprise of a supply voltage, an earth and a variable voltage output. The waveform on this page is monitoring the variable output voltage.

The three electrical connections will comprise of a supply voltage, an earth and a variable voltage output. The waveform on this page is monitoring the variable output voltage.

With the example waveform displayed on the screen you can now hit the space bar to start looking at live readings. Snap the accelerator quickly from idle to full throttle and observe the waveform.

Example waveform

Waveform notes

The output from the external MAP sensor will show a square wave its frequency will be lower at idle than when the throttle is opened. The example waveform clearly demonstrates the output signal and when the frequency is also displayed can be compared against the model's specification.

A higher frequency could result from any number of problems, but could be as simple as a split vacuum hose or incorrectly adjusted tappet clearances. Prolonged exposure to this higher frequency can result in damage to the catalytic converter.

Further guidance

The Manifold Absolute Pressure (MAP) sensor is employed to measure the pressure in the inlet manifold, and it is this output that when sent back into the engine management system can determine either the fueling or the amount of vacuum (light load) ignition timing advance. The MAP sensor will in most cases measure a negative pressure, but can also be used to measure a positive boost pressure on a vehicle equipped with a turbo charger.

The sensor is a 3 wire device that will have:

  • A 5 V supply voltage
  • An earth connection
  • A varying output that can be either analogue or digital
  • A vacuum connection to the inlet manifold

This particular component can be either an integral part of the Electronic Control Module (ECM), or an individual component.

The output from the analogue version will show a rise and fall of voltage depending upon the vacuum seen. When the engine is stationary or the throttle is in the wide open position, zero vacuum will be recorded and a voltage approaching 5 volts will be seen, as a vacuum is applied this voltage will reduce. This voltage can be recorded on either a multimeter or an oscilloscope.

A digital map sensor will produce a square wave signal to the engine management ECM, this square wave will change frequency with varying engine vacuum readings. This output waveform can also be monitored on an oscilloscope, or the frequency measured on certain multimeters that have the appropriate setting (Hz). Examples of both the digital and the analogue outputs can be seen below.

Figure 2 shows a MAP Sensor.

Typical readings whilst testing a MAP Sensor:

Vacuum (inches Hg) Vacuum (mbar) Frequency (Hz)
29.6 1000 80
23.6 800 93
17.6 600 110
11.8 400 123
5.9 200 140
0 0 160

GT025-2

Disclaimer
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.

Suitable accessories

  • Premium 6-way breakout lead set

    £269.00

  • Back-pinning Probe Set

    £48.00

  • Flexible Back-pinning Probe

    £4.25

  • PicoScope Battery Clip

    £2.75

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Guided test: Testing a Digital MAP Sensor