You will require a PicoScope to perform this test. A list of suitable accessories can be found at the bottom of this page.
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.
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.
Figure 2
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:
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 |
AT025-2
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.
If you have any suggestions to improve this guided test please do so using the 'add comment' button.
