Manifold absolute pressure (MAP) sensors

Figure 1

Figure 2

How to perform the test

Channel A

Plug a BNC test lead into Channel A of the scope. Place a small black clip on the lead end with a black moulding (negative) and a Back-pinning Probe onto the end with a red moulding (positive). Place the small black clip on to the motorcycle frame or battery negative, as in Figure 1. Check the manufacturer's pin data for the fuel injection Electronic Control Module (ECM) MAP sensor connection terminal. Back-probe the correct multiplug pin with the Back-pinning Probe as in Figure 2. Ensure that a good connection is made through to the wire or terminal and the probe has pierced through the wire and plug insulation.

With the example waveform displayed on the screen you can now hit the space bar to start looking at live readings.

Example waveform

Waveform notes

The Manifold Absolute Pressure (MAP) sensor waveform seen in the above illustration is a typical output from an analogue sensor. With the motorcycle's engine at idle, the voltage shown on the example waveform is at 1.0 volts. The throttle is then snapped open and the voltage rises to just under 5.0 volts. The voltage achieved depends on how fast the throttle is opened, as a slower reaction time results in a lower voltage. While the engine is slowing back to idle with the throttle closed, the increased vacuum within the inlet manifold results in a lower voltage than that seen at idle. This lower voltage returns to the original voltage when the speed returns to idle.

Technical information

MAP sensors can be either integral to the motorcycle's ECM or separate units. An ECM that is fitted with an internal MAP sensor can be identified by a vacuum pipe connection.

External MAP sensors are three wire devices which have:

• a 5 volt supply voltage
• an earth connection
• a varying analogue output
• a vacuum connection to the inlet manifold

The output from the sensor shows a rise and fall in voltage depending on the vacuum.

When the engine is stationary or the throttle is wide open, zero vacuum will be recorded and a voltage approaching 5 volts will be seen; as a vacuum is applied the voltage will reduce. The example waveform shows that at idle a voltage of around 1 volt is seen, and as the throttle is opened the vacuum in the manifold drops and a higher voltage for these conditions is seen. In this case the voltage rises to almost 5 volts.

The 'hash' on the waveform is due to the vacuum change from the induction pulses as the engine is running.

All voltages are similar between different motorcycle manufacturers and a lower-than-anticipated voltage will produce a loss of power due to fuel starvation. Conversely a higher voltage will cause overfuelling and could eventually result in the failure of the catalytic converter (if fitted). This high voltage could result from any number of problems but may be as simple as a split vacuum hose, or incorrectly adjusted tappet clearances. A simple way to test an external MAP sensor is to remove the vacuum pipe, switch the ignition on and observe the voltage. Suck on the pipe and the voltage should drop. The voltage from an integral MAP sensor cannot be evaluated due to the lack of access to the output voltage.

Faulty waveforms - troubleshooting

Observe the voltage under varying throttle openings. If the voltage fails to change but the MAP sensor's vacuum connection is intact, renew the sensor. If the voltage observed is at battery volts, check the sensor's earth connection.

Pin data

Our test vehicle was a Honda motorbike. Below is the ECM multiplug pin data. Pin data is manufacturer- and model-specific and this data is shown for illustration purposes only.

AT110-2