Atmospheric pressure sensors

Figure 1

Figure 2

How to perform the test

Channel A

Plug a BNC test lead in to Channel A of the scope. Place a small black clip on the lead end with a black moulding (negative) and a Back-pinning Probe on the end with a red moulding (positive). Place the small black clip on the motorcycle frame or battery negative, as illustrated in Figure 1. Check the manufacturer's pin data for the fuel injection Electronic Control Module (ECM) atmospheric pressure 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 waveform illustrated above shows a continual voltage that is representative of the atmospheric pressure seen at the time of capture.

Technical information

The signal seen by the motorcycle's ECM adjusts the engine's fuelling according to the altitude or atmospheric pressure. The change in atmospheric pressure causes the engines AFR (Air Fuel Ratio) to be altered. The adjustment to the motorcycle's fuelling thus gives the exhaust-mounted lambda sensor better control over the final emissions seen by the catalytic converter.

Faulty waveforms - troubleshooting

The voltage seen in the waveform must be referenced against manufacturer's data and it is therefore important that atmospheric pressure can be measured accurately with a barometric pressure gauge. If the voltage remains at battery voltage, ensure that the earth return via the ECM is complete. If the voltage remains at 0 volts, check the supply to the sensor.

In most instances, the atmospheric pressure sensor is integral to the ECM.

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.

AT107-2