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PWM measurement

PWM measurement


PWM measurement

PWM signals , i.e. communication between control modules, are usually positive-triggered, that is to say the pulse is a Batt+ pulse. Other PWM outputs (inputs) are usually negative-triggered. One example is the injectors, which are supplied with a constant positive voltage and pulsed with ground from the control module.

Measurement using a voltmeter

If we measure PWM by means of a voltmeter (AC), the instrument will show the average voltage carried by the cable so that a higher pulse ratio will show a higher average voltage reading. Using the voltmeter, we can obtain a rough estimate of the pulse ratio. A pulse ratio of 9%% will often give a reading of about 1.2 V (0.09 x 13 V = 1.17 V). In the case of positive-triggered PWM, connect the red test lead to the cable and the black test lead to a good ground. In the case of negative-triggered PWM, connect the black test lead to the cable and the red test lead to Batt+. Select "Smooth" on the voltmeter if it has this function.

Measurement using an ISAT or a multimeter with pulse measurement capability

Connect the red test lead to the cable and the black test lead to a good ground. Then select positive or negative triggering. Frequency, pulse duration and pulse ratio readings can then be obtained.

Measurement using the Logic Probe

Connect the Batt+ supply voltage and a good ground to the Logic Probe and then apply the tip of the probe to the cable. The Logic Probe will indicate whether or not pulses are present and give a rough estimate of the pulse ratio.

Measurement using an oscilloscope

Frequency and pulse duration readings can be obtained at the same time as we see the pulses on the screen. We can now see how the pulse width changes when the input data is changed.

Additional PWM information

If you would like to read more about PWM, you will find further information in Service Manuals 3:2 and 2:5.