PWM Measurement

PWM signals, i.e. communication between control modules, are most often triggered by a positive pulse, that is to say a battery positive (Batt+) pulse. Other PWM outputs (inputs) are most often triggered by a negative pulse. For instance, the injectors are supplied with a constant positive voltage and pulsed to ground from the control module.

Measurement With A Voltmeter

If we measure PWM with a voltmeter, the meter will show the mean voltage applied to the lead so that a higher pulse ratio will give rise to a higher mean voltage. By means of the voltmeter we can obtain a rough estimate of the pulse ratio. At a pulse ratio of 9%, a reading of about 1.2v is usually obtained (0.09 x 13v = 1.17v). In the case of PWM triggered by a positive pulse, connected the red test lead to the electric lead and the black test lead to a good ground. In the case of PWM triggered by a negative pulse, connect the black test lead to the electric lead and the red test lead to battery positive (Batt+). Select the "Smooth" function on the voltmeter if it has this feature.

Measurement With An ISAT Scan Tool Or Multi-meter With A Pulse Measuring Function

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

Measurement With Logic Probe

Connect battery positive (Batt+) and a good ground. Then apply the tip of the probe to the electric lead. The Logic Probe will show the presence of pulses, if any, and give a rough estimate of the pulse ratio.

Measurement With An Oscilloscope

Pulse frequency and duration are shown at the same time as we see a picture of the pulses. We Can see how pulse width changes when the input information changes.