Procedures For Checking PWM Signals

GENERAL
PWM signals, ie communication between control modules, are always positive-triggered, ie the pulse is a positive voltage pulse from the battery.

Other PWM outputs (inputs) are often negative triggered, for example the injectors which are constantly positively powered and pulsed with ground from the control module.

MEASURING WITH VOLTMETER
If you measure PWM with a voltmeter, the voltmeter will show the average voltage present on the cable, ie if the pulse ratio increases we obtain a higher average voltage.

With the voltmeter we can obtain a rough estimate of the pulse ratio. At a pulse ratio of 9%, the result is often about 1.2 Volt (0.09 x 13 V = 1.17 Volt).

If the PWM is positive-triggered, connect the red measuring cable to the cable and the black measuring cable to safe ground.

If the PWM is negative-triggered, connect the black measuring cable to the cable and the red measuring cable to battery positive voltage.

If the voltmeter is equipped with a SMOOTH function, select this function.

MEASURING WITH Intelligent Saab Tester (ISAT) SCAN TOOL OR MULTIMETER WITH PULSE MEASURING FUNCTION
Connect the red measuring cable to the wire and the black measuring cable to safe ground. Then select positive or negative-triggering. You can now measure frequency, pulse time and pulse ratio.

MEASURING WITH LOGIC PROBE
Connect the positive terminal of the power supply battery and safe ground and then place the measuring point on the cable. Logic Probe shows if there are any pulses and gives a rough estimate of the pulse ratio.

MEASURING WITH OSCILLOSCOPE
Frequency and pulse time can be read at the same time as you obtain a picture of the pulses. You can then see how the pulse width changes when the value of the information changes.