Information on diagnostics

Electrical motor vehicle signals

Increasing importance of the oscilloscope

An ever-increasing number of vehicle components communicate using clocked signals.
What is the difference between PWM (pulse width modulation) and PFM (pulse frequency modulation) signals?
Can clocked signals be measured with a multimeter?

Analogue signals can be measured with any commercially available multimeter. Synchronised signals require the use of an oscilloscope or the appropriate function on an engine tester.

In automotive technology, increasing use is being made of signals that consist of a regularly synchronised voltage. Multimeters only show the average voltage over a period.

Pulse width modulation (PWM)

  • The frequency is constant.
  • The pulse duty factor, i.e. the width of the pulses, varies.

Pulse width modulation may be used as a signal input factor or for the power control system, e.g. for EGR valves, throttle valves, electro-pneumatic valves, idling actuators or for demand-controlled fuel pumps.

Video 1: Signal on the oscilloscope and multimeter

Pulse frequency modulation (PFM)

  • The frequency varies, i.e. the signal curves are compressed or elongated.
  • The pulse duty factor is constant.

On some PIERBURG air mass sensors the output factors may be output as PFM signals.

Video 2: Signal on the oscilloscope and multimeter

Synchronised signals on the oscilloscope screen

Parameters:

  1. Voltage U, in volts
  2. Pulse or ON time
  3. OFF time
  4. Period duration T
  5. Time axis, in seconds
  6. The frequency is inverse of the period duration: f = 1/T
  7. “Pulse duty factor”

The term “pulse duty factor” is not always defi ned in the same way. Generally speaking it is understood to mean the relationship of ON time (2) to period duration (4).
The pulse duty factor is shown as a number between 0 and 1 or a percentage value between 0% and 100%.
Some oscilloscopes, as in the example here, show the pulse duty factor “upside”, in other words the OFF time (3) in relation to the period duration (4).

Synchronised signals are relatively insensitive to faults. Faults in the signal fl ow, for example due to corrosion or moisture in the plug-in connections, may cause the voltage level (8) to vary. This does not aff ect the actual “pulse duty factor” or “frequency” data.

Video 3: Faults do not have an eff ect on the transferred data.

In automotive technology frequencies of 100 Hz are usual. This is equivalent to 100 periods per second. Signal forms with these high frequencies can only be displayed on an oscilloscope.