Test of High-Pressure Pump

Test Of High-pressure Pump

The high-pressure pump in the common rail system delivers fuel under high pressure into the rail. During operation, the three pistons of the high pressure pump generate pressure peaks in the rail which can be made visible by means of measuring the rail pressure sensor signal with an oscilloscope. The shape of the signal makes it possible to determine the mechanical status of the high pressure pump.
In order to obtain a meaningful signal, the rail pressure control must be active. The test module automatically switches from volumetric flow regulation to rail pressure control. As a result, two faults are stored in the DDE fault memory; these should be ignored and must be deleted once diagnosis has been completed.

Test preconditions
- Fuel level sufficiently full (reserve fuel lamp must not light)
- No fault in fuel supply system
- The common rail system displays no leaks to the outside
- No fault code stored for rail pressure sensor, volumetric flow regulating valve or rail pressure control valve

Evaluating the oscilloscope measurement
The figure below shows a rail pressure sensor signal that is in order thus indicating an intact high pressure pump. The signal pulsating uniformly at a period of approx. 25 ms and an amplitude of approx. 20 mV can be clearly seen. The maximum permissible value for the amplitude is 40 mV.







In contrast to this, the next figure shows a signal that has been generated by a defective high pressure pump (one piston of the pump failed). The following factors characterize the difference compared to Fig. 1:
- Considerably longer periods (approx. 100 ms)
- Higher amplitude (max. permissible: 40 mV)