02 March 2018: Current Measurement of Pump Motors in a Common Rail Direct Fuel Injection System

Current Measurement of Pump Motors in a Common Rail Direct Fuel Injection System 

Common rail direct fuel injection is a direct fuel injection system for diesel engines developed in the 1960s and further developed and adopted by virtually every automotive manufacturer in the world. The system uses a high-pressure fuel rail to pump diesel fuel to individual solenoid or piezoelectric valves, where high pressure provides for better fuel dispersion. Because it uses a single or “common” fuel rail to feed up to 5 fuel injectors, the high-pressure pump only needs to maintain a specified pressure at a target, regardless of the number of rotations at the engine or its load. The pressure is normally controlled by the vehicle’s ECU.

In order to monitor and inspect the performance of a common rail direct fuel injection system, customers can record the current signal of the pump motor as it controls the pressure being applied while fueling the common rail by using the Hioki MR8875 Memory HiCorder and 3273-50 Current Probe. For example, as shown in the graph below, a reference pulse represented by the waveform on the top is used as a trigger to record one cycle of waveforms continuously over 2ms.

Then, the time it takes the current to reach the lowest point of the waveform (at point “2”) and the variation around that point is analyzed.

This enables the customer to check the rate of rail pressure rise and fall which gives information about the functionality of high pressure pump, because the value of rail pressure drop is directly proportional to the real injector opening duration. By comparing the current waveforms during different injector openings, you can infer the following, for example:

  • A difference in pressure drop can mean that one injector is delivering significantly lower amount of fuel than This may be caused by injector blockage, malfunction, or other failures, and may need maintenance or repair.
  • If the pressure drop at one injector is much greater than the others, there may be a leak at the injector, which should be examined more
  • Lack of any pressure drop can mean that an injector is not working at The reason for this can be an open injector circuit due to lack of contact in the plug, a broken wire, or burnt injector solenoid, or even external causes.


The portability and multi-channel characteristics of the MR8875 make it the ideal recorder for this application. In addition, the 3273-50 with its wideband frequency and DC measurement capabilities lets you capture sudden current waveform fluctuations that may not be possible with a more general sensor.