MAXIMUM PRESSURE MEASUREMENT
PREPARATION (see Figure 2)
There are high pressure pumps that switch off one of 3 pistons if a lower pressure is required. The
third piston is activated by a magnet if a higher pressure is required.
1. Attention: allow the common rail system to become depressurized before connecting the dial
gauge unit (see Fig. 2).
2. Connect measurement hose (2) to the pressure gauge (1) and outlet of the high pressure pump.
3. As an alternative to this, the measurement hose (2) can also be connected to the rail outlet.
4. When the fuel system is depressurized, disconnect the accessible high pressure line and connect
the pressure gauge either with the M12 or M14 measurement hose (2).
5. Connect the blind plug to dial gauge unit.
6. Make sure that the hose connections are screwed tight before continuing.
7. Connect the diesel collecting bottle to the transparent hose at the bottom connection of the dial
gauge unit. The bottle collects the diesel fuel when the pressure relief valve in the dial gauge unit
opens.
MEASUREMENT
1. Start the engine. Note: the engine does not start, the measurement is carried out at the alternator
speed.
2. The pressure should increase to more than 1050 bar if the pump operates correctly.
3. If the pressure is too low, this could be because of a defective pump or a defective pressure
regulator.
4. To determine which of both components is faulty, carry out a pressure regulator test.
NOTE: a defective pressure sensor (on the fuel distributor pipe) can send incorrect information to the
PCM and the PCM sends the data calculated incorrectly to the pressure regulator. In this case, the
pressure regulator would open too early and as a result, lead to insufficient pressure.
A comparison measurement from the output value of the pressure sensor to the measured pressure
value can be carried out with an EOBD tester by reading out the live data.
PRESSURE REGULATOR TEST
This is an essential test to determine if the fault is caused by a defective pump or a defective
regulator.
Connect the dial gauge unit. Observe the steps in chapter "MAXIMUM PRESSURE
MEASUREMENT".
1. When the fuel system is depressurized, remove the pressure regulator.
2. Select a matching pressure regulator dummy (A-I) and install this instead of the pressure
regulator.
3. Carry out a "MAXIMUM PRESSURE MEASUREMENT". If the pressure now increases to more
than 1050 bar, the pressure regulator is defective. If the pressure is always too low, the pump is
defective.
NOTE: a defective pressure sensor (on the fuel distributor pipe) can send incorrect information to the
PCM and the PCM sends the data calculated incorrectly to the pressure regulator. In this case, the
pressure regulator would open too early and as a result, lead to insufficient pressure.
A comparison measurement from the output value of the pressure sensor to the measured pressure
value can be carried out with an EOBD tester by reading out the live data.
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