The diesel injection system with Bosch distributor-type injection pump has two control units for electronic diesel control. A Bosch pump control unit (installed on the pump) and an engine control unit. This configuration prevents overheating of certain electronic components and also interference from signals that are generated by very high currents (up to 20A) in the distributor-type injection pump.
Fig. 1 shows the VP44 pump.
Principle of operation of the VP44 Timing Control Solenoid
VP44 is a rotary style medium high pressure injection pump that is mostly mechanical with two electronically controlled components in it – the fuel metering solenoid and the timing advance solenoid. The timing advance solenoid is pulse width modulated by the ECM to control timing piston travel against a spring in the housing of the VP44. This piston moves the wavy ring inside the pump which is what forces the pistons in the rotor inward as it turns and creates high pressure to pop off, or open, the injector that the rotor is pointed to, to get fuel to flow. Fuel only flows through the injector as long as its pop off pressure is exceeded. If the high spot on the wavy ring is moved one way to the point where pop off pressure is exceeded and fuel flows sooner, the injection event is advanced. If it moves the other way it makes pop off pressure come later and therefore retards the injection event timing. The distributor portion of an injection pump is basically the same as a distributor cap in a gas scenario except that it has holes in it going to each delivery valve and injector line in the correct firing order in direction of rotation. The rotor in this pump does the same job as a rotor in a distributor in a gas car application. Instead of directing electricity to the contact in the distributor cap and spark plug wire, in an injection pump it is hydraulic and the rotor turns past a round hole in the so called distributor so fuel flows to the individual injector. The hole in the rotor, that mates up to the round distributor hole, is slotted so fuel can flow for a period of time as the rotor turns.
Order to verify functionality of theVP44 Timing Control Solenoid
• Testing the VP44 Timing Control Solenoid by oscilloscope
- Connect an AC/DC current clamp to the first oscilloscope channel.
Set the AC/DC current clamp range to ±20A.
Important note: Only one of the two wires should be clamped, and not both of them. It doesn’t matter which wire will be clipped with the current clamp: the positive or the negative one. This will only affect the polarity of the measured current.
- Start the engine and leave it idling.
- Watch the oscilloscope screen and compare it to the waveform in fig. 2.
• Possible failures in the VP44 advance valve
- Mechanical fault
- Broken valve solenoid
- Missing control signal – usually due to blown pump control unit