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 Fuel Metering Valve
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 fuel metering solenoid is both the fuel fill valve and the pressure relief valve for the rotor. The rotor is hollow with three pistons mounted radially in it, that mate up with and run over the highs and lows of the wavy ring on the inside of the pump housing. The solenoid is actually a valve on the end of the rotor. When it is open, low fuel pressure fills the hollow part of the rotor with fuel as centrifugal force and fuel pressure push the pistons outward to the lowest spot on the wavy ring allowing the rotor to completely fill with fuel. At a computer determined magic moment, the solenoid closes the fill point or fuel solenoid, and then as the rotor turns, the wavy ring makes the pistons compress as they go over the high spots. When the pistons are compressed, pressure builds up in the rotor and when it exceeds injector pop off pressure, fuel flows through the injector until the computer on the top of the injection pump shuts off the solenoid valve, allowing it to open, which relieves the pressure in the rotor to below pop off pressure and fuel stops flowing. As the solenoid is now open, the rotor is refilled for the next injection event. The longer the fuel solenoid is kept closed during each injection event, the more fuel is injected into the cylinder. This is how you make more or less fuel come out of this pump. A fueling style performance box like our Fuel Management System works on this principle. The performance box holds the solenoid closed longer than the computer on the pump tells it to, and fuel continues to flow, making more power until the rotor is empty or the solenoid is shut off by the box and the fill valve is opened!
Order to verify functionality of the VP44 fuel metering valve
• Testing the VP44 fuel metering valve 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 left it idling.
- Watch the oscilloscope screen and compare it to the waveform in fig
• Possible failures in the VP44 fuel metering valve
- Mechanical fault
- Broken valve solenoid
- Missing control signal – usually due to blown pump control unit