Needle drivers (NHG/NBF), G80, Construction and Function (Articles)

The needle valve, abbreviated NHG or NBF (needle movement sensor), is installed in TDI engines with distributor injection pumps. He has the designation G80 in the schematic.

The function of the fuel injector pulse sender is to report the actual injection start time to the engine control unit, so that it can adjust the actual value to match the desired injection start time.

The engine control unit can adjust the injection start time, for example, on the Bosch VP37 fuel pump using PWM, the adjustment is made hydraulically via the internal pressure of the pump.
Details about the VP37 can be found here: /viewtopic.php?t=5517

The needle driver is essentially comparable to a standard nozzle holder combination, the only difference being a "extension" of the nozzle needle and a coil into which the needle tip dives when moving.

Once the nozzle of the fuel injector opens, its movement is mechanically transferred to the actual sensing needle, which is immersed in the coil, through various pressure components.
By immersing the iron needle probe into the coil, its inductance changes, which can be detected by the motor control unit using a test current.

In four-cylinder engines, such as AHF, ALH, AFN is the third nozzle holder combination with needle lift actuator, so it measures the injection start only on one of the four injection nozzles.
Therefore, it is also important that the nozzle opening pressures of the individual nozzle holder combinations are relatively similar.

Defects in the needle position sensor are most often winding breaks or interruptions in the measuring coil, or loose contacts and chafing points on the connection cable.
Permanently defective coils/connections can be determined using resistance measurement with a multimeter. The "normal" resistance is around 100 Ohms and varies by approximately +/- 15 Ohms depending on the model.

Unfortunately, electrical resistance measurement is not sufficient for a functional assessment. Loose connections or winding shorts within the coil often only become apparent sporadically during operation, making error messages in the engine control unit (e.g., readable with VCDS) more informative.

If you keep the engine idling at around 3000 rpm and, for example, tap the sensor coil with the handle of a screwdriver, engine misfires are a sure sign of a faulty sensor. Conversely, there are no failures when tapping, but there is no guarantee of proper function.