Circuit of a PWM-controlled valve:
The two images below show the inside of an engine control unit. The covers have been removed. In this section we use a diagram and the connections to show an example of the circuit of a PWM-controlled valve. First, take a look at the top and bottom of the circuit board.
The PWM-controlled pressure regulator is located on the high-pressure line of the common rail. The image below shows the solenoid valve that is opened by a PWM signal. Next to it you can see an overview of the common rail system.
The diagram below is of a 3.0 common-rail diesel engine (VAG). We look up the component code of the fuel pressure regulator: the N276.
The purpose of this fuel metering valve is to regulate the fuel pressure in the rail. In this engine the pressure varies between 300 and 1600 bar, depending on the operating conditions.
The N276 receives a supply voltage on pin 2 (grey) that is equal to the vehicle’s system voltage (between 13 and 14.6 volts with the engine running). Pin 1 is connected via a brown/white wire to pin 45 in connector T60 on the ECU.
When the ECU switches the valve to ground, current flows through the coil. In that case the valve is energized and opens. When the ECU interrupts the ground, a spring in the fuel metering valve ensures that it closes again. By doing this in very quick succession and varying the period during which the valve opens and closes, we can speak of a PWM control.
We will examine the circuit of this PWM control using the diagram below and measurements in the connector and on the ECU circuit board. How are the components actually connected? How are they visible on the circuit board? And what are the components used for? This will become clear in this section.
The image below shows both the inside of the connector and the underside of the circuit board. Using measurements with the multimeter, the solder joint on the circuit board to which connector terminal T60/45 is connected has been located. These solder points are indicated by the purple arrows.
The negative terminal of the fuel metering valve (1) is connected via connector terminal T60/45 to the drain of the FET and the anode of the freewheeling diode. The red lines in the images indicate the solder connections. For clarity, an enlarged view of the above image is shown here.
The source is connected to ground via connector terminal T94/1 and is indicated by the blue line.
The microprocessor switches the FET on and off by applying a control voltage to the gate of the FET. The orange line shows the connection between the microprocessor and the gate of the FET.
At the moment the gate receives a control voltage from the microprocessor, the FET conducts and current can flow from drain to source and therefore also through the coil. Due to the magnetic field, the coil is energized and the fuel pressure control valve closes.
As soon as the control voltage on the gate disappears, the connection between drain and source is broken. The freewheeling diode ensures that the inductive current, resulting from the residual energy in the coil, is routed to the positive supply. This ensures a gradual current decay and prevents an induction surge from occurring.
The diagram with the fault shows the transition resistance in the positive wire of the coil. The red arrows indicate the direction of current flow with the FET switched off. Thanks to this circuit, the current can decrease slowly.
Now that we have gone through the circuits and components of the fuel pressure regulator, we can also look at the oscilloscope images when we are dealing with a fault. How do we recognize a fault in a PWM signal? What are the consequences for the operation of the pressure regulator? You can read this on the page duty cycle and PWM control.
