Look at this, I have my meter hooked up to this fuel pump, and it only gets five volts when the vehicle is idling! Something must be wrong. But, when I hit this button for the minimum and maximum voltage, I get zero volts for the minimum and 13.85 volts for the max. If I hit this percentage button, I get 35.6 percent. What is going on here? Shouldn’t it be 12 volts or more all the time?
No, this fuel pump in this car uses a pulse-width modulated voltage signal to drive the fuel pump in the tank. This type of pump uses voltage switched on and off to control the speed of the pump and the pressure it produces.
Pulse-width modulation works like this. The longer the power is left on compared to the off time, the more fuel pressure is generated by the pump. The amount of on time compared to the off time can be expressed as a percentage. That percentage is often called a duty cycle or commanded percentage or rate on a scan tool. This is what it looks like on a scope. You can see the square wave as the voltage switches on and off.
With the engine at idle, the duty cycle on the scan tool and the meter is 35.6 percent. Since the injectors have a very short open time, the pressure at the rail remains at 58psi.
If we revved or increased the load on the engine, the duty cycle increases to keep the pressure at the fuel rail between 55 to 60 psi. So on the meter, the voltage might read higher, but the minimum voltage is still zero and the maximum is still 13.85.
How do you use this knowledge for fuel pump diagnostics? Using a scan tool that can graph the fuel pressure and duty cycle together, you can chart the health of the fuel pump. The fuel pressure should remain in a constant range specified by the service information. What will change is the duty cycle of the fuel pump as you accelerate. If the fuel pressure drops below the specified range while the duty cycle is increasing, chances are the fuel pump needs to be replaced.
This video is sponsored by Carter Engineering.