The fuel system you’ve become familiar has a pump that flows constantly, providing fuel to the engine with the same flow at all times. It’s designed with a return line that sends unused fuel back to the fuel tank to be used later. Since around 2004, domestic car manufacturers have begun using a different style of fuel pump system with fuel injection systems known as pulse width modulated fuel pumps, or PWM fuel pumps.
What is a Pulse Width Modulated Fuel Pump?
To understand pulse width modulation requires basic electrical understanding. In electric pumps, there are typically two settings – on and off. Simply put, a pump is on when it receives electrical power and it’s off when there is no power. When it’s on, it’s also known as 100% duty cycle.
Pulse width modulation is vastly different. Instead of being constantly powered at 100% duty cycle, PWM fuel pumps have a varying fuel pressure due to controlled electrical operation. With the ignition key on, full system voltage is supplied to the fuel pump. A module, either the engine control module or a PWM module, determines the desired fuel pressure at the engine and pulses the negative side of the fuel pump circuit to provide the desired amount of fuel.
Keep in mind, the pulse width does not change the voltage supplied to the fuel pump – that value remains the same regardless of the duty cycle. Instead, think of pulse width modulation like an on-off switch. If 30% duty cycle is required, ground is supplied to the fuel pump at full blast for a very short period of time, and measured in milliseconds. If 60% duty cycle is requested, the pulse width is twice as long as the 30% duty cycle, yet both are at the same voltage.
The Complexities of a PWM Fuel Pump
In a PWM fuel pump system, other components must perform with exacting precision for the system to operate. A fuel pressure sensor on either the fuel rail or on the fuel line measures the real-time fuel pressure, and a control module monitors that reading. The desired fuel pressure is determined elsewhere, influenced by the vehicle operation.
Each of these components must be operating with complete accuracy for the PWM fuel pump to supply the correct amount of fuel. One defect at any point in the system will cause fuel-related problems like running lean, misfires, or a fuel leak.
Why Do Manufacturers Use Pulse Width Modulated Fuel Pumps?
It’s not a fluke that carmakers have moved from traditional fuel pump designs to a pulse width modulated system. At its core, the reason for the switch has to do with reliability.
PWM Fuel Pumps Last Longer
Modern vehicles with in-tank fuel pump modules are expensive to service. A fuel pump that’s operating at 100% duty cycle at all times will undoubtedly fail sooner than one operating at a lower duty cycle. PWM fuel pumps offer increased reliability for carmakers, lowering the cost of warranty repairs and providing higher customer satisfaction due to lower failure rates.
The trade-off for PWM fuel pumps is on the electrical side. Additional sensors and a control module are potential failure points, although the expected failure rate is much lower than fuel pumps operating at 100% duty cycle.
Fewer Potential Fuel System Leaks
With traditional fuel pump designs, a return line from the fuel rail sends unused fuel back to the fuel tank. This line is also a potential failure point, either due to a weak seal, weak clamp, or a corroded line.
Doing away with a return line in a pulse width-driven system reduces the likelihood of fuel system leaks at the engine and near the exhaust system. That means a drastically reduced risk of fire from fuel spraying on hot surfaces.
Because the fuel pressure fluctuates based on demand, the fuel systems seals are under less strain. Leaks are avoided and seals last longer in vehicles using a PWM fuel pump.
Lower Power Consumption
A major challenge for car manufacturers today is reducing electrical draw. Because virtually every system in a vehicle is controlled by or consumes electricity, the demand on the alternator and battery is much higher. Not only does that lead to premature failure and replacement on these parts, there is a good chance that electronics may not function properly or may experience intermittent operation when electrical demands are high.
With PWM fuel pumps, less power is required to drive the fuel system. That leaves more available for other high-demand operations such as air conditioning, blower motor, and lighting.
Challenges with PWM Fuel Pumps
You should know that typical fuel system diagnosis equipment and techniques wouldn’t work with PWM fuel pumps. Most vehicles will not have a Schrader valve on the fuel rail to check fuel pressure and a digital volt-ohmmeter (DVOM) won’t determine inaccuracies like traditional systems.
Diagnosing a pulse width modulated fuel pump must be done by a live-monitoring scan tool or a digital storage oscilloscope. Using other methods will often result in a misdiagnosis and needless component replacement that does not solve your problem.