Diagnosing Pulse-Width Modulated PWM Fuel Pumps

Diagnosing Pulse-Width Modulated PWM Fuel Pumps

Here are five things you should know about pulse-modulated fuel delivery systems.

pwm fuel pump control module

Just this summer, I experienced a situation in which the fuel pump had been replaced four different times on a single vehicle with no result because the various repair shops attempting to repair the vehicle didn’t understand how to diagnose a pulse-modulated fuel delivery system. Instead of a defective fuel pump, the problem was caused by a defective electronic fuel pump control module (FPCM). Here are five things you should know about pulse-modulated fuel delivery systems:

1. Supply and Demand

Conventional electric fuel pumps constantly operate at 100 percent capacity, which wears the fuel pump out relatively early in the vehicle’s life. To remedy this situation and establish more accurate fuel control, engineers designed a fuel pump that runs on electrical impulses rather than a steady current. Running the fuel pump on electrical impulses not only allows the Powertrain Control Module (PCM) to control fuel pump speed and pressure according to engine demand, it also extends fuel pump life and reliability.

2. System Identification

Pulse-modulated systems can be identified because they generally have no external Schrader valve port for testing fuel pressure. Instead, when the technician connects his professional scan tool, he will usually see fuel pressure as a data display transmitted from an electronic fuel pressure sensor located on the fuel injector manifold or “rail.” In addition, he will observe the “duty cycle” or the length of pulse modulation available at the fuel pump.

pwm fuel pump electrical probe

3. Pulse Modulation Principles

The basic operating principles of pulse-modulated fuel pumps are relatively simple. When the PCM senses that the engine is demanding more fuel, it commands another key component, the fuel pump control module (FCM), to increase the fuel pump speed by increasing the length of the electrical pulses being sent to the fuel pump. Similarly, when the engine’s fuel demand decreases, the length of the electrical pulses sent to the fuel pump is decreased. Electronically managing the fuel pump speed and fuel pressure through pulse modulation is a convenient method for helping the fuel injectors efficiently meet minimum and maximum fuel demands.

4. Fuel Delivery Diagnostics

Most failures in a pulse-modulated fuel system are instantly recognized by the on-board diagnostics built into the engine’s PCM. The on-board diagnostic process is relatively simple. When the PCM senses that a longer-than-usual pulse width is required to meet a commanded fuel pressure value, it stores one or more DTCs in its diagnostic memory indicating that the fuel pump isn’t performing at its commanded level. Keep in mind, of course, that common failures such as a restricted external or in-tank fuel filter or pinched fuel line also can store these and other trouble codes in the PCM’s diagnostic memory.

5. Selling Pulse-Modulated Parts

Pulse-modulated systems can be accurately diagnosed only by using a professional scan tool to recover DTCs and perform specified testing procedures, such as bi-directional pump control.

Article courtesy Underhood Service.

You May Also Like

Throttle-By-Wire Diagnostics

On older throttle cable systems, the carburetor or fuel injection system reacted to the throttle angle.

Throttle by wire was once a mystery. It was first used on a German V12 to synchronize the two banks of cylinders with two throttle bodies. Over the past 20 years, throttle-by-wire has become the standard technology on all gasoline engines. So, what can go wrong? A lot!

On older throttle cable systems, the carburetor or fuel injection system reacted to the throttle angle. When the driver smashed the throttle to the floor, the air flowing through the venturis would cause more fuel to be sucked into the engine. On older fuel injection systems, the injector open time would increase. The opening of the throttle was never synchronized with the fuel. As a result, the engine might experience rich or lean conditions when the throttle is opened or closed. These throttle “tip-in” conditions can increase emissions, and the driver might notice the lack of power.

Brake Rotor Quality Check

The brake rotor might look fine, but the problem could be internal.

PCV System DTC

PCV systems are far more complex now, but they are also much better at controlling crankcase pressure.

Transmission Valve Body Replacement

Learning how to perform drivetrain diagnostics and “in-the-car” repairs is important.

Adjusting Parking Brakes

You should never estimate when it comes to parking brake adjustments.

Other Posts

What’s All The Noise About? (VIDEO)

Inspect the vehicle and review all the things that can cause a noisy fuel pump. Sponsored by Carter Fuel Systems.

Mercedes-Benz Alignment

Here are some tips and tricks that should help you to work on just about any Mercedes-Benz from the past few decades.

High-Pressure GDI Fuel Pumps

When solving a fuel pump issue, the scan tool is your best friend.

Diesel Turbocharging Basics

There’s a reason diesels typically utilize compound turbo setups instead of twin turbochargers.