Pulse-Width Fuel Pump Diagnostics -

Pulse-Width Fuel Pump Diagnostics

You will see only one fuel line going to the fuel rail on most late-model vehicles. Instead of using an engine vacuum routed to a pressure regulator under the hood, returnless systems use engine data and vary the speed of the electric pump to meet fuel pressure and volume requirements without having to bleed off and return it to the tank when the engine does not need it.

You will see only one fuel line going to the fuel rail on most late-model vehicles. Instead of using an engine vacuum routed to a pressure regulator under the hood, returnless systems use engine data and vary the speed of the electric pump to meet fuel pressure and volume requirements without having to bleed off and return it to the tank when the engine does not need it.

Modern pumps do not vary the voltage to the pump; rather, they pulse the power to the pump. Instead of pumping at 100% for 100% of the time at 12 volts, the pulses are changed so the pump supplies enough pressure and volume for the different conditions. This strategy means less current going to the pump, which means less heat and a longer life for the pump.

Managing the speed of the pump electronically instead of using a mechanical pressure regulator on the engine simplifies the fuel system by getting rid of the return line and vacuum pressure regulator under the hood. Other benefits include lower fuel temperatures in the tank and less of a chance of vapor lock.

A driver in a module is turning the pump off and on to regulate speed.

How does a pulse-width modulated pump work?
The driver for the fuel pump switches 12 volts off and on to control the speed of the fuel pump. The longer the time the voltage is turned on, the faster the pump will turn and more fuel will be pumped. This is done with a solid-state transistor and there are no moving parts inside a module. When the circuit driver fails, the pump will stop working.

How is it measured?
The ratio of “on to off time” is called the duty cycle. The duty cycle is the fraction of one period in which the voltage is on compared to the off time. A constant 12 volts is a 100% duty cycle. If you had a pump with a 1-second cycle and the driver switches the voltage on for 0.3-seconds, it would be a 30% duty cycle.

Don’t confuse duty cycle with Hertz (HZ). Hertz is measured on a sinusoidal wave of alternating current, where pulse-width voltage waves are square and represent a positive voltage switching.

Measure the voltage as DC and AC voltage. If they are similar, it is a sign you are dealing with a pulse-width modulated fuel pump. If the AC volts were zero, it would be just DC voltage.

How does the fuel pump look on a meter?
A full-feature voltmeter is critical to diagnosing modern fuel pumps. The meter should be able to measure the duty cycle and have a min/max function.
If you use a meter and back probe into a pulse-width modulated fuel pump circuit, it might display only 3 to 7 volts in DC volt mode. Also, the last two digits on the display might be jumping around. If you switch your meter to AC voltage, the voltage reading will be close to the DC voltage.

Back probing with pins is not a good idea due to the corrosive road salt and debris that can build up on top of the tank. Break out leads are recommended.

To the untrained technician, they might assume the power being delivered is not sufficient to run the pump due to the 3- to 7-voltage reading. The truth is the 3- to 7-volts is just an average of the high and low peaks of the pulse powering the fuel pump. This is what the Root-Means-Square (RMS) feature is designed to do when reading some DC and AC voltages and displaying an average.

If the meter has a min/max function, you can see a peak voltage that should be close to the battery voltage and low voltage would be between 0 and 0.5 volts. This is a sign that the voltage and driver powering the fuel pump are healthy. If you saw a max voltage lower than 12 volts, you might want to check for high resistance in the circuit.

A meter with a min/max function can show if the pump is getting full battery voltage when the power is switched on.

If the meter has a duty-cycle function, it is possible to measure the on and off times and determine the ratio and display the duty cycle as a percentage. The duty cycle percentage can be compared to the scan tools data and the PIDs for commanded fuel pump speed or duty cycle.

The duty-cycle measured with a meter can be compared to the scan tool data to see if there is a possible glitch like a short to power.

With a scope, it is possible to graph the switching of the duty cycle. The scope can be used to spot glitches and the dropping out of the pulse-width modulated signal. You should be able to see if the driver is switching between 0 and 12 volts. The scope can also monitor the current consumption of the fuel pump with a current clamp.

The waveform should have a clear square wave. If you have a fast scope with a high sample rate, you can see small ripples when the transistor turns the power on or off. As fuel demands increase, the upper part of the wave will increase in length.

Scan Tools
A scan tool is essential to solving pulse-width modulated fuel pump problems. With a scan tool, you can observe the commanded duty cycle or speed for the fuel pump.

If you can graph the PIDs for the commanded speed/duty cycle, fuel pressure and engine speed, you can get an indication of the health of the fuel pump. If you hit the throttle, the speed/duty cycle of the pump will jump upward, while the pressure remains within an 8-psi range. A healthy fuel pump’s pressure reading will not fall below the specified minimum range. If you see the fuel pressure dropping or taking an extended period to recover after a load is put on the engine, the pump is worn out.

Scan tools can also capture codes for the fuel system that can hold freeze-frame information that can show fuel pressure readings.

Article courtesy Underhood Service.

You May Also Like

Differences in Brake Fluid Matter To Cars – And Owners

Though DOT 3, 4 and 5.1 brake fluids are technically compatible with each other, when mixed, fluid properties can change.

Trying to match the brake fluid to a product that is available can quickly have your head spinning when trying to service a late-model BMW, VW or Mercedes. Finding the correct specification can be ambiguous because some manufacturers provide a part number instead of a specification in the service information or owner’s manual. Brake fluids can have several confusing names like LV, Super or DOT 4 Plus.

Brake Hardware

Specific hardware does not do its job if it is left in the box.

Throttle-By-Wire Diagnostics

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

Spotting Brake System Failures By Inspecting The Old Pads

The main culprit of friction material separation is typically corrosion.

Diagnostic Strategies For Stop/Start Systems

This function of the power management system uses several modules to decide when the engine needs to stop and start.

Other Posts

Brake Pad Ecology

The leading source of particulate emissions come from brake pads and tires.

Lightweight Brake Components

Engineering teams have certain goals that they need to meet or exceed.

Power Steering Pull

Every driver has a different threshold for a pull.

Fuel Pressure Diagnostic Service

Use a scan tool that can look at special direct fuel injection parameters and perform bidirectional tests.