What you'll learn:
- Where you will see BLDC fuel pumps
- Proper PWM frequencies
- How to diagnose three-phase BLDC issues
- Common causes of failure
You might consider a multimeter to be your best tool for diagnosing electrical circuits that power in-tank electric fuel pumps. Those pumps are powered with two wires – 12-volt ignition and ground...right? Not necessarily! If you miss reading this article you might just experience a misdiagnosis of the growing number of newly designed electric fuel pumps. We’ll give you a close look at three-phase brushless DC (BLDC) fuel pumps and pass along the knowledge required to diagnose their complex control circuits.
Three-phase BLDC?
We’ve all heard or used that phrase “two’s company, three’s a crowd.” Traditional low voltage DC (12-volt) electric motors use two wires to power them. One wire carries the 12 volts (positive) and the other wire (or metal bracket) carries the ground (negative) side of the circuit. Today, that technology is on a road to obsolescence. Increased requirements for fuel efficiency are the primary reason behind the shift to these more expensive (and complicated) three-phase brushless (BLDC) fuel pumps. Side benefits are longevity with no brushes to wear out and robustness to corrosive fuels (E85), thanks to no armature commutator for brushes to ride on (Figure 1).
Starting around 2011, select European vehicles began using three-phase BLDC fuel pumps in place of traditional (and simpler to diagnose) two-wire fuel pumps. By 2018, some Asian OEMs followed suit. North American car and LD truck manufacturers joined the three-phase BLDC fuel pump club on select models in 2019. Even some newer LD truck diesel applications are going with the three-phase BLDC for in-tank lift pumps, as well. Before we go on to the newer design, let’s make sure we fully understand the “lower tech” fuel pumps that are controlled with “high tech” modules.
Understanding fuel pump duty cycle, frequency, and the mysterious world of EMI
I’ve always had better luck diagnosing and repairing things that I can understand (at least partially). If I don’t understand it, I either have to have a photographic memory to keep track of all the possible fixes for every vehicle or rely on factory diagnostic trouble trees. My memory isn’t perfect, and trouble trees certainly aren’t perfect either! So let's explain some established electronic theories used before the new three-phase BLDC pump controls.
Even before the adaptation of Gasoline Direct Injection (GDI), vehicle manufacturers began moving away from simple relay-powered in-tank fuel pumps. Removing the mechanical/vacuum adjusted fuel pressure regulator from the rail in favor of a returnless fuel system did not eliminate the need to vary the fuel pressure applied to SFI injectors. Most fuel pump control modules (FPCMs) use either a data bus like CAN or LIN, or digital pulse width modulation (PWM) input from the PCM to request the proper average voltage output to the fuel pump (Figure 2). Varying the pump’s input voltage varies its speed. Pump speed creates the correct fuel pressure. A mechanical pressure regulator built into the modular electric fuel pump assembly keeps the fuel pressure from exceeding a safe limit and allows the fuel to recirculate within the pump assembly/fuel tank. The FPCM’s output is a varying duty cycle to control the speed of the in-tank electric pump. This allows for more precise fuel pressure control. A variable duty cycle is a much more efficient means of controlling the speed of anything (lights, solenoids, motors) compared to dropping the voltage to where you need it via power resistors. Resistors that drop voltage can also suffer from heat damage and eventually drop dead! Solid-state controlled PWM is the way to go!
V-8 vs. V-6 engines help explain the PWM high-frequency mystery
Ford has been varying fuel pump speed with their FPCM outputs on SFI models for several years (Figure 3). Ford FPCMs use a varying duty cycle on a fixed frequency signal wire from the PCM to the FPCM. The FPCM then sends an output with a different varying duty cycle to the fuel pump’s power feed. Variable PWM in/variable PWM out; simple, right? The FPCM’s fuel pump power output has a much higher frequency compared to its input from the PCM (Figure 4). Why? Technically, it’s all about calculating the PWM frequency based on the electric motor's time constant, inductance, and low-pass filter effect. The correct PWM frequency can help the motor to benefit mainly from the DC element of the PWM signal. If that bit of techno-babble is hurting your head, let's apply an engine analogy that will help make the point much clearer.
Eight-cylinder engines typically run smoother than 6-cylinder engines (of like designs) due to the higher frequency of power contributions afforded by their extra two cylinders (more combustion events per cycle). Also, a lower idle RPM (a.k.a. frequency) on any engine will result in fewer power contributions per second and therefore make the engine idle rougher. With electric motor PWM outputs, the higher frequency smooths out those motor speeds, too (Figure 5)! As with any rough running motor (engine or electric fuel pump), if the noise goes up the reliability goes down.
What’s the big deal about three wires going to a fuel pump?
All the fuel pumps we’ve covered to this point have been two-wire models. Even though modular pump assemblies may add to that number of wires with a float sending unit and fuel vapor pressure sensor circuits, the actual fuel pump motors themselves have been simply two wires – positive and negative (ground). If you've worked in the hybrid/EV world of high voltage electric motors, you know they are three-phase AC-powered motors. The three wires to these motors receive high voltage AC (Alternating Current) from the vehicle's inverter (which changes high voltage DC into high voltage three-phase AC). These motors can also create three-phase AC power when the vehicle is coasting or braking (called regenerative braking). Similar (but smaller) three-phase AC motors are used in most hybrids (and all EVs) to run the air conditioning compressors. That’s old news to most techs, but did you know there may be numerous other "low" voltage DC applications of three-phase motors in many of the vehicles you work on? Probably not!
I say that because most of the time, these motors have their control modules built onto the 12-volt three-phase BLDC motor assemblies. Many vehicles with electric water pumps, advanced electric radiator cooling fans, and HVAC blower motors have been using these three-wire/three-phase BLDC 12-volt motors for years (Figure 6). If the schematic shows a single non-serviceable assembly containing the motor and the motor's control module, you don't need to know anything about these motors. They run or they don't. Their control modules either have the correct power, ground, and signal circuits (bussed or PWM) or they don't. Diagnosis is business as usual.
Diagnosing three-phase BLDC fuel pumps
A minority of three-phase BLDC fuel pump applications include an integrated FPCM. On those, just diagnose as you would with any other module with power, ground, and signal circuits. If the FPCM is remote from the modular pump assembly, as are the majority of three-phase BLDC pumps, you'll have three wires carrying the three phases along with an RFI shield wire (Figure 7).
The three-phase BLDC harness between the FPCM and the in-tank module fuel pump assembly may only be a couple of feet (Chevy Equinox) or the harness may be several feet in length as with the newer Toyota Sienna (hybrid) minivan. The longer the harness, the more things can go wrong electrically. The resistance between each phase is very low, so don’t even think about employing a PowerProbe between a pair of the three-phase connections (Figure 8). When diagnosing no starts/no fuel pressure, most FPCMs will set a DTC to help give you a clue but that’s never guaranteed. Checking for voltage supply to the pump/doing voltage drop testing becomes a whole new world on these three-phase BLDC fuel pumps. The typical multimeter will be almost useless unless it can show frequency/duty cycle (Figure 9).
What goes wrong with BLDC fuel pumps?
The root causes of failures vary. Fuel contamination (any pump’s enemy) along with mixed-up phases (wiring repair induced), faulty FPCMs, and probing (jumping) with power and ground can cause no starts, DTCs, and pump failures. These pumps are unlikely to wear out (no brushes) and they use less current (their main advantage). Misdiagnosis due to lack of training will inevitably be the main enemy of these fuel pumps. On Toyota and Lexus models using three-phase BLDC fuel pumps, the factory scan tool (Toyota Techstream) works well with a laptop, factory subscription and J2534 pass-through device. That factory tool allows for bi-directional requested pump speed control (so you can watch the results of the fuel pressure PID/gauge reading). That tool can request a single-phase activation, enabling you to monitor with a meter or scope as each phase is activated with a pulse. Overall analysis with a multi-channel lab scope will show strange and new voltage and current ramping patterns we’ll be comparing notes on in various technician forums for many years to come (Figure 10).
