Much has been said and done with the modern fuel supply system, particularly the fuel pump. Today's supply pumps are usually in-tank mounted to provide good priming characteristics and reduce vapor lock. Fuel handling is improved as fuel is pushed directly from the tank. Many systems are in use today, but they can all be divided into return and returnless.
This supply system is a closed-loop system as fuel is picked up from the in-tank mounted fuel pump strainer sock and pushed through a steel line into an engine mounted fuel rail. The fuel then moves through a rail-mounted pressure regulator and back through a steel transfer line to the tank.
Although this system has been used for years, it will more than likely go away as evaporative emissions requirements tighten. Fuel-return contributes to vapor formation, as any fuel flowing through a return system absorbs heat from the rail in the engine compartment. This fuel is allowed to depressurize in the return line and will raise the bulk fuel tank temperature.
Returnless fuel delivery systems
With tighter emissions standards, more of the 'returnless' fuel supply systems are being produced. Using a demand system with regulator mounted inside the fuel tank, fuel pressure is regulated inside the tank instead of on the engine-mounted fuel rail. This does a number of things, one of which is to lower actual hydrocarbon emissions caused by the heated fuel returning to tank from the engine. It also would require the OEMs to only use a single fuel supply line to the engine compartment.
Vapor lock with no return is solved as the electro-mechanical injector units open to allow fuel to travel through the system. Look for this to become an industry standard, because almost all OEMs are using at least one version of this system today.
A typical return-type fuel system consists of a tank-mounted electric fuel pump to deliver the fuel to the fuel rail. Filtration is started with a 50- to 70-micron tank-mounted sock designed to filter the fuel and act as a water separator. Fuel is pulled through this sock up into the fuel pump to a 10- to 20-micron frame-mounted fuel filter and into the fuel rail. At the end of the rail is a pressure regulator designed to maintain a constant fuel pressure at the injector. Each injector has a 10-micron final basket filter to protect the injector. Excess fuel above the regulated volume, usually 50 to 80 percent, then is returned to the fuel tank.
Filtration varies from model to model and brand to brand. For example, a late General Motors (GM) Gerotor fuel pump strainer is 33 micron, which is much finer than the usual 70-micron filter. This finer filter protects the gerotor pump from contamination, as clearances on this unit are much smaller than the usual vane throttle body injection (TBI) pump.
There are many fuel pump/module designs, but the basics apply to most all systems. Test procedures differ very little from system to system. Total fuel pump testing includes fuel pump current draw, pump rpm, fuel pump waveform (Scope test); fuel pressure; and fuel volume (in U.S. gallons per minute [GPM]).
Fuel pump testing with a current probe
Why use a low amp probe? Automotive engineers have used the low-current probe for years to measure actual current of circuits and design features. Only recently have these probes become part of the automotive technician's toolbox. When used with a digital storage oscilloscope (DSO), the probe will help a trained technician quickly analyze an electrical fault and perform a 'real world' failure analysis, therefore preventing costly comebacks. This is all done with an inductive hookup without disturbing the circuit.
Current vs. voltage: Technicians routinely view voltage signals either with a DSO or graphing multimeter (GMM) to ensure proper dynamic potential energy within the circuit or component. Now, current will be graphed vs. time and a new understanding of circuit analysis will be required.
Remember from basic electrical theory that one volt is required to push one amp through a one ohm resistor. Ohm's Law, as most technicians should recall, states the relationship between these three elements and tells us that as resistance increases, current decreases inversely. However, Ohm's Law by itself does not completely explain many automotive circuits, including fuel pumps, injectors, coils and semiconductor controls. An electromechanical device is more complicated due to mutual- and self-induction, terms that help define how these devices work. Let's begin with motor basics:
Amps = Volts x Resistance
Although the working resistance of an electrical motor is difficult to figure (based on electromagnet operation and electromotive/counterelectromotive force), it is still a good starting point for actual current draw.
Electromotive force (EMF) equals the force that, in fact, causes current.
Counterelectromotive force (CEMF) equals the forces that resist the current once started.
For example, a battery or generator may provide the EMF and its strength is measured in volts or voltage, and a magnetic field created by the spinning motor may offer the CEMF. This is why an electrical motor doesn't just keep gaining rpm with current supplied.
Higher fuel pressure, such as multipoint fuel injection (MPFI), equals higher motor loads that, in turn, require higher amps.
Lower fuel pressure, such as TBI, equals lower motor loads and use fewer amps.
Fuel pump basics
Diagnosing fuel pump faults begins with understanding how a motor operates. Automotive pumps use horizontal commutator bars and vertical bars as well, with the brushes contacting from the side. A commutator bar is the brass/copper segment of the commutator that connects each end of a field winding. They usually are an even number as pairs are most often used.
Armatures and EMF
The soft steel laminate cylinder is mounted on a shaft so that it can rotate, converting electrical energy from the battery into mechanical energy, which drives the pump, by using current carrying coils of copper wire embedded longitudinally. The current is led in and out of the device by way of spring-tensioned graphite brushes riding on the auto-switching device known as the commutator. Contrary to popular belief, automotive pumps are DC devices, meaning the current is direct and always flows in the same direction with respect to the control circuit. Also, the resistance of the pump is not in the armature windings, but in the CEMF generated when the voltage is applied and the armature starts to spin. This EMF opposes the applied voltage, and includes the driven mechanical device -- in this case, the pump that accounts for the suction/pressure of fuel (or whatever is in the tank).
Fields and conductors
When current flows in a conductor, a magnetic field is created that surrounds the wire. When placed next to more conductors, the field strength is increased. If this entire assembly is placed in an existing field created when two magnets are placed opposite each other, it tends to rotate so that fields align. All that is left is for the commutator to switch the current flow through the different windings. This allows the rotation to continue. Without the commutator, the device would just stop when the fields align. This would be called a galvanometer (think of an old analog meter).
There are several main reasons why motors fail. Most of the electrical failures involve the negative brush and commutator device. Since the brushes are spring loaded, commutator wear is normal. Also, the graphite brushes will wear. It is where the negative brush contacts the commutator that electrons first move from the stationary brushes to the rotating armature. (See Figure 4.) If the contact pattern is good, current flow will be higher. If contact is poor, lower current will result due to the high-resistance voltage drop across the point between the brushes, and the commutator will heat up. This increased heat causes spring and brush failure. As a note, all electromechanical devices will fail.
Testing fuel pump current draw is nothing new; understanding the results is another story. If wear decreases current, what causes an increase in current? Take for example a blower motor (which also can be tested by examining the current ramp) spinning a blower cage. If we exert a force that counters the spin of the cage -- CEMF -- the current draw will increase. A fuel pump is the same: Any force that counters armature spin increases current.
Since the pump's job is to convert electrical energy to mechanical energy, the output shaft of the armature is what is doing the work. The pump (not to be confused with the motor) draws in the fuel and compresses the liquid pushing it past and around the armature.
Designed similar to an oil pump, the fuel pump also can wear out or fail. Excessive drag in the gears will cause an increase in current and slow the rate of spin of the armature shaft. As gears wear out, the reduction of designed-in drag will decrease the current and increase the speed of the motor. This is why the pattern may look good electrically, but the output will be low. Four main types of pumps are used, individually and in combination. They are roller, gerotor, turbine and vane. The current level, pattern quality, motor speed and known good patterns for each type must be used in diagnostics.
Motor speed is determined by the strength of the magnetic field and the applied CEMF, not by the number of bars. The field strength is adjusted by the number of windings used and the strength of the magnetic field generated (magnets). Different types of pumps have different drag -- CEMF -- on the output shaft. To determine the motor speed, first capture and analyze the pattern looking for matching 'humps' which represent the same bar, and count the bars in between; then use the equation below. Typically automotive motors use three-, six-, eight-, 10-, 12-, and 14-bar armatures.
Motor speed equation:
RPM = 60,000 4 time in milliseconds (ms) for one armature revolution
There are many good ways to test fuel pump current draw and several poor ways. Let's look at each type.
Good, from accurate to convenient:
Measuring the current at the pump using its own wiring and controls. Where access is easy, this is the preferred method since it takes into account oil pressure switches, relays and the wiring faults.
Using the fuel pump fuse socket. Consult a wiring diagram to ensure no other loads are on the circuit. Also, use a fused jumper wire being careful not to short the terminals together.
Powering-up the test connector, if the car is equipped with one. Several manufacturers have used these for years, including GM, Mazda and Hyundai. Many are installed on the output side on the power relay, but this does not take into account the operation of the relay. A large voltage drop across the contacts would lower source voltage, thus lowering current and slowing the motor, but could be missed by just powering-up the pump and not testing it as it normally operates. Also, pump speed is changed because of the difference in static vs. dynamic charging voltage.
Static voltage (engine off) would be somewhat lower than the dynamic voltage (engine running with alternator charging), and would cause a different operational speed of the motor.
Unplugging any connector and powering the pump through a jumper wire or a digital multimeter (DMM) set on amps. First, using a DMM is a last resort only. A locked pump, which is not uncommon, can give a good reading while the armature is not spinning. Second, poor wiring or controls will be bypassed, delaying accurate diagnosis.
Direction is needed and provided
Many service technicians just starting with a current probe and a lab scope are somewhat happy with this new form of circuit analysis. But care must be taken as the waveform only provides direction for the additional testing that must take place.
A quick non-intrusive current probe test on the power feed of a fuel pump quickly shows:
Pump current draw.
Pump brush contact and motor condition.
Pump working rpm, or speed.
Now, understanding these important factors with regard to the pump, technicians must go a step further when any and all of these items are seen to be out-of-range or displaying a problem. This next level is an investigation of the fuel supply system supply pressure and volume.
Fuel pressure is improperly used quite often on injected vehicles with most technicians simply hooking up -- through a restrictive Schrader valve -- and reading the pressure with a gauge. Specs sometimes are difficult, for example, stating 35 to 45 psi is considered correct. One might as well state that some is needed!
When a late model engine will shift fuel trim for a 1 to 2 psi difference, the fuel pressure becomes much more important. All fuel pumps have an 'as designed' current draw and flow rated in U.S. gallons per hour (GPH) or GPM. With the current cost of modular fuel pumps and difficulty of installation, we must be more than accurate in our analysis of a defective fuel pump unit.
A case study would be a 1987 Chevrolet Camaro 305 CI V-8 VIN 'F' that was sent to us with a driveability complaint and a long history of problems. Believing the problem to be a lean mixture -- high, long-term fuel trim (blm) numbers and a lean O2 sensor -- we tested the fuel pump amp draw and current waveform, and didn't like what we saw. At this point, it was time for further analysis and a volume meter was hooked up to the vehicle.
Testing the unit, we knew that the specification given for a rollervane in-tank pump (Airtex/Master catalog FP99) is 45 psi (no vacuum at regulator) and flow specification is 40 GPH or 0.67 GPM.
The first test for the pump is open-ended with fuel flowing into the tool from the inlet to return with no rail or regulator attached. This unit flowed 0.67 -- exactly spec -- with no resistance to flow from regulator. Very good!
Lines were then hooked up to allow the flowmeter to measure flow in series with the injector rail and regulator allowing return to the tank. This test showed a flow of 0.35 GPM with the added resistance of the regulator. Regulated flow specification usually is 50 to 60 percent of open flow, based on return to the tank. This more than makes up the reserve needs of the engine's fuel demands.
We now knew that we went the wrong direction with this lean fuel problem, as this pump was very capable of supplying fuel to this engine. This vehicle was lean at speed as well as idle. A normal fuel consumption for 305 CI is just 30cc per minute at idle. Again, direction is needed and provided with the proper tools and training.
Further discussion with the customer (after discovering the entire engine assembly was yellow) revealed that the engine in fact had been replaced with a new engine unit. The actual problem was the fact that shop 'A' had installed a 350 CI engine and used the old 305 CI injectors.
Checking our flow specs we found that the 305/5.0 injectors flowed 50 mils on our flow bench while the 350/5.7 injectors flowed 60 mils. That explained the high blm and lean O2 on acceleration.