In order to understand how electronic fuel injection works you need to understand the basic concepts of electricity. The first concept that must be learned is that of "Magic Smoke."

A Native-American friend explained to me, shortly after something I was working on stopped working, that everything that uses electricity has "magic smoke. " If you let an objects magic smoke out, whatever you are working on will probably never work again. So, whatever you do, don't let the magic smoke out.

Voltage Drops

Voltage drops occur when current and voltage meet some kind of resistance. Resistance can be there intentionally, such as a light bulb, or it can be unintentional, like a bad connection.

Resistance and voltage drops have a proportional relationship. As the resistance increases the voltage drop increases, or if it is easier to conceptualize--as the resistance ncreases the voltage decreases.

This can be shown mathematically using the equation:


I: Intensity (Amps), E: Electrical Potential (Volts), R: Resistance (Ohms)

I= 6A, E=12V, R= 2 ohms

6 amps x 2 ohms = 12 volt drop.


Reference Voltage

A reference voltage is simply preset voltage sent to some component by the computer. The computer reads the amount of drop in that voltage after it has gone to the component. A reference voltage can be a full 12 volts or it can be as little as 5 volts or less. This voltage drop is used to determine the state of a given resistor.

Variable Resistors

Cars use two basic types of variable resistors--thermistors and potentiometers. The two types have similar end results but achieve those results differently.

Thermistors change resistance based on temperature. This is accomplished chemically. Thermistors are constructed of a semi-conductive material--such as silicone--that has been engineered to give a specific resistance when cold. When heat is applied, the resistance goes down. In some cases, the resistor itself will produce a small voltage by releasing electrons from the compound. This small voltage produced by the resistor can also be used to indicate temperature--such as a simple automotive temperature gauge. Thermistors are sometimes referred to as-- Negative Temperature Coefficients (NTC) or Negative Coefficient Thermistors (NCT).

Alternately, a potentiometer changes resistance mechanically. It is a large resistor--such as a spring--and three electrical contacts: A positive, a negative and a variable return contact.

The computer sends a reference voltage that can be measured in its entirety by checking the end connections. Or, by checking the variable return, the actual resistance can be measured. With this type of resistor the computer can determine airflow, manifold vacuum and position of the throttle plate.


A simple definition of Watts is the ability to do work. For our discussion purposes we will be looking at heat as a property of work. Watts can be described in the equation:

W = I x E

(W: watts, I: amps, E: volts)

As the current or the voltage increases the Watts increase. For example, a 12-volt system with initial amperage of 2 will produce 24 watts. Increasing the amps flowing through a circuit will produce more Watts, thus more heat. A circuit with 12 volts and 4 amps will produce 48 watts etc.


When a current is sent through a wire a small magnetic field develops. This magnetic field can be increased by looping wires together in such a way that the fields combine. If these fields are surrounding an iron object--such as a bar--that object will also become magnetized.

The strength of the magnet can be increased two ways. 1. Increase in the number of turns in the wire (more fields to combine). 2. Increase the current flow through the wire.

Electromagnetism drives the operation of solenoids. Electricity magnetizes the core, which is anchored so that it does not move. When the magnet is energized, it pulls the plunger toward it, creating a mechanical motion that can be put to work.

Starter Solenoid

Fuel Injection Systems

Modern gasoline engines use two basic types of fuel injection. These two types of injection systems operate by the same principles, yet look very different. The two systems are Throttle Body Injection and Port Injection.

Port Injection systems send fuel directly to the individual cylinders. Often these systems have various sensors located throughout the engine compartment to detect airflow, vacuum, temperature and exhaust.

Throttle Body systems usually have two injectors mounted in an aluminum housing, which is then mounted to the intake manifold. Often Throttle body systems have nearly a ll the required sensors located in the same aluminum housing as the injectors.

There are two significant differences between these systems--cost and performance. Port injection systems are far superior to throttle body in terms of performance. Since each cylinder receives fuel individually, the computer is able to control fuel consumption as well as power much more efficiently. However, to manufacture, this system is much more expensive. The individual sensors must be manufactured so that they can be mounted independently. Throttle body systems on the other hand, offer lower performance, but are cheaper to manufacture.



 Throttle Body Injection System




Port Injection System


Fuel Injection Components

The key to an efficient electronic fuel injection system is a computer. All modern injection systems (as well as some carburetor systems) utilize some type of computer. The automotive industry has many names for these computers. Depending on the manufacturer, the computer may be called:

  • Electronic Control Module (ECM)
  • Power Control Module (PCM)
  • Electronic Control Unit (ECU)
  • Brain Box (common in the used parts industry)

All of these names accurately reflect the computer's purpose. The computer receives information from the sensors. Then, based on the information received sends a signal in the form of voltage to the fuel injectors (or other mechanical components).

The concept of a computer is quite simple. However, actual implementation of those concepts is not so simple. The computer distributes battery voltage through a complex system of resistors and computer chips. The "input" is merely voltage and current that the computer interprets based on where that "signal" is coming from. For example, when a component sends voltage (or does not send voltage) that is outside acceptable range the computer will store a trouble code, which in turn often triggers t he "check engine" light.

Modern computers can actually compensate for faulty components by extrapolating needed input from other sources. For example, if the air flow meter malfunctions, the computer will extract he information it needs to keep the vehicle operating from the throttle position sensor and the oxygen sensor. The change in performance may be slight enough that the driver will no notice the difference, but will trigger a trouble code.

A trouble code is a computer's internal system for recording errors. Scanners, used by mechanics, look for these trouble in order to diagnose a specific problem.

Air Flow Metering

Multiple airflow devices are available. The first is called a Mass Air Flow Sensor (MAF). An MAF measures airflow with heat. Two types of heating elements are used in MAF systems. One type uses a titanium wire. The other type uses a conductive heating film. Both types work the same way. The MAF generally has a built-in thermistor to measure the actual air temperature in relation to the heating element. The heating element is heated to a temperature 70 degrees Celsius above the ambient air temperature. For example, if the ambient temperature is 10 degrees Celsius the computer--using Watt's law W=IxE--will send sufficient current through t he heating element to maintain the 70 degree difference; in this case the temperature of the element would be 80 degrees Celsius. As air flow increases the element is cooled. Accordingly the module inside of the MAF housing will increase the amount of current going through the heating element. The module converts those current fluctuations into a voltage signal that is sent to the computer in waveforms. The computer uses this information to extrapolate the volume of air going through the system, then adjusts the duty cycle of the fuel injectors to match.






Another type of airflow sensor is called an Air Flow Meter (AFM). An air flow meter also has a built thermistor to measure ambient air temperature. However, an AFM measures airflow mechanically using a potentiometer.





An AFM uses a movable vane to indicate where the contact is on the resistor in relation to the amount of air passing through it.




The Throttle Position Sensor (TPS) is another device used to measure airflow. However, rather than measuring the actual airflow, the TPS measures the position of the throttle plate. This is vital for the computer to know for two reasons. First, this is how the computer knows when the vehicle is at idle and when the operator is trying to accelerate. Second, this is how the computer can determine if enough airflow will be possible in order to maintain the proper stoichiometric ratio. Again a TPS is nothing more than a potentiometer. The computer sends a reference voltage then reads the amount of voltage drop--which is determined by the position of the throttle plate.

The final air-sensing device that will be discussed here is called the Manifold Absolute Pressure Sensor (MAP). The MAP sensor is a combination of a vacuum diaphragm and a potentiometer. The MAP is attached to the intake manifold or plenum in some way, either directly mounted to the manifold or via a vacuum hose. Engine vacuum pulls on the diaphragm, which is connected, to the potentiometer. Again, via the voltage drop the computer can determine the vacuum at any given moment.

Fuel Injectors


With information from various sensors the computer can control fuel delivery to meet the needs at a given instant. A Fuel Injector is used to deliver that fuel. A fuel injector is a solenoid that opens and closes allowing fuel to pass through it. The computer controls how long the injector stays open or in some cases how quickly it opens and closes.

A basic understanding of how components work will allow testing of nearly all parts of a fuel injection system. Before any testing, a good service manual should be obtained. Many components work on the same principles, however they do not operate within the same tolerances. For example, one system may use a 5-volt reference voltage for the AFM with a resistance range of .05 ohms to 2 ohms. Where as another system may use an 8-volt reference voltage with a resistance range of 3 ohms to 5 ohms.


Below is a typical example of what would be found in a service manual.


To test the various sensors one must have the ability to recognize what type of sensor is being worked on. Once that is determined, testing is quite simple.

Resistor--Use an ohmmeter to determine if the resistor is within specifications.

Thermistor--Test resistance with an ohmmeter, apply heat to test range.

Potentiometer-- First, check resistance across the entire resistor, make sure it has continuity as well as being within specifications. Then test across the variable connection. Watch carefully for any "dead" spots. Again, determine if it is within specifications.

Solenoids--First, check resistance across the two terminals. Then ensure resistance is within specifications. If possible, apply power to see if it is actually functioning.


Conceptual Questions

  • What are some of the factors that could cause an MAF to fail?
  • What could cause an AFM to fail?
  • What could cause a TPS to fail?
  • What could cause a fuel injector to fail?


Something to ponder

Imagine that you just purchased a new CD player for your car. With your trusty test light in hand you poke around under the dash until you find a wire that has power when the key is on. You install your new stereo and it works fine. You put the dash back together and start your car. As you are pulling out of the garage you turn on the stereo, and the car dies. It turns over but will not start.

What could be the cause?

One possibility is that the wire spliced into was a reference voltage circuit. That circuit is designed to have a set current running through it. This same circuit is fed by the computer, which is designed to handle a certain amount of current. What happens when the load on the circuit is increased? There is a good chance that the computer has been damaged.

The question is how to avoid such mistakes. The solution is simple--wiring diagrams.






When diagnosing a problem it is necessary to perform the proper tests before buying new parts. Imagine that the computer indicates--through a trouble code--that the MAP is malfunctioning. The MAP is replaced at a cost of $150.00, yet the problem persists--now what?

Consider how a computer functions. If, for example, the resistor in the reference voltage circuit itself is defective and sends 8 volts to a 4-volt system. The voltage drop the computer reads will be incorrect. Generally speaking computers are not programmed to detect an internal malfunction. Another consideration is wiring. The wiring in a vehicle has been chosen based on a specific resistance. The wiring to the AFM must be of a certain resistance in order to maintain the proper voltage drop reading. If that wiring is damaged or has a bad connection the computer may interpret the increased voltage drop as a defective component.

New and used electrical parts are non-returnable in most cases. That means buying a new MAP for $150, "just to try it" becomes very expensive very quickly. Individual components should always be tested before replacing.


Modern fuel injection systems seem very complicated. However, with a little bit of knowledge and time spent analyzing, they are not as terrible as they look. Nearly all the components in an entire vehicle utilize the same principles. Understanding these principles is key to understanding how to diagnose and repair a car.

Some of these concepts are not easily understood the first time through. The key to working with electrical components is to take a little time and think about how they work, once that is clear then testing should not be a problem.