DIY Auto Service: ECM PCM Automotive Computer System Operation

Updated on January 23, 2019
Michael M Thomas profile image

Michael Thomas, automotive and diesel technician, is the author of the McGraw-Hill textbook "Truck and Trailer Systems."

Computer Components

The computer contains memory, voltage regulators, microprocessors and output drivers.
The computer contains memory, voltage regulators, microprocessors and output drivers.

Computer System Introduction

Today’s vehicles have computers to monitor or control almost every system on the vehicle. Many of the computer systems share information from common sensors. The computers are interconnected thru a data link called multiplexing. Some of the computers work together, to control different aspects of the vehicle. Automatic Traction Control is a part of the Anti-lock Brake System but, if the system senses a wheel spinning it can communicate with the engine computer to reduce engine power, and it can apply the brake at the spinning wheel. It can process this information and make decisions 5 to 100 times per second depending on the system.

The engine computer system referred to as an Engine Control Module (ECM), or Powertrain Control Module (PCM) uses a variety of input sensors and switches for information. The processor in the ECM uses the information to make decisions. The computer controls outputs like the injectors, fans, solenoids, and relays to operate the various components.

An old saying about computers rings true on our vehicles today, “Garbage in, Garbage out.” If the information fed to the computer is faulty, the system will malfunction and not operate properly.

See my articles "DIY Auto Service: Basic Electrical and Electronic Testing" for basic electrical theory and "DIY Auto Service: Basic Digital Volt Ohm Meter (DVOM) Electrical and Electronics Testing" for meter usage.

See DIY Auto Service: Computer Sensor Diagnosis and Testing for the testing procedures for the sensors.

Automotive Computers

A computer is a processing and control device that has a variety of complexity levels depending on the system it is monitoring or controlling. Computers house a variety of components inside the infamous “Black Box.” The computer also has a self-check system built in to monitor the function of the system and will set and store fault or trouble codes. A computer can have many names depending on the manufacturer and what it is controlling. An engine computer may be referred to as an; Engine Control Module (ECM), Powertrain Control Module (PCM), Engine Control Assembly (ECA), Motor Control Module (MCM), Body Control Module (BCM) or Electronic Control Unit (ECU). The main sections of the computer are the processor, memory, voltage regulators, analog to digital converters, signal conditioners, and output drivers.

Computer Memory

Memory falls into categories that reflect how volatile or erasable the memory is. This also has to do with how important the information is and whether it needs to be changed.

  • ROM (Read Only Memory) is memory that cannot be changed and is not lost if the battery is disconnected. This is where the basic operating system and other vital information is contained.

  • PROM (Programmable Read Only Memory) is memory that was installed at the factory and cannot be changed. PROM’s used to be a removable chip that had to be replaced if a change needed to be made. Caterpillar called it a Personality Module that contained specific information about the engine and vehicle.

  • E-PROM (Erasable PROM) has the same information as a PROM but can be changed once or twice.

  • EE-PROM (Electronically Erasable PROM) has the same information as a PROM but, can be changed over and over. Most vehicles today use the EE-PROM because it can be reprogrammed an infinite amount of times.

  • RAM (Random Access Memory) is the notepad of the computer, with this information constantly changing. Many vehicles today have the capability to “learn” driving styles, sensor minimums/maximums and idle speeds. This information is retained until the computer loses power or the batteries are disconnected. After losing power, this information could be lost. On some occasions, the vehicle may run or shift “funny” until it “re-learns.” KAM (Keep-Alive Memory) is another form of volatile memory.

Voltage Regulators

The computer runs on a lower voltage than the rest of the vehicle. Earlier in this article, we spoke of 5-Volt VREF. This is a 5-Volt Reference Voltage sent out to many of the sensors. This voltage must be very accurate and stable; otherwise, the sensor readings would not be accurate. Some manufacturers allow a 0.2-Volt variance; others as small as .04 Volt variance. If for some reason the voltage regulators are bad, you may have fault codes for several sensors due to the sensor voltages being off.

A to D Convertors: Analog to Digital Convertors

An “A to D” Converter changes a steady voltage into digital numbers (pulses) or an AC voltage into a DC Digital signal. Computers are DC digital and do not understand anything else. The A to D Converter is like a translator which translates signals the computer cannot understand into something it can use. The computer actually operates on binary code which is “0” and “1” or on/off. It is much easier to translate a digital signal than an analog signal. Some signals are a weak voltage and may need to be amplified as well.

Output Drivers

Output drivers are transistors used to control power or ground (usually ground) to an; injector solenoid, modulator valve solenoid, AC relay, fan solenoid, intake heater relay, starter relay, EGR solenoids, just to name a few things the computer can control. The transistor is like a solid state relay with no moving parts. Some engine computers have a separate Driver Module. Since these output devices generate the most heat in the computer, many of the computers use a finned design or a cooling plate to dissipate the heat.

Service Tip: The output drivers are very sensitive to over-current (amperage) and can easily be burnt out by someone using a jumper wire or the wrong test procedure. If you have a bad Injector #5 Driver fault code, the driver for #5 Injector has probably been burnt out. Check the resistance in injector #5 solenoid. Low resistance or a short could cause the amperage to increase and damage the injector driver.

Computer System Operation

Computer systems are divided into three zones: input, processing, and output. The input into the computer revolves around sensors and switches. The processing is done inside the computer. The outputs are devices like solenoids, injectors, pressure control valves, relays and indicator lights.

The computer system operates entirely off of voltages. Most of the sensors change a voltage signal, typically a 5-volt signal, into a voltage between 0 and 5 volts. The voltage is then interpreted as a temperature, pressure or position by the computer. Some of the sensors send an Analog voltage to the computer, which means it is a steady or varying voltage and other sensors send a DC Digital signal, which is an on/off or high low signal. When a technician connects a scan tool or computer to the system, it will read out temperatures in degrees, pressures in PSI and positions in percentages. The computer has translated these readings for us. The computer plugs the sensor information along with other information into an algorithm and comes up with an answer for what to do next to operate the system.

The computer is a collection of components such as memory, processors, analog to digital converters, voltage regulators, circuit boards and transistors much the same as a home computer. The vehicle computers have to endure a lot more variables in temperature, vibrations, and environment. That being said, the computers running the systems on our vehicles are very reliable.

When the computer makes a decision from the information given, it will operate devices like solenoids, relays, motors and indicator lights. Many of the output device circuits are controlled on the ground side of the circuit. Since most computer circuits are low amperage circuits, the computer may use a relay to control a higher amperage circuit.

Example: The AC clutch coil is a high amperage electromagnet (10A). The AC clutch is turned on or off by the computer. The computer controls the ground path for the AC relay coil (pins 85 and 86), which is low amperage (.15A). The magnetism generated by the relay coil closes the NO contact (30 to 87) in the relay and connects the high amperage to the AC coil. This allows the computer to control a high amperage device with a low amperage control circuit.

When the computer senses a problem, it will turn on the Malfunction Indicator Lamp or MIL. When this happens, it will store a trouble code for the technician to retrieve. A problem in a circuit typically has one of three causes; the computer (very rare), the component (40% or better) and the wiring (40% or better). This means, most of the time the problem is going to be the components or wiring.

Temperature Sensor

Temperature Sensors use a temperature sensitive resister (thermistor) to change voltage into temperature.
Temperature Sensors use a temperature sensitive resister (thermistor) to change voltage into temperature.

Temperature Sensors

A temperature sensor is also called a thermistor. The resistor inside is sensitive to temperature. As the temperature changes around the resistor, the resistance value changes. There are two types of thermistors used in vehicles; Negative Temperature Coefficient or NTC, which is the most common, and Positive Temperature Coefficient or PTC which is rarely used in vehicles.

  • NTC Thermistors increase resistance as the temperature goes DOWN and decrease resistance as temperature goes UP. Resistance and temperature go in opposite directions.

  • PTC Thermistors increase resistance as the temperature goes UP and decreases resistance as temperature goes DOWN. Resistance and temperature go in the same direction.

  • The computer sends a 5-Volt voltage to the sensor and monitors the voltage.

  • The sensor has two wires going to it: the 5-Volt wire and a ground wire.

  • The sensor has two pins that connect to the resistor in the sensor.

  • The circuit must be complete for the sensor to read properly. If either the 5-volt or the ground wire has a problem, the sensor cannot be read properly. This will typically give a temperature readout of about -36°F. This is a dead giveaway the sensor circuit has an open circuit.

  • The sensor must be exposed to the air or fluid temperature it is sensing.

  • NTC Thermistors are used for, among other temperature readings:

    • Engine Coolant Temperature or ECT
    • Ambient Air Temperature Sensor or AAT
    • Manifold Air Temperature or MAT
    • Transmission Oil Temperature or TOT
    • Drive Axle Temperature
    • Fuel Temperature or FTS


The most common potentiometer is the Throttle Position Sensor located on the throttle body or accelerator pedal.
The most common potentiometer is the Throttle Position Sensor located on the throttle body or accelerator pedal.


Potentiometers are position sensors. The most popular potentiometer is the throttle position sensor on the accelerator pedal. In this section, we will use the Throttle Position Sensor (TPS) also referred to as the Accelerator Pedal Position Sensor or Electronic Foot Pedal Assembly as our example, since it is the most common.

Note: On many gas engines, the TPS is located on the throttle body on the engine. With electronic throttles or electronic diesel engines, the TPS is located on the throttle pedal.

  • The TPS uses a three-wire connection.

    1. 5 Volt reference voltage or VREF is sent to the sensor from the computer.

    2. Ground circuit either to a chassis ground or grounds back thru the computer.

    3. Signal wire is the varying voltage between 0 and 5 Volts as the sweeping sensor arm scrapes across the resistor in the sensor.

  • The 5Volt VREF is the source voltage for the sensor. This 5 Volt may be shared by other sensors. This means a problem with this circuit may affect multiple sensors.

  • The ground circuit completes the 5 Volt circuit thru the resistor to ground. The ground also may be shared with other sensors.

  • The signal wire is like a voltmeter doing a voltage drop across the resistor. At idle or pedal in the released position, the voltage is a low voltage reading, typically between 0.5 and 1.0 Volts.

  • At wide open throttle or WOT, the voltage is high, between 4.5 and 4.8 Volts.

  • The TPS will usually set a Fault Code if the signal wire voltage is 0 or 5 Volts. The signal should never reach these extremes.

As the pedal is pushed down, the voltage increases from the minimum of about .5Volts to about 4.8 Volts. Early TPS sensors were adjustable and later TPS sensors the computer “learns” the minimum and maximum voltages. Thru a scan tool or PC, this would usually be 0% to 100% throttle position.

Pressure Sensors

Pressure Sensors change pressure into a voltage. Oil, Fuel, Air, Boost, Atmospheric, AC system pressures are typically measured by pressure sensors.
Pressure Sensors change pressure into a voltage. Oil, Fuel, Air, Boost, Atmospheric, AC system pressures are typically measured by pressure sensors.

Pressure Sensors

Pressure sensors are referred to as variable-capacitance or piezoresistive sensors. They are used to measure a variety of pressures on vehicles. Some of the pressure this type of sensor can measure are; atmospheric pressure (BARO or Barometric pressure), intake manifold pressure or vacuum, manifold absolute pressure (MAP) or Turbo Boost Pressure (BPS), oil pressures (OPS), fuel pressures, AC system pressures, and transmission oil pressure.

[B] Pressure Sensor Wiring

Pressure sensors are a three wire sensor that is set up similar to the TPS.

A pressure sensor uses a three wire connection.

  1. 5 Volt reference voltage or VREF is sent to the sensor from the computer.

  2. Ground circuit either to a chassis ground or grounds back thru the computer.

  3. Signal wire is the varying voltage between 0 and 5Volts as the pressure changes the resistance thru the sensor and its signal voltage.

The 5 Volt VREF is the source voltage for the sensor. This 5 Volt may be shared by other sensors. This means a problem with this circuit may affect multiple sensors.

The ground circuit completes the 5 Volt circuit thru the resistor to ground. The ground also may be shared with other sensors.

The signal wire will send a signal back to the computer between 0.5 and 4.8 Volts depending on the pressure it has sensed.

Two types of pressure sensors are the most popular:

  1. A variable-capacitance sensor uses a ceramic disc that, as pressure is applied, moves closer or farther from a steel disc. This changes the capacitance value and changes the signal voltage back to the computer.

  2. A piezoresistive type sensor is also called a Wheatstone bridge. It uses a silicon chip that flexes and changes its resistance value to alter the signal back to the computer.

The computer changes this voltage signal into a pressure reading.

Permanent Magnet Sensors

Permanent Magnet Sensors produce AC voltage.
Permanent Magnet Sensors produce AC voltage.
Notches on a tone ring or reluctor create and collapse the magnetic field as they pass by.
Notches on a tone ring or reluctor create and collapse the magnetic field as they pass by.

Speed and Position Sensors

There are many areas of the vehicle where the speed of rotation needs to be measured. On the electronic engines, speed and engine position are important to fuel injection timing. Some of the speed sensors are:

  • wheel speed (WSS),
  • vehicle speed (VSS),
  • transmission input speed (TIS),
  • transmission output speed (TOS),
  • crankshaft speed and position (CKT)
  • and camshaft speed and position (CMP).

Two types of speed and position sensors used in today’s vehicles; the Permanent Magnet Generator and the Hall Effect Sensor. The Permanent Magnet Generator or Induction Pulse Generator is widely used for Vehicle Speed Sensors, Wheel Speed Sensors, Transmission Speed Sensors, and Crankshaft Speed Sensors. The Hall Effect Sensor is used as an accurate speed and position sensor for crankshaft and camshaft position and speed.

Permanent Magnet Generators Operation

Permanent Magnet Generators use a permanent magnet surrounded by a fine coil of wire. A tone ring or reluctor wheel has a raised edge followed by a low space. The magnet is very close to the ring. As the ring rotates past the magnet, the raised edge creates a magnetic field around the magnet. As the ring continues to rotate the low space comes by the magnet. This weakens the magnetic field, and it collapses thru the coil of wire generating a low AC current. As the ring speeds up the pulses get stronger, more frequent and produce more current. For most applications, the high and low spaces are evenly distributed around the ring. If the sensor is used for engine speed and position, an odd notch will identify that position to the computer by varying the signal every time the odd notch rotates past the sensor. Clearance between the sensor and the ring is critical. Too much clearance and the signal will be weak or lost. Too close and the sensor may be damaged.

Hall Effect Sensor Operation

A Hall Effect Sensor is a DC Digital Sensor. This means it produces a high low signal that is a square wave and not an analog signal. It also produces a DC Voltage signal. This type of sensor requires three wires to operate. The Hall Effect is the rotation of a tone wheel or pulse wheel that contains a solid portion or vane and a window. As the wheel rotates alternately between the window and vane, the sensor sends out a digital on/off signal. An odd window shape allows the computer to pick up a location on the tone wheel for engine position. This type of sensor is used most often for camshaft and crankshaft position sensors. Since this signal is higher voltage and a digital signal, the signal to the computer is more accurate and more dependable. Clearance between the tone wheel and the sensor is extremely important.

The three wires needed to operate a Hall Effect sensor are:

  1. Power supply may be 5 to 12 volts depending on the manufacturer. Check the specifications.

  2. The ground wire may ground thru the computer or share ground with other sensors.

  3. The signal wire has an on/off or high/low voltage signal from zero to source voltage.

Service Tip: Some Hall Effect sensors are actually two sensors in one. This will be typically a 4, 5 or 6 wire sensor.

Hall Effect Sensor

Hall Effect sensors produce a DC digital on/off signal.
Hall Effect sensors produce a DC digital on/off signal.

Output Devices

Out of all the wiring going to an engine ECM (150 to 180 wires), only about 10% is used for the actuators. The rest of the wiring is power, ground, switch and sensor wiring. With an engine computer, the outputs will generally be:

  • Fuel injector solenoids
  • Fan solenoids
  • Compression brake solenoids
  • Emission control solenoids
  • Starter controls
  • Turbocharger controls

With ABS/ATC computers, the outputs will typically be two control solenoids per modulator valve and an Automatic Traction Control solenoid.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

© 2014 Mike Thomas


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    • Michael M Thomas profile imageAUTHOR

      Mike Thomas 

      17 months ago from Phoenix, AZ

      You can purchase an online version of Alldata or Mitchell on Demand for the vehicle you're working on.

    • profile image


      17 months ago

      You can usually buy a repair manual online for the car you're working on from Alldata or Mitchell on Demand.


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