How to Fix a
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Dodge engine computer repair #2
Keenan Laws
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The computer, also known as the "brains" or Eelectronic Control Module (ECM) is a microprocessor that converts electrical inputs from sensors into electrical outputs. These outputs are used to control actuators, relays, switches, and various solenoids found throughout the vehicle.

The computer system consists of inputs, processing, and outputs...

  • Inputs - When an engine is started, the computer uses information from the sensors to regulated the air-fuel mixture: namely coolant temperature, barometric pressure sensor, and the air temperature sensor.
  • Processing - As the engine starts, information about idle speed, timing, and fuel mixture are stored in adaptive memory to keep  the engine running while it is cold or running in 'Open Loop' mode.
  • Outputs - While the engine is running, it heats up quickly and the computer goes into 'Closed Loop' mode.' In this mode, the oxygen sensors (located in the exhaust system) sends a voltage signal to the computer, and the computer starts adjusting fuel at the injectors, spark timing at the spark plugs, and idle speed to control the engine. 

A typical automotive computer consists of six main divisions: reference voltage, signal amplifier,  analog to digital converter, output drivers, memory, and ECM grounds. As soon as the engine starts, the ECM quickly goes into self-test mode to verify if all sensors and actuators are working properly. If a fault occurs, the ECM turns on the “check engine” light and a code is stored in memory. See photos - Click Here for More Information.

Automotive computers are easy to diagnose. If they are good - they work. If they are bad - they don't work! One of best ways to tell if your computer is bad is to eliminate POWER and GROUND problems. Use an electrical schematic to identify and test for good B+ or battery positive ,  B- or battery negative, and key-on ignition switch voltage. If there is no voltage on the B+ side, check for blown fuses or damage wires and connectors. 

If you suspect a poor ground circuit at the computer, run a voltage drop test at the computer and check for ground at the B- or battery negative terminal. Use an electrical schematic to check for multiple ground connections and test each ground separately. 

Another easy test for computers is to verify there is communication with your scan tool. If there is no communication at the Data Link Connector (DLC) - this is where you plug-in your scan tool to the vehicle's computer, check for corrosion at the computer pins, bent pins, or blown fuses. Once you have eliminated all problems listed above, you can now suspect a bad computer.

Keep in mind that if a computer fails, there is always a reason WHY. The quicker you find out, the happier you and your customers will be.


Automotive computers work on the principle of digital electronics and binary coding. In digital electronics, the computer uses billions of transistors and semi-conductors to turn electrical circuits ON and OFF. A computer does not understand our system of communication through spoken or written languages. Nor does it understand our sense of sight, sound, and touch. These human senses can be communicated electrically, through the use of a simple computer language called binary coding.

Binary coding uses a series of 1's (switch ON) and 0's (switch OFF) to give machines intelligence or logic. This computer language system enables machines to do complex tasks such as monitoring, translating, interpreting, deciding, and even manipulating or controlling motors, relays, and other devices.

Now it's your turn! See if you can determine how computers change our decimal numbering system into binary codes in order to energize fuel injectors during various power needs of the engine. 

If you notice in the diagram below...

  • 1 ms injector pulse width equals a binary code of 0001 for IDLE LOW POWER and a 10% duty cycle or narrow injector pulse width
  • 3 ms injector pulse width equals a binary code of 0011 for MEDIUM POWER and a 50% duty cycle or medium injector pulse width
  • 5 ms injector pulse width equals a binary code of 0101 for WIDE OPEN THROTTLE HIGH POWER and a 90% duty cycle or wide injector pulse width

The computer uses binary tables stored in memory  to adjust air-fuel ratios of the engine. Binay tables can also be used to provide data for the computer to energize a starter motor when the ignition switch is turned-on and the neutral safety switch is in Park or Neutral positions.

Read the binary table below (interpet each row and column)...


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