Electronic Control Module/ ECM (Caterpillar Engines)

Electronic Control Unit (ECU)

The terms Electronic Control Module (ECM) and Electronic Control Unit (ECU) are interchangeable terms. ECMs are sophisticated computers. They contain electronic power supplies, central processing units, memory, sensor input circuits, and output driver circuits. They communicate with other electronic controls over a bi-directional data link.

Most ECMs used in Cat electronic control systems use three types of inputs.
  • Switch-type: measures the state of a switch (open or grounded)
  • Analog-type: measures the amplitude of a signal (generally between 0 - 5 volts)
  • Digital-type: measures a frequency (speed) or the pulse width of a periodic signal.
Digital input signals are sent to the ECM’s processor, while analog signals are filtered, amplified, and converted to a digital signal before being sent to the ECM’s processor.

An electronically controlled engine cannot operate without the ECM. All aspects of the system, from the fuel system, controls, engine performance, etc. are all controlled by an ECM.

Electronic Control Unit (ECM) (CONT.)
The ECM software contains control formulas. Depending on the data, an almost unlimited number of logical operations can be established for storage and processing in the form of parameters, characteristic curves, and multidimensional program maps.

The type of ECM used is determined by engineering and is based on the types of inputs and outputs. Most output controls are called “drivers,” which identifies the output characteristics, such as current and voltage drivers.

NOTE: ECMs are not serviceable, therefore, understanding ECM internal operation is not as important as how the ECM interacts with the engine. It is important to know how to use the ECM to troubleshoot the wiring harness and electronic component faults. Since ECMs are programmable, it is also important to understand the parameters that can be programmed into the ECM, which can change engine or machine performance.

ECMs can:
• Accept input data (sensors, switches, etc.)
• Control outputs (solenoids, relays, lamps)
• Indicate active faults and events, log faults and events
• Provide insight to circuit integrity
• Record data (Datalog, Histogram)

ECMs cannot:
• Troubleshoot themselves
• Fix themselves
• Identify marginal component performance
• Identify inaccurate sensor readings
• Create horsepower

Programmable Electronic Engine Controller (PEEC)

The D3500B series engine first used the Programmable Electronic Engine Controller (PEEC) ECM. It was introduced in 1992 and was used on the early D3500B series electronic engines from approximately 1992 to 1995.

The PEEC has a personality module, which is the software for the computer. It contains the fuel setting information such as horsepower, torque rise, and air/fuel ratios.

The PEEC requires a 23 to 27 volt DC power supply.


The D3500B series engine uses the ADEM 2 (A2) ECM. It was introduced to D3500 engines in 1995, but had been used in other Cat engine applications for several years prior. It has two 40-pin connectors for 80 pins of inputs and outputs. It features 128k memory. Most are fuel cooled, with
unfiltered fuel running through a channel in the ECM case and back out (1).

Early A2 ECMs contained a software chip, which contained the ECM software. The chip was located beneath a cover (2) and could be changed.  Later versions of the A2 no longer used the software chip. The ECM software was “flashed” into the ECM over a communications data link.


The D3500C series engine uses the A3 ECM.

The ADEM III ECM has increased memory (1M), computing power, and improved reliability
compared to the previous ECMs. The ADEM III also uses flash programming to modify or update ECM parameters. The ADEM III ECM connector allocations, including power supplies, grounds, and data link connections, are common across applications. Diagnostic and programming functions must be performed with a laptop computer with Cat ET software installed. The ECMs can be reprogrammed with a Flash File using the WinFlash application of Cat ET.

-Some of the ADEM III ECM features are:
• Engine speed governing
• Fuel limiting
• Fuel injection timing control
• Engine retarder control
• Ether start control
• Engine monitoring
• Voltage monitoring
• Active and logged diagnostics


Some D3500C series engines utilize the A4 ECM. With this generation of ECM, the term "ADEM" has been dropped, so A4 is not short for ADEM 4. The A4 ECM started being used on D3500C engines in 2007 and will be used on Tier 3 and Tier 4 emission applications in the future.

The A4 features one 70-pin connector and one 120-pin connector, for a total of 190 pins of inputs and outputs.

ECM Functions

The ECM has three main functions; it provides power for components, monitors inputs, and controls outputs.

The ECM provides a regulated voltage supply to power sensors, typically 5 V, 8 V, and 12 V. Sensor returns (grounds) are also provided by the ECM.

The ECM monitors inputs from several sources including sensors, switches, and data links. The ECM software uses this input information to determine how to control the outputs. Input information is available to a technician though Cat ET in the status screens.

Based on inputs, the ECM software determines how to control outputs. Examples of outputs are injector solenoids, actuators, lamps, gauges, and data link information.

ECM Power

Shown is a diagram of the power circuit for an ADEM III ECM. All ECMs require power and ground circuits in order to function. Most machine and all engine ECMs require both switched (key switch) and unswitched battery power. Unswitched battery negative is also required.

All ECM power, switched or unswitched, must be fuse protected. Check service information for the application you are working with to determine the proper fuse rating required. The switched power also provides voltage to communication ports (typically found as a 9-pin Deutsch HD connector). A Communications Adapter will not communicate with an ECM unless the Adapter and the ECM has switched power.

Typically, more than one unswitched battery power and battery negative wires are required. Check the electrical schematic of the application you are working with for specific information. The unswitched power and negative provides two functions. The first is to keep internal memory circuits powered while the ECM is not in use. Second, unswitched powers and grounds provide additional current to the ECM under heavy loads. An example of this is when multiple output drivers, such as injector solenoids, are under heavy load.

The battery negative wires are internally grounded to the ECM casing. It is important that the ECM grounding strap is properly installed to ensure the ECM has a proper bond to the chassis ground.

ECU Connector

An example of multiple ECM powers and grounds is illustrated above. Shown is an illustration of the J1/P1 connector for an ADEM III or A4 Engine ECM. The pins for power and ground are filled in black:
• Pins 48, 52, and 53 are unswitched power
• Pins 61, 63, and 65 are unswitched ground
• Pin 70 is switched (key switch) power
NOTE: The illustration above is an example of multiple power and ground circuits on an engine ECM. Be sure to check the appropriate service literature to identify ECM power circuits for your application.

Service Tool

Shown is a diagram of a typical data link wiring schematic. All electronic Cat engines and machines will have a Service Tool Connector, typically a round 9-pin Deutsch HD connector. This connector is used to connect a Communications Adapter into the ECM data link. Each pair must be twisted together to protect the data link from electrical interference, or "noise", which can interfere with the data being transmitted. To power up and communicate with Cat ET, all an ECM needs is keyswitch power, ground, and the data link properly installed.

The Service Tool Connector pin-out is the same throughout the Cat product line. The pin-out is as follows:
A - Power (powers the Communications Adapter)
B - Ground (ground for the Communications Adapter)
C - CAN Shield (J1939)
D - Cat Data Link +
E - Cat Data Link
F - CAN (J1939) Data Link
G - CAN (J1939) Data Link +
H - ATA (J1587) Data Link -
J - ATA (J1587) Data Link +

ECM Software Flash File
In order to operate a specific engine application, the ECM must have the proper software Flash File installed. This is also known as the "personality module". Flash Files can be located and downloaded through the SIS website.

The Personality Module Code, also called the Interlock Code, "marries" the ECM to the Flash File.
Each Flash File has its own Interlock Code, if you chose to flash another software application (install a different Flash File) you will have to change the interlock code to marry the new Flash File to the ECM. This requires Reason Code 5 passwords and likely, a fee.

Flashing the Flash File is performed using WinFlash, which is a program within Cat ET.

Configuration Parameters
The ECM needs the parameters to be properly defined in order to perform its functions correctly.  Each 3500 engine application is set up different. Many parameters can be customized to fit the customer’s needs. Some examples are, Low Idle Speed, Crank Duration, Cooldown Time, Start Aid, Pre-lube Oil, etc.

The Personality Module/Interlock Code is also found in the Configuration screen in Cat ET. Parameters available will vary based on engine application. Some configuration parameters will require a password in order to change.

Read More:
Identify Types of Basic Circuit Faults and Wiring Harness Test
PULL-UP Voltage and How the ECM Uses it to Determine Diagnostic Conditions


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