Access Cummins Engine Controllers without a Wiring Harness

This page presents a method to extract data from an ECM without the use of bench calibration harness. This method should only be used if all other attemts to connect to the module fail. Other techniques include: direct connection through the diaglostic link connector in the cab, swapping the module to a surrogate platform, or use an OEM bench calibration harness. The extraction method below will use the SAE J1939 protocol as opposed to the older J1708. The method shows how to determine the electrical connections needed to extract the data from the ECM without purchasing the Cummins bench calibration harnesses. Specialty cables were made at a fraction of the cost of the Cummins OEM cables. The procedure should also apply to other ECMs, but extreme caution must be used to not accidentally corrupt or delete the data. It is recommended to prtactice on an inconsequential module first.

Page last modified: 31 July 2011


  1. Obtain Wiring Diagrams for the ECM.
    1. Identify Battery Power (+)
    2. Identify Battery Power (-)
    3. Locate Pin for Key Switch Circuit
    4. Determine J1939 High and Low pins
  2. Connect the Data Link Adapter (DLA) to a breakout box.
  3. Wire the breakout box to the ECM using cables with sockets on one end and banana plugs on the other.
  4. Connect the power to a reliable source.

Obtaining Wiring Diagrams

The module for this example is a Cummins CM870 from a Signature ISX engine. Armed with just this knowledge, a Google search with the phase, "Cummins CM870 wiring circuit diagram filetype:pdf" (without quotes) produces to a link from SuperMiller: that gives a PDF of the schematic for a Peterbilt 379 with a Cummins ISX. With that diagram, the following information is available:

Battery +12V (ECM Supply (+)) is pin 4 on the 4 pin plug.
Ground (ECM Supply Return) is pin 1 on the 4 pin plug.

J1939 (H) DATA LINK is a yellow wire on pin 46.
J1939 (L) DATA LINK is a green wire on pin 47.
The schematic says the connector has details described by the 8 surrounded by a triangle. This gives a complete description of the connector.

OEM 50 Pin Connector for Cummins

The other piece of information needed is the circuit for the ignition key switch circuit because the key needs to be on for communications.The Ignition Key Switch is pin 39 and normally uses a red wire.

Confirm Wiring Diagram

Another source of information is always beneficial. To confirm the wiring information, pull up the Cummins Insite Fault Information System:

Cummins Insite Fault Information System.

Expand the tabs to find circuit diagrams. Below is a circuit that shows the pins for the J1939 connection. Sure enough, the J1939 connection is on pins 46 and 47.

J1939 Circuit

The power connection diagram is shown below with Pin 1 being on the upper right of the 4-pin connector of the ECM when reading the label on the ECM.

Power Connection Image

The key switch circuit is not available in the Fault Information System under this tree. However, a similar module, the CM870 for an ISM engine, has the information. Looking at the 870 module for an ISM, a diagram containing the key switch circuit confirms pin 39 is for the key switch.

Ignition Key Switch Circuit for CM870

With two sources of information, we can confidently connect the wires to the correct pins. To summarize, we need the following 5 connections:

+12 V: Pin 4
Ground: Pin 1
J1939 (H): Pin 46
J1939 (L): Pin 47
Key switch: Pin 39

Connecting the Module to the Computer

The quality of the mechanical connections are important when connecting the ECM to the computer. The OEM connectors are the best, since they make use of the keys, screws, and insulating blocks. Aside from that, the next best thing is to use female sockets that are sized for the proper pin. Furthermore, the socket should be have heat shrink tubing around the outside to prevent accidental shorting. A photo of a socket connected to a banana plug is shown below. The sockets are the from a female computer serial connector that you can buy at Radio Shack.

Banana Plug to Socket

The power connectors use a size 12 socket and are connected to the module at the 4 pin power connector as shown below. The 12 gauge sockets are Digi-key part number 889-1057-ND.

Power Plug Connector

The pin connectors need to be connected to the module at the correct pins as determined by looking at circuit diagrams. All numbers refer to the OEM harness. An example is shown below. Note the marking "04" on the ECM matches the schematic drawing of the connector. Pin 1 is the upper left and pin 50 is the lower right with the numbers increasing in rows going from left to right. This numbering scheme is opposite from the 4-pin power connector.

Connecting Pins -2 Connecting Pins

All pin connectors terminate to a banana plug for easy hookup. An interface adapter is a very useful tool for this project. Click on the image of the interface tool below to see part numbers if you desire to make your own. The 120 ohm J1939 terminating resistors are attached to the breakout box as well.

Breakout box for J1939

External power is required to supply power to the module and the DPA5 connector. The overall setup is shown below where the computer connects to the DPA5, the DPA5 connects to the Breakout Box through the J1939 9-pin adapter, the breakout box connects to the module and to the power supply (a battery in this case).

Overall Setup

It is recommended to apply battery power first to energize the ECM. Then connect the key switch circuit to wake up the ECM. Always be sure to use a fused link when applying power. A cigarette lighter adapter often has a fuse in the barrel of the connector. AC/DC converters will often limit current to safe levels by design.

Powerspec and Insite can be used to access the data stored on one of these modules. Be aware that fault codes may be set because normal sensors and signals are not present.