Many owners like to add various equipment to help understand what the engine is doing while running down the road. While the intention is good, the outcome can sometimes be very bad (and expensive). Some of this added equipment is attached to the ECM Data Port, the same port used to connect the engine ECM to the CAT diagnostic equipment used to troubleshoot engine problems and supply upgrades if required. Often there are multiple devices connected that may include dash displays, trip recorders and transmission controls. Be aware that these devices may cause problems with the functioning of the ECM and disrupt communication with it. Many times I have attempted to connect the ET to the ECM and find the data communication channel disrupted, with the ECM not recognized, resulting in a ECM Comm failure on the diagnostic equipment.
The ECM data link is wired without fusing. There are different data links, a 6 pin named J-1708 and a 9 pin called J-1939 which provide engine data. The ECM has 1 switched and 3 unswitched 12v lines. Additionally it has 4 ground wires, running to different ECM pins.
If you connect a piece of after-market equipment, you must be sure it is wired the same as the data link. A good example of this is I found a case where the 12v unswitched lines were wired to ground in the after market equipment. The equipment’s directions stated just plug it into the data port and turn it on. The ECM failed.
Many companies build equipment that connects to the ECM data port and in most cases operate trouble free. However, trouble can arise when connecting multiple equipment, especially if the equipment is to be “hard wired” to the data port lines. J-1708 and J-1939 data link wiring is shielded twisted pair. This is to eliminate or greatly reduce cross talk and electrical interference from external sources that could disrupt ECM functions. If theses wires are cut, or otherwise damaged, they need to be replaced.
We often try to save $ by the DYI approach, but in cases like this, the downside can be very expensive.
Dave Atherton; Retired CAT Mechanic