[jclark003]

Ok I believe that is a Cat messenger. It uses a j 1939 interface to get engine data from the ECM. The attached link has the customer connector wiring diagram in appendix. It lists the C7 but should be the same for 3126B
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Possible you pulled a wire in the harness or have a bad connection in one of the connectors. The ECM has two plugs/harnesses. One goes to the sensors and injectors and one leads up to the customer connector and the cat diagnostic port. Check the harness that runs up to the customer connector and the harness that runs up to dash from customer connector. They come apart simply with an Allen wrench.

Also possible the j1939 circuitry went bad in the ECM. Apparently this happens from time to time. Hope it's as simple as a wiring problem.