Also, on IH's, if you pull the ECM power fuse, ID the in/out terminals, and run a jumper wire to the output of the fuse it powers the ECM with the key off. In an extreme case you may need to overlay the J1939 wires to the ECM and run them directly to the scan tool connector so that only the ECM is on the bus.
Diag is pretty straight forward- disconnect extra modules until normal communication returns. If your scan tool has a "sniffer" function that lists modules on the bus, get familiar with what's normally there so you can identify aftermarket modules quickly. Many times the truck will run and drive fine without DTCs but the scan tool won't communicate or drops out. Fuel minders, GPS systems, maintenance modules, QuallComm style modules, etc are all a nightmare. In addition to this good info, MD/HD trucks have a lot more aftermarket modules that ride on the bus than LD trucks do. No joy there and I lost at least an hour screwing around with my scan tool before I realized what the hell was happening.
The reason I posted this is because if you suspect a network problem, Cummins has you monitor the fault code display which has an incredibly slow update rate AND, if the network goes down, your INLINE 6 adapter and Insite will lose connection to the ECM. Then disconnected modules on the network until I pulled the connector for the SCR temperature sensor module and the voltage dropped back to 2.6 volts and all but one code changed back to "inactive." Network voltage is normally about 2.6 volts on the CAN+ - I measured 9.8 volts! Using the scope I performed a wiggle test. I took a page from my Ford training and decided to use the oscilloscope to monitor the network and was able to catch it in failure - and luckily it finally stayed broken long enough to diagnose.
The actuator was working fine however and I had a difficult time diagnosing this intermittent network concern. This might make sense if we take into account that it contains one of the two termination resisters. Normal diagnostics lead to a faulty turbo actuator. The SCR system, DPF sensors, SCR level sensor and the turbocharger actuator are all "smart devices" which communicate with the ECM via the J1939 network, or CAN. If you are not familiar with Cummins, only sensors mounted on the engine itself are hard wired to the ECM. Aside from an EGR valve hanging open there were about 8 fault codes for data update time from the aftertreatment modules, DPF level sensor and the turbocharger actuator. A 2013 F650 with a 6.7 ISB CM2350 in for an engine derate, both amber and the red engine lights on.
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