Please help! Cat 3406e not powering up or communicating with truck

If you have power in to the ECM (and good ground) at the connector then I would say it's the ECM. Try a spare and see what you get.

 

Have you manually put 12v and ground directly to the ECM from the batteries?

 

If all of this actually checks out, and I didn’t have a CEL come on on two different ecms, when key is turned on, I would have to suspect the OEM harness or plug. Terminal pin bent or some such. It’s wasn’t mentioned whether this happened suddenly on a good performing truck or is this a resurrection of some fashion. Everything you have described having been checked has been on the engine harness side except the key on power. In other words if all checks good as you say and you have no CEL illuminating I would have to suspect, once again the OEM side.
Another thing. If this was brought to me in a no start condition as this, first thing I would do would be connect a Pro Link or similar and see if the ECM was in fact communicating via the J1939, and whether I was receiving any outputs from the ECM. When the CEL comes on at start up, the ECM is saying, hello I’m awake and ready.

 

was the truck running as a daily driver? im thinking wire harness as an offhand guess by what we have so far.

 

Yes the 1939 is the data link that transmits information (bytes), between devices, components,sensors and whatnot. You would do well if you haven’t already, is to google 98 Freightliner basic wiring diagram, print it off and use it as a guide. There are things that could cause an issue with data transmission, opens, shorts, short to voltage, as was mentioned bent or pushed back pins to name a few. At the time and I would imagine now as well Ftl wiring harnesses were without a doubt one of the best, using good terminals and pin designs with all connections soldered. We found very very few issues with a harness problem that was not caused by being worn, cut or broken. Get a good DVOM and get after it. Good luck.

 

you should not try to ohm a wire harness or any other wire for continuity. a tiny strand will ohm out to 0. you need to load test each and every wire from connector to connector. it aint fun.