J1939 datalink and ABS

I've got 2002 9200i with an ABS light on. I just bought it. Light came on when I replaced the bulb. Blink code 71 says datalink communication lost. I've read a lot about wiring problems on the datalink but don't understand how it is actually put together. Is it one pair (or set) of wires that runs all over the truck that systems like the ABS module and satellite systems hook into? Is there one connection point for this bus to plug into the ECM (Detroit 60) and other systems splice into this pair of wires? If the connection for J1939 is bad at the ECM would that cause other problems (there are several posts about instrument cluster problems that mention the J1939 bus)? If only the ABS system appears to have a problem would that suggest the wires from the ABS module to somewhere on the J1939 are bad but probably not where the J1939 connects to the ECM? (of course, it could be a bad ABS ECM too).

 

I checked that and saw no sign of corrosion at all. Looking at schematics I saw that pins 11-13 of that plug go to J1939 and the ECM. I checked the negative (-) pin (13 I think it was) for continuity to ground and had none so figured I have an open somewhere but figured I’d try to see what I would be looking for once I start crawling around to look at the ECM wiring. Is the negative side of the J1939 bus grounded? If I unplug that big connector from the engine side of the firewall is it safe to check for voltage across the pins (11 and 13)?

 

I looked around for terminating resistors. But being not sure what I was looking for I didn’t find any. I did where the wires coming from the firewall connector had abraded at the frame enough to expose the wire. It looks like aluminum wire was used and about 8” of it oxidized inside the insulation. But I still get no continuity between the yellow and green at the firewall connector. I checked the connector that comes off the Detroit ECM and get 120 ohms between yellow and green. I get 0 ohms between the green on the ECM connector and the green at the firewall so I’ve lost the connection on the yellow wire probably where it spliced into the bus itself. I haven’t cut that open to get to it.

120 ohms means the other 120 ohm terminating resistor isn’t connected. I did read on another post that the ECM might contain the other other resistor, so pulling the plug at the ECM and getting 120 would be a good reading of the J1939 bus. Any idea if this is correct?

 

That would be great. I have a very slow internet connection but could find a public wi-fi to download it. But, it appears that I haven't posted enough to be allowed to send a message to you. Can you put a link in this thread? Or enough of a link that I could use to find the 2002 guide? Closest I found was for 2005 but there appear to have been enough bus changes to make it not very good for figuring out a 2002.

Thanks.