Just a little more on this subject. It's not uncommon for vehicles of this vintage to have multiple diagnostic connectors. Round 6 or 9 Pin or oblong 12 pin connectors are connection points for different data link protocols. Different subsystems (transmission, engine, ABS, displays) can communicate diagnostic information on different data links (J1939, J1507/1708, J1850 or even ISO15765). In addition to the SAE J and ISO standards, there are RPs (recommended practices) that further define implementation requirements to attempt to drive commonality. The heavy duty industry in North America frequently uses J1939 as the comm link default standard but European OEMs have different protocols depending on whether or not the product is marketed in NA or not. Further, if you look "under the hood" of these protocols, they are actually defined by a
cluster of different SAE or ISO documents that describe in painful detail the physical layer (wires), communication protocol (code level, non destructive communication between devices, priorities....), diagnostic codes, and connector configuration of each data link. In my previous life I was involved with diagnostic tools for Allison transmission and trying to make sense of this stuff is why I am gray and bald. Its enough to make your head explode
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