Bill,
The data port is sometime confusing as there are two side by side and many times not marked. You will see a 6 pin, sometime marked J-1708 and a 9 pin J-1939. In most cases the data Port you want is the 6 pin J-1708. The other is 9 pin j-1939 which is the Allison transmission data port. Toward the end when Caterpillar stopped supplying engines in motorhome they started using a 9 pin J-1939 data connector.
Dave Atherton Retired Cat Mechanic.