I know the J2K100 receives messages from the J1939 network and subsequently transmits them over the NMEA 2000 network, but does the J2K100 transmit any messages on the J1939 network?

By default, the J2K100 transmits no messages onto the J1939 network. As shipped from the factory, the J2K100 only listens for J1939 messages and creates no bus traffic on the J1939 network. The J2K100 does however acknowledge received messages, which is required when the J2K100 is the only other component on the J1939 network besides the transmitting device (i.e., the engine or generator that is outputting J1939 information).

The exception to this is if the  "Request Engine Hours" feature has been enabled. In this case, the J2K100 will periodically transmit an ISO request message for the Engine Hours on the J1939 network.

The J2K100 is also completely optically isolated between the J1939 network from the NMEA 2000 network (i.e., no ohmic connection between any J1939 pins and NMEA 2000 pins).