HDS puzzle

[blackhawk180]

I have a merc monitor and two HDS 7's connected via the NMEA 2K network. Only one of the HDS units is wired directly to a transponder but both units (should) display sonar data. My question is, why does HDS #2 show a constant -34F water temp? The unit #1 directly connected to the transponder is correct and the merc monitor displays that same temp as unit 1 so I know the data is being sent over the NMEA network correctly. Is there a setting I'm missing?
BTW, if you own a merc, I highly recommend the merc monitor. It displays an amazing amount of data and in 50 years, I'll probably save enough gas to pay for the darn thing! /sarcasm/