Disconnect the transducer and test your temp sensor of the transducer. Between pins 4 and 7 the temp sensor should be roughly around 10 to 12 K ohms. What does yours read?
Kokinidis1,
The 1971F has three places it will look for water temperature, NMEA 0183, NMEA 2000 or the transducer. Did you program the 1871F to look at the transducer for temperature?