Ok, this my be a little confusing:
My friends boat is boat A, My boat is boat B
1) With the 727 on boat A, The volts read high up to 20 volts and temp anywhere from 75 to -4 degrees with the motor NOT running. Voltage settled at 16.3 v. Sonar readings were normal.
2) I put the 727 head on boat B with my installed SI transducer. Absolutly no problems for over 6 hours.
3) Took the 727 head back to Boat A and plugged in using existing mount. Same results as #1.
4) Took the mount apart to check each cable connection, wiggle them, tug them. Everything straight and correct. No change in indication. Disconnected the transducer and started the unit in Simulator mode. The sonar image looked great as it always does in Simulator mode but the voltage still read 16.3 volts. I thought if the TD was the problem, the voltage would read normal. Not so.
5) Used a DVM to check the voltage at the plug and the battery terminals with the engine off. Solid 12.5 vdc. Plugged in power and TD cables to the unit and turned it on. 727 read 16.3 v while DVM reading 12.5 at battery terminals.
6) Brought down my old Quad Beam transducer from my Matrix 37 to boat A. Plugged in the QB TD to the 727 head while powered from boat A. Temp and voltage read normal.
7) Plugged in boat A TD again powered from boat A. Results of #1 returned.
8 ) It will take a couple days to get the software version. I doubt that it has ever been updated since leaving the factory.