When VP changed from the D2-40A to the D2-40B the main change was the introduction
of the MDI (mechanical diesel
interface) system. This system allows a mechanical engine
to interface with the same canbus gauge instrumentation system as used in the larger electronic engines in the VP line, even though the D2-40 engine
itself is not electronic.
What this means is that the "temp sender" which is already installed in your engine is not a normal temp alarm
switch which opens and closes, but is a thermistor which sends varying resistance values to the MDI box depending on the engine temp. When the MDI circuit sees a resistance value that corresponds to "over heat" it sounds an audible alarm
and lights the high temp light in the alarm instrument or illuminates the alarm in the tach window.
If you want to install a temp gauge, all you have to do is source the proper gauge, (metric or US), and hook it up to the daisy chain harness which is already connected to the tachometer. The temp information is already flowing on the canbus, and the gauge will display it.
If you want to get a bit fancier, you could buy an NMEA 2000
gateway and connect the engine data into your vessel NMEA 2000
bus and display all the engine data on your compatible chart plotter.