Auto transmission temperature gauges are probably closer to the mark. There temperature senders, like this one
from Summit Racing, for VDO gauges and others for different brand gauges. The trick is to find one that will fit the threaded fitting on the transmission and is the right length. I have done this for the transmission on a truck but had not thought about it for the transmissions on the boat.
Might be a good idea to check with Brian at American Diesel to see if they have one made to fit and also to find out where to install it.
Darn, now you have me thinking about it - guess I have to look into it.
Edit - I looked in my Velvet Drive manuals and found this:
On Page 36 of the Service Manual I found reference to a Optional Drive Gear Alarm Kit, with two temperature switches, but no indication where the two switched would be installed.
In the Marine Installation Manual
Page 19 - Under Test Cooler Circuit -
The cooler size affects the oil temperature and lubrication pressures within the transmission; therefore, a test run should be made to insure that the transmission sump oil temperature falls between 14OoF (60~) and 19OoF (88~). The 19OoF maximum sump temperature should not be exceeded when running at full throttle for an extended period of time. Overheating can cause transmission failure.
Continuous monitoring of sump temperatures is possible when a thermocouple is installed in the cooler out circuit near the transmission. The thermocouple should always be placed in the oil circuit so the oil passes over the sending unit.
TRANSMISSION ALARM KIT A4867HN - This is the recommended method for monitoring transmission functions. The temperature will rise to indicate low oil level, low pressure or mechanical problems quicker than a pressure gauge will indicate a drop in line pressure.
TRANSMISSION ALARM KIT A4867HS - This kit is used in conjunction with the A4867HN kit. This kit provides extra components for making a dual station installation.