My 1995 and 1997 960s have a water level sensor in the reservoir that activates a light on the dashboard. The 940s use the same reservoir tank. There is a very nice write-up on Brickboard that tells how to install on in the 940 and 240s: https://www.brickboard.com/FAQ/700-900/Cooling.htm#LossofCoola...

Did your trusted specialist actually look at them to determine that they are Nivomats? A lot of people assume that because it is a V90, that it came with Nivomats, but actually they were an option, and most don't have them.
John