[Lynn Eades]

Thanks Tony. I had a gut feeling there were 12 volt ones, but my quick search never found them. The mixing of components was what I was concerned about and that's why I touched on it.
My personal experience with the senders is that they are very reliable and the biggest reason an old one is no good is because someone has damaged the terminal end.
When it comes to "on-off" my point was that the 2 post one is either open or closed, showing either cold or hot (for the right bank) and nothing in between. With the left hand single terminal unit, Yes it has points, but the ratio of open verses closed gives a specific temperature reading at the gauge which varies with L.H. cyl. head water temp. This then gives the operator a real time indication of the (L.H.) engine temperature across the range of the gauge. Basically the left bank indication is a reasonable indication of the whole engine until the 2 post sender is activated, which sends the needle to hot.
Addendum:
Five minutes after this the operator sees the steam coming from under the bonnet (hood) and looks at the gauge. Then, depending on how much water has gone, the temp gauge could be reading normal, because the sender bulbs are in the air (inside the heads)
Later in the workshop whilst looking at the scored bores he declares " I always watch my gauges, religiously!" (mechanic thinks to himself "yeah right!")