I totally agree with you, Frank, that this mechanism is somewhat strange, and I still can’t see why it is needed at all. However, I did’t misquote the manual (which is an old manual from the 80’s). In the meantime, I had the opportunity to double check the information by flipping through the Jaguar Dealer Training Environmental Control System Manual, which is just a newer version of the manual for the Delanair AC system.
This manual describes a “Ranco Override” microswitch as well, it is part of the servo unit. An illustration of the camshaft sequence shows that this microswitch is activated only within the last 25 degrees of the camshaft movement towards the full cooling limit (p. 7). The circuit diagram for the compressor clutch (p. 19) shows the Ranco and this arcane switch as two parallel options for activating the clutch: either by a closed circle in the Ranco (typical case, above 2°C) or by microswitch closing – or both. This microswitch indeed overrides the Ranco in the sense that the microswitch can engage the clutch when the Ranco refuses to do so.
What I found most intriguing is a short passage on p. 18. There it is stated in a quite laconic manner:
“The magnetic clutch will energise through the Ranco thermostat except when the temperature of the evaporator matrix falls below 2°C. The Ranco thermostat opens at this temperature to prevent excessive temperature drop. The Ranco switch is overridden, however, when the camshaft travels the last 25 degrees of rotation in the full cooling mode. Cooling should not be selected if the exterior (ambient) weather conditions are cold enough to cause evaporator matrix icing.”
There we have it: Seems they intentionally installed a desert mechanism which effectively endangers the evaporator unit – relying on us Jag drivers being reasonable human beings and turning off the AC when the time has come to do so? A small hint in the driver’s manual would have been a kind gesture.
Anyway, I installed my switch in the meantime in the same manner as you, Aristides, did. My switch cuts off the Ranco circuit, but it doesn’t interfere with the override mechanism. I also installed a small control lamp which will give me a hint when the clutch engages. So, I will know exactly what the servo is up to!
All in all, I have to admit that this has become a purely academic issue, but it still puzzles me. After all, the manuals I could obtain refuse to explain which ambient conditions allow the camshaft to enter those famous 25 degrees befor max. cooling. Maybe I’ll find out next summer.