Hello
What is the difference between switches-water temp. JLM 763 (red) and JLM 2121 (black).
I have a black switch on my xjs and I have no idea what to replace it with and if the red one has the same parameters.
Can the correct function of this switch be measured?
Jakub Drobílek
Welcome Jakub,
As far as I know the Water Temp By-pass Switch, as it’s officially called, prevents turning on the blower fans until the AC cools the evaporator.
If the switches in question are physically the same, which I think they are cause according to https://parts.jaguarlandroverclassic.com both are listed for the same models, either will work.
Just the opposite. Blowers are defeated until the coolant entering the heater core reaches xxx-temperature. This way, if heating is called for, there’s no blast of cold air into the cabin. The blowers come on when the coolant is hot enough to actually give some warmth. The trigger point is about 40ºC I think.
Disclosure: It’s 4:20AM and I’m just two sips into my first cuppa coffee 
I can’t think of any possible difference except the trigger point. I’m inclined to agree that either would work. The 2121 switch was used on facelift models; the 763 on pre-facelift, as I recall.
The MKIII system went thru several minor changes over the years. This is one of them. I think it’s discussed in The Book.
Cheers
DD
Yes, my mistake, I called the switch the wrong name… but the switch Jakub is referring to is the equivalent in the AC side and does what I described.