Jaguar Repair: Heater, climate control system, cold mind

Question
Hi Howard,

While I am on a roll.  1987 XJ6, Defrost blows hot, when I turn the dial to Low-Auto or High it might blow hot it might blow cold.  Mind you nothing has been changed on the temperature setting.  I am new to the car and continually learning.  What is the purpose of the 2 temperature settings, one is bottom center and the other on the left of center?  Thanks again.  Bob

Answer
HI Bob,
The left knob totally controls temp.
The right knob is "Mode" and controls fan and modes with vacuum controlled doors.

The AC is on all the time that any heat is on to help cut humidity down.

The series III Jag has a vacuum controlled heater valve in the center of the firewall in the engine compartment and it is a common item to fail,so I would check it for freedom of movement first. If however, the switching from hot to cold and cold back to hot is a quick change, it is more likely a blending of the door movements as a cause. So first check that the vacuum is good and steady supply. (Two ports through the lower right firewall)

The control of the vacuum doors is done by a rotary drum that operates all of the vacuum valves.

The complete system is a climate control system and is complex. If the items above don't correct the problem you would be better off puchasing an AC manual from Jaguar. It has a series of complex tests to isolate problems.
Howard