I have a 1976 jaguar XJ6C in for repair on the Hvac system, Ive replaced one of the mico switch on the mode control c for the low speed mico switch and have no high speed and no control for the defrost mode, if anyone can help me that know the system, I also bought the tool from pektron to help dignose the system, but not really a jag teck, but have worked in the automotive for thirty years, thanks

 

Hello, Dave,
Try this link: Delanair Mark II Automatic Climate Control
It contains quite a lot of information about system which was built into 1976 Jaguars.

I just tried the link and it's still good.
Welcome to the section, and the forums from ElinorB.
(';')

 

in the 1980-1987 (the Series 3) XJ-6, the 4 fan speeds are controlled by a large Relay which contains 4 small relays and is located on the driver's side, behind the Vent Panel next to the Emergency Brake Handle.

I am not familiar with the 1976 a/c system but if it is the same Mark 2 system used in the Series 3 XJ as LnrB says, then you have two possibilities:

1) the Mode switch itself, (Low High.AUTO. Defrost)
2) the large relay mentioned above.

One of those components is going or gone. I doubt it is the Mode switch, those rarely go bad, though I am not saying they don't. The servo in particular controls the opening and closing of different Vent Flap Actuators via vacuum and roller switches, not the speeds of the blowers.

To check the large relay, remove the driver's side Vent panel to find the large Relay;

with the engine running, put the mode switch in HIGH, then wiggle each wire connected to this large relay, particularly make sure that the center wire with the screw is tight. See if there's any change in the response of the blower speeds.

As a last resort, this large relay can be opened and checked inside, there are tiny wires connected/soldered from one small relay to the other, and one of those could have come disconnected.

the picture below shows this relay.

 

A good electrical supplier will have all of the micro switches in stock.
I paid less than $20.00 for all, for replacing the cheesy stock switches.
Once I replaced all of the old vacuum tubing, micro switches, cleaned and greased the gears, installed a John's Cars lifetime warranty AC amplifier and replaced the sensors, I have satisfactory air-conditioning.