My '89 Coupe is not getting vacuum to the Heater Control Valve. I checked the Vacuum Check Valve is operating correctly (checked due to tip in Kirby's book). There are two vacuum lines that go through the firewall to the environmentals on the right side of the engine compartment. There is good vacuum on the larger line, but no vacuum on the small line that goes to the Heater Control Valve at any control setting.


Also, the A/C clutch wants to engage continuously regardless of the selected temperature. I am wondering if these symptoms point to a particular cause. Anyone familiar? Thank you.

 

Possibly the 'water valve solenoid' isn't working? Or not connected? I'll try to find a pic for you



Normal operation. This is done to dehumidify cabin air


Cheers
DD

 

This shows two solenoids assemblies, Item #1 and Item #2, each having two actual solenoids

Air Conditioning Vacuum Solenoid and Pipes - Parts For XJS from (V)139052 to (V)179736 | Jaguar Heritage Parts UK


I can't remember offhand which one pertains specifically to the heater valve. Someone here will know. But at least you have a rough idea of what to look for, visually.

Does the center dash vent open when you select maximum cooling? Here's why I ask....

Vacuum is applied to the heater valve when cold air is requested. The center dashboard vent opens when max cold air is requested. If the center vent *does* open then you know, at minimum, that the system brain has received the request for cold air and the system is trying to comply.

Cheers
DD

 

The A/C is always on regardless of the chosen heating setting. It works by maintaining the a/c matrix at a set temp a,d then blending hot and a/c air into the car using the flaps of the heater box. You will find that the a/c compressor will automatically cut in and out from time to time when and if the matrix is cold enough.

The two vac pipes that come though to the engine bay from the heater unit can be accessed by removing a plate on the sloping part of the engine bay firewall on the passenger side where the pedal box fixings would be on the driver's side. Under this plate is a round vac reservoir to which the big vac line goes from the inlet manifold. Make sure this line is OK, if you run the car and then turn it off and pull the line off the reservoir you should hear a hiss. If not that is one thing to fix.

The reservoir has a second tube from it (NOT the second tube to the heater valve) going into the interior, this is the vac feed to the a/c unit. The second engine bay vac tube goes from the heater valve directly to the a/c unit and does not stop at the vac reservoir. This tube closes the heater valve at FULL COLD.

IF you turn your LH temp selector to full cold, this tube should have a vac on it and the heater valve should close. If under this condition (a) there is vac to the unit from the reservoir and (b) no vac on the heater valve line, then either the vac is escaping in between the reservoir and the unit, or the line is punctured somewhere, or a line has come off its spigot somewhere, or the vacuum solenoid is jammed not working, or something else!

It is very possible that a vac line has come off its spigot on the unit, so worth a poke about inside, but it is very hard to see what's what without lots of trim being removed.

greg