There is no vacuum to the heater core valve and because of this the ac system has a tough time keeping up. I replaced the heater core valve as it was faulty as well. Is there a trouble shoot method to test the hvac vacuum servo or could this be an issue caused by something else. As always thanks for your help. 1995 XJS 2+2

 

If you can get to it, unbolt the plate over the passenger side footwell tunnel in the engine bay, where the pedal box would bolt up if the car was steered on the other side. Under here, on pre-facelift cars at least and I think, repeat think, facelifts are the same, is a round vac reservoir with two vac tubes going into it. One goes to the interior, and one goes out of the cover plate via a one way valve to the inlet manifold to get vac. There is also a third tube that comes through the plate and goes direct to the heater valve vac take off. This is the vac signal that closes the heater valve on full cooling.

One or more of these may be leaking, unseated, disconnected to the manifold take off, etc etc. Check them out and also that the one way valve is not jammed shut.

Greg