Now its my turn to fix the heater and ac controls on my '90 XJS.

Here are the problems;

1. Hot not hot. I get some warm air but not as hot as it should be.
2. Cannot get air to blow through the cent vents.
3. No vacuum at the hot water valve when in ac mode.
4. No voltage to the compressor clutch when in ac mode.

What works;

1. I have already replace the hot water valve.
2. Temperature and Fan controls appear to work. Air will flow at the foot wells or the defrost slots but will not flow from the center vents.

Troubleshooting suggestions greatly appreciated.

Thanks!

Mark

 

The center vents are for max ac only.
If there is a vac leak on same circuit as the water valve it will not operate. I had vac leak in cruise control and it knocked out heater as there wasn't enough vacuum to operate valve.

 

I have spent a lot of time under the bonnet fixing and replacing nearly every hose I could find. Like many old cars, mine had been worked on by people who made changes to some of the original hoses and wiring. I have restored all of them to OEM condition.

If there is a vacuum leak then it must be in the cabin. I will pull out the interior panels next week to get as much access as possible to the ac/heater system. I will check all of the wires and hoses and see if I have any success.

Mark

 

Start with the vac reservoir. This is under a plate in the equivalent position on the passenger side to the pedal box. Accessible from the engine bay by undoing four bolts. The vac reservoir is a round flattish cannister in the small compartment revealed. It should have a vac tube going to it from the manifold and another tube going from it to the cabin. In addition there is a third tube going though this compartment which is the vac tube to the water valve.

When you pull the vac tube from the manifold off the cannister there should be an audible hiss. If not, you either are not getting vac to it, or it is broken, or the vac tubes from it to the cabin are leaking. there should be a non return valve in the tube between the cannister and the manifold to prevent vac leaking back to the manifold. All this must be ther and working for the system to work.

Let us know what you find and then we go on from there!

Greg

 

I'm glad you reminded me about the vac reservoir. I will check that first.

Thank you!

Mark