Hi all,

I have a 90 XJS convertible.

Last winter, I got great heat for a short time, but it gradually turned cold. I learned to put the defrost on and that is how I had heat. I figured I'd address it later.

I had to go in to the AC unit over the summer to drill out those drain holes so the AC wouldn't short out. The AC works great.

Since the weather is turning cooler here in NM, I noticed the heater isn't working on any setting. I only get cool air and I have never gotten anything coming from the footwells. It when the heat is on, the air comes from the dash vents.

I checked the vacum at the water valve and in either hot or cold, there was no vacum.

Any suggestions??

Tony

 

Tony, if you are still able to get heat from the dash vents at full defrost, then the blend flaps in the unit that mix the air that goes to the cabin and footwell vents are not working properly. Could be they are jammed or that their actuators are not working.

The unit has full heat going through the heater matrix at nearly all times, and produces the selected temperature by blending the heater and the aircon airstreams together. So if you can get heat on the defrost setting, this proves the heater matrix is getting hot.

The vacuum at the heater valve unit on the engine bay bulkhead centre should only be there when full cold is selected, at which point the unit sends vac to it to switch off the hot water to help the unit get cold. Check to see if it does get vac at that setting.

If not, it could be that the vac reservoir is faulty or the vac lines are perforated, so if at full cold there is no vac at the heater valve, open up the cover on the passenger side engine bay slope, (the place where where the pedal box casting is on the other side) and check the round vac reservoir you will find under there.

It should have one feed from the manifold and one to the interior. In addition, you should see a vac line going to the heater valve directly from the cabin. If you pull off the vac line to the manifold, there should be a hiss as the stored vac escapes. If no hiss, that is part of the problem.
Greg

 

No vacuum to the heater valve is a good thing when you want heat. The system is designed to apply vacuum to the valve when max cooling is required.

Anyhow.....

I would begin by cleaning the connectors and grounds related to the system. Look for the tiny wires and tiny connectors on either side of the climate control case....a couple of them are up pretty high. Open them up...and don't be surprised if they are packed with green crud. There are about 6-7 connectors to seek out.

The system grounds are on either side of the case. Look for the vertical dashboard supports. You'll see ground wires on each one...quite possibly corroded.

I've fixed a couple if semi-dead systems by cleaning the connections and grounds. Even if the problem remains unsolved the connections and grounds need to be clean for any testing measurements to be trusted.

Cheers
DD

 

IN the DA Mk111 the only time the heater tap (under the bonnet) is turned off is when the AC temp is at max cold.

Greg is correct the unit blends cold and hot air to deliver the desired temperature,

Pull the side covers (the ones with the foot well vents) and check all the connectors, I will almost guarantee you will find engine coolant has leaked into these connectors causing issues with the HVAC. Jaguar recognized this issue and modified the O rings in the XJ40.

If this this the case you will need to replace the O rings in the LH side of the heater matrix, this is where the DA Mk111 leaks, you may need to replace the heater core as well.