No it's not the solenoid. The solenoid supplies vacuum to the actuator. The actuator will be a metal can with a pipe at one end (vacuum) and a rod at the other end to move the flap.

To test if it is leaking use a large syringe, disconnect the hose from the solenoid and put the hose on the syringe and suck. The flap should move and hold vacuum. If the flap stays open for 3-4 min the actuator should be good.

 

Thanks. I do this with the a/c running, right?

 

In the photo, the metallic round device is an actuator. It's connected to a solenoid vacuum valve. The actuator has a air (vacuum) connection on one end, and a mechanical linkage on the other...usually a rod. The other end of the rod is connected to a flap, which blocks or unblocks airflow to a particular vent. The solenoid vacuum valve is connected to the actuator by a hollow tube. When the solenoid vacuum valve is closed, the actuator is in its normal state (no vacuum on it). When the solenoid vacuum valve is "open" (which exposes the innards of the actuator to vacuum), then the actuator causes the mechanical linkage to move, which in turn moves the flap, which in turn opens or closes the path of hot air, or cold air, or whatever.

As you can see by the photo, this particular actuator is for the center vent. Short of removing the dash, I've no idea how to replace it if it were leaking. In my recent case, the problem was that one of the solenoid vacuum valves failed. They are much easier to replace.

 

You can do it with the engine shut off. Since the syringe, or you, or a hand-held vacuum pump is taking the place of engine vacuum, there is no need to have the engine running. You just need to have the ignition on so you can run the fans and feel if the air is coming out of where you expect it to come out.

And warrjon, thanks for all the wisdom you've provided me over many posts on this topic.

Thanks,

John

 

Thanks for all that detail you've give to someone who only recently (after getting the XJS) even knew that cars used vacuums at all.
I am a little confused about one thing, however. If I'm reading you correctly, you say that the actuator controls the flaps using mechanical links, but according to KWE's nice detailed account, the Mark III (which I have) uses motors. ("The Mark 2 used a motor-driven rotary controller which operated the air blend flaps directly with levers, and the various other flaps and valves by vacuum. It senses the difference between the temperature of the cabin and that which the control knob is set to, and varies the proportion of hot to cold air accordingly. The Mark 3 system carries out the same functions but the controller is a microcomputer and the main air blend flaps are operated by electric motors.") That... doesn't make sense to me. If the flaps are operated by motors, what is the vacuum for in the first place? That my car uses vacuum is not in doubt. So what's KWE on about here? Is there a distinction between the curved doors inside the blend box that vary the temperature of what comes out, which are motor-driven with the MkIII (because even without air from the centre, what air does come out the other vents is cool), and whatever flaps actually direct the air, and are vacuum controlled? That makes sense to me.
Anyway, I'm going to read your advice several times until I understand it, and also look for a large syringe--I only have small ones at home. Mind you, I can always try a small one first I guess....

On this diagram, No. 6 are referred to as "vacuum motors" - are these the actuators? If so, would Vacuum Tube 7 leading down from Elbow Joint 9 be the black hose that connects to the restrictor and solenoid for the centre vent? If so, I can see how it would be not easy to access....

 

Yes 6 on that diagram are the vacuum actuators, Jaguar may refer to them as motors but they are not electric, they are operated by vacuum and open and close the flaps controlling air to the screen and centre vents.

The electric motors are on the RH side of the evaporator, circled in red in the diagram. These motors control the blend flaps that are inside the AC unit to control the temperature of the air. The lower blend flap also controls air to the footwell.

The pictures below are all of the AC unit the last one shows the blend flaps and how they work. I hope this hepls

 

Very helpful, thank Warrjon. So the lower blend door controls temp of footwell air, while the upper controls temp of fascia and windscreen air, which in turn is directed where it should go using the vacuum motors/actuators? So changing from, say, cooling to defrost means that the lower blend door rotates to block off cold air to the footwells while the upper rotates from allowing all cold no hot to allowing all hot no cold while at the same time the actuators close the vents to the front/fascia and open the vents to the windscreen?
Anyway, I need to do that syringe testing soon. Not today, however, as it's very wet over here and I'll need the door open. At least the rain makes things cooler....

 

You could also try it by just sucking one one of the lines, if you can figure out how to extend it so you can reach it. My actuators worked at 15" Hg vacuum, which I could pull with my lungs (but I have strong lungs). I get 15" Hg vacuum at idle (see measurements in an earlier post) so it seems like that is at least the minimum vacuum they are supposed to work at. So, if you can pull 15" Hg by sucking on the tube (no jokes, please) you should be good.

Thanks,

John

 

I honestly have no idea how strong a vacuum I can suck, but I know I don't have any leeway for extending the tube, as it's only just long enough to get over the nipple (no jokes here either) on the solenoid. I suppose I could find some extra tubing and see if it fits over the end, mind.

 

Okay, I have proven I don't suck much....
Or at least, I can't suck much. I couldn't get the vent flaps to move by sucking on the hose (via a thin straw) directly. I could feel a mild vacuum when I attached the straw to the blue and the yellow resistors in turn and sucked, so the yellow resistor does let air flow through it, definitely. Which maybe suggests the actuator, I fear. Still, wouldn't hurt to get another yellow one and see, as that would be cheaper and easier than removing the dash. And frankly, if the only issue is a clunk when the a/c's turned off, I can live with that.

I tested the magnetic clutch on the compressor by listening as, with the engine on, I turned the a/c on an off. I could hear a definite click--actually a double click--though 4 times out of 6 (or so) the compressor itself wasn't spinning--at other times I could see the front of it whizzing around, but mostly not. I didn't run the a/c for more than a minute or two each time - just enough to see if the compressor engaged. At all times the aircon was set on full manual override cool (knob at top and pulled out a notch). So the aircon appears to make its own decisions on whether to engage the compressor or not. I didn't leave it on long enough to see if the compressor cycled on and off and on, however.

Between all that sucking in awkward poses, replacing the footwell vent and knee pad stuff under the dash, and then replacing some of the retaining sockets and studs on the driver's side cantrail, I have decided sod it--I will not work on my car in summer. Far too sodding hot. It's not even thirty degrees today, but the sun is something fierce....