XJ40 A/C is hissing at me!

[BritCarFan]

Hello forum,

When I turn on the a/c, a hissing sound comes from the vents. Earlier in the year it hissed occasionally, but now it's constant.

Any help or thoughts on this?

 

[vettedog72]

Well if it is constant, it rules out snake! I suspect you have a vacuum leak in the control. I am not knowledgeable about the system but I just had to take a swipe at this. Hope you find a little relief in the humor.

 

[Exotexs]

the hissing could be a vent-flap not fully opening, and that means a vaccum valve or "solenoid" going bad or gone.

The XJ-40 (unofficially called the Series 4 XJ) has a Delanair MK-III climate system, which is somewhat similar to the Delanair MK-II system in my Series 3 XJ.

The Center, Side, and Demister Vent FLAPS are operated by little vacuum valves in each vent box and these are known to fail.

Ideally you want to remove the Dash Cap and operate the climate system in all modes to see if all flaps move to their fully open and closed positions.

The Center flap might require pulling the plastic vent assembly to watch it opening or closing.

If you have a Service Manual CD (about $25. at Jaguar dealers) it will guide you through removal of those parts and troubleshooting the system.

Ex