The first hot day of the year came and I turned on the A/C. What I got out of the vents of my 2001 XK8 convertible was HOT air! Not good. I noticed that the air was cool when I first started the car, but then the air got hot. A little JTIS research and deductive reasoning identified the problem.
The heater valve, or water valve, was stuck open. This valve isolates the heater matrix (heater core) from the cooling system when cold air is desired from the A/C. The valve is on the left (driver's) side of the firewall, just below and behind the coolant tank. It has a two wire connector and 4 heater hoses going to it.
From what I just learned, this is how I understand this system. Normally, for heat, air flowing through the air distribution box passes through the evaporator, which does nothing if the A/C compressor is disengaged. The air then passes through the heater matrix to be heated. If you want defrost, the A/C compressor engages and the evaporator dries the air before it's heated in the heater matrix.
If you select cold air, the A/C compressor engages, the heater valve closes to keep the hot coolant out of the air distribution box, and the cool air bypass flap opens (it's in the air distribution box next to the heater matrix). This allows the air or at least most of it, to bypass the heater matrix. The heater matrix has ambient temperature coolant in it at this point, basically it's doing nothing.
When my heater valve stayed open, there was hot coolant flowing through the heater matrix in the air distribution box, heating everything in the area, so I got hot air. I did not get hot air immediately, the air was cool when I first started the car, so I knew that the A/C system was capable of producing cold air. But as the engine warmed up, the coolant getting into the heater matrix got hotter and finally overcame the A/C system's ability to cool the air.
The heater valve is normally relaxed open. It is energized closed when commanded by the A/C control computer. So, it was open all winter long.
My solution was to start the car and turn on the A/C. Assuming that there was probably electrical power to the heater valve so it was trying to close but probably stuck, I gave it a love tap. A long piece of wood or other soft material like a brass punch, one end against the valve and a hammer applied to the other end and, Voila! Cold air.
The valve stuck again so I repeated this but lately it's working fine. I hope that the more I exercise the valve, the better it will work.

 

Moved to the correct section Stu

 

If you don't run A/C all the time, it's a good plan to switch it on about once a month and cycle through from max hot to max cold to prevent this kind of issue.

Graham

 

Agreed. Due to the complexity and failure-prone components in our Jaguars' HVAC systems, I believe it is better to keep the HVAC system on and running throughout the year. A frequently-operated HVAC system is less likely to suffer major failures than one which is used only sporadically, at least in these cars....

On the contrary, the very simple HVAC system in my 1999 Dodge Ram pickup rarely gets used during our cold-weather months because I'm a polar bear and love a frigid cabin. However, that truck's HVAC system functions just as well as it did when I acquired the truck in December 2000 and has never needed any service or R-134a top-up at all....

 

hmmm, good tip, I'm getting a little 'warm' A/C air when sitting at idle. Strangely, immediately starting the car at idle, the air is really cold...then later on, idling gets me like warm. A little twist though, at highway speeds, the air cools back down... I'll check my heater valve, just in case a love tap diagnoses this annoying temperature change.

 

Just guessing here but it seems that the increased airflow across the condenser at highway speeds may be helping, which would mean that your A/C system can produce cold air before your engine warms up, or when there is lots of ram air through the condenser. No other part of the A/C system senses vehicle speed, nor can I see any other part of the A/C system being sensitive to engine temperature. The heater valve that I had a problem with gives heat no matter what the vehicle speed so that's probably not your problem.
The good news is that your A/C system is capable of producing cold air.
On startup, maybe the condenser can dissipate enough heat to work for a little while. Maybe one (or neither) of the two fans is running. When the engine is warm, the coolant in the radiator may be reducing the efficiency of the condenser if the fans aren't working properly to scavenge the warm air and the vehicle is sitting still. The heat can radiate from the radiator to the condenser and a condenser surrounded by warm air doesn't work well. At speed, the cool ram air takes over.
I'd make sure that both radiator fans are working, especially when your lose your cold air. Since the fans are thermostatically controlled, on startup they're not running. Your engine has to warm up to get the fans to turn on, right? If they don't, and you're sitting still, you're losing the cooling needed at the condenser and radiator.
This is just me thinking out loud.

 

Mine also gets warm air at idle and cool when rolling down the highway. The thing that I don't understand with mine is that if I rev the engine at idle to 1700 RPM or above it gets cold then goes warm when I let it idle down.

 

You need to check your fans, fan relay and perhaps most likely, the air-conditioning 4-way pressure switch, which controls the fans when the A/C is on.

 

I agree. My condenser fan had been out, allowing hot air to come out of the vents at stop lights (idling) and cold air when at speed. Driving with it this way for a while allowed the compressor to compress the material at hotter and hotter temperatures while idling. It resulted in the destruction of my compressor while on the road during my trip from Tucson to Boulder. A GREAT Jag technician at Autowerks-East in the Boulder, CO, area replaced almost my entire A/C system in 2 days, getting me back on the road in time. Of course, he also replaced the fan that blows on the condenser! Total cost for this complete rebuild was $2007. It blows incredibly cold air, even at idling! In this record 107 degree heat, I have had to turn down the A/C after the interior cools. A new relay and air dryer were also installed.

 

My problem turned out to be three leaking a/c hoses and the leaking low charge port atop the dryer...my local Christian brothers repair shop charged $350 for labor, and I got the new hoses fromjag (gaudinjaguar) for $320. I am cold at idle as well as driving.

Since I had hardly enough freon left in the system to do anything, the pressure switch wouldn't work at idle, but at highway speeds, the switch worked n compressor functioned fine.