Just a few thoughts.....

When I had a fairly low whining noise on my previous car, a Rover 75, I soon found out that the whine disappeared when I turned off the AC (climate control in my case). It then became easy to diagnose, because if the connection to the compressor clutch was disconnected, the whine never appeared with AC on or off. At that time, some very cheap, but brand new compressors came up on eBay, so I bought one and had it fitted, and.... no more whining !

Now here's the thing - the Technical Notes I have for the X350 describe the AC compressor as a "variable displacement" (clutchless) unit. There is still an electrical connection to the compressor, but the technology uses the variable swashplate pump principle.

As Brutal has said, getting the noise you describe when the AC is on and it disappearing when off ihas to be a very strong indicator of compressor failure. When turned on, the AC controller will command the compressor to output refrigerant liquid, and the swash-plate pump will be activated to provide the required output. When turned off the compressor is commanded to cease output and the swashplate pump will be set back to Nil.

When the compressor is producing output of refrigerant it will put a load on the engine, which is fed back to the ECU to compensate engine power as necessary.

This article describes the compressor in more detail

http://delphi.com/pdf/contact/brochu...ompressors.pdf

There is apparently a checking tool using electronics, which air con maintenance workshops should have.

http://www.compressortech.co.uk/down...A4_CLT1_06.pdf

Just using the "screwdriver stethoscope" test will point you in the right direction. You could also disconnect the electric plug at the compressor, then turn on the AC, and see if the trouble occurs or not. If it still does, it is not the compressor.

Have you taken the car to an air con specialist ?

 

While there may be an issue with the compressor, I am still not convinced that is the issue I am describing here.

As I mentioned before, the vibration is worse towards the rear of the motor, perhaps a mount or transmission issue?

I tried something else today. I do not notice the roughness or noise while underway, however, for example if driving at 40 mph I gently apply the brakes, the vibration and roughness starts and continues even while I am stopped, as does the noise. If I shift to neutral, the noise continues and the roughness lessens but it is still there. Now if I do the exact same thing with the AC compressor turned off, none of those symptoms exist, for the most part. I say that because to clarify perhaps about 10% of the time everything operates normally and also, the amount of roughness is variable. And increasing RPMs while in neutral seems to make no difference either.

Hope that helps to getting closer to the root of the problem and a solution

 

Call Car Talk on NPR on Saturday morning. I would love to hear this!

 

Problem persists...yesterday I thought the car was going to stall twice it was running so rough...

Anyone else with ideas?

 

Go back to the beginning........tell us what was happening with the car when you first noticed this business. Had ANYTHING been done to the car recently, any maintenance, any impacts, big chuck holes, accessories added. Anything, no matter how seemingly insignificant could be important.

Cheers,

 

It has been like this for a couple of years at least, just getting more frequent and pronounced. I thought replacing the throttle body would help, but it did not. The car has not had many issues in 6 years until recently, so it has had little warranty work done other than maintenance.

 

Did it again, 2 blocks from the house, cold start, ambient temp 84 F,

 

I wish you success in finding the cause.

 

Oh how I wish it were that simple. As the title of this thread suggests, there has been nothing for the dealer to go on. The CEL light would be something. If there was a leak, would not the dealer have been able to detect it?

 

Which sadly can make Jaguar ownership an unpleasant experience. They won't even let me speak with the techs.

 

HELL I talk to customers all the time and some have my cell number.

 

Would you like to live in beautiful Tampabay area Florida? There are 3 Jag dealers for you to chose from....

 

jahummer, I've been pondering this one for a few days, and I'm wondering about the possibility you may have a fractured or broken flexplate. I know from experience a fractured flexplate can be somewhat temperature sensitive and sort of come and go. The reluctor for the crankshaft speed sensor is on the flexplate, and irregular movement in a broken part can cause running irregularities and noise. Unfortunately the only sure way to determine this is to remove the transmission and look; although someone very good with an oscilloscope might be able to detect an issue in the crank sensor signal. That could be problematic though.

Might be worth discussing with your dealer guys, they have the car in front of them, we don't.

Good luck!

 

I thought of this originally but it can't explain why the problem only comes with the AC compressor running.

 

Actually that's why I keep coming back to this possibility. Things like flexplate or balancer fractures have always been very load sensitive in my experience. Sometimes it's the in and out of gear that affects it, sometimes it's the A/C on and off; I've even zeroed in on a flexplate before because of the alternator load caused by turning the A/C blower motor from high to low, you just never know. By the same token, once he resolves this we may find this whole idea was way out in left field.

Mechanical quirks can be as strange as electrical quirks at times.

 

I'm in Cincinati if you want to come look at it Small update, while stopped in gear as mentioned before noise and vibration is bad, shift to neutral and it goes away. So perhaps this could be the problem? Anyone have experience with White-Allen Jaguar?

 

If you don't have any luck there, we're only a couple hours away. Just about any day would be OK, except I'll be away 12/30 to 1/6/12.

Good luck!

 

Another tech has suggested the serpentine belt and a pulley might be the culprit. There is a chirp from the front in cold weather. I brought it to Jaguar a couple of years ago but they said it was nothing.

What do you think?

 

That actually crossed my mind, but I just couldn't imagine that it had not been confirmed or eliminated by someone at this point. In my mind a simple squirt of water would tell whether the belt was noisy. Water on a noisy serpentine belt will either temporarily worsen or eliminate any noise it is making, confirming it as the source.

But I've been wrong before......

 

What about the vibration?