Short Vibration on 2001 Windstar whenever A/C compressor starts

I have a 2001 Ford Windstar with 102,000 miles. Sometime over the past few months it started to get a periodic vibration. It lasts about 1 second (maybe a bit less), and feels like you just rolled over a
series of 2" pipes. It's most noticeable on smooth highway driving, but I'm pretty sure I've felt it on surface streets also. They are just so bumpy around here that often it is hard to tell. Eventually I realized it only happens when the A/C is on, and is more frequent when it is hotter outside. Generally it happens every 30 seconds to 3 minutes, and it usually happens within a couple of seconds of first turning on the A/C. For this reason I'm assuming it happens whenever the compressor starts up.
The Ford dealer is guessing that it is the torque converter, but that's only a guess. It will take a $2200 repair to find out, so for now I don't plan on doing anything unless the torque converter fails. I asked what the tie-in with the A/C is, and the best they could do was speculate that the added engine strain somehow triggered it. It sounds pretty fishy to me, especially since the transmission is rock solid (by both my experience and their testing). A transmission flush didn't help at all.
Any other ideas as to what the problem might be?
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snipped-for-privacy@sun.com wrote:

A mechanic who doesn't know what he is talking about.
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wrote:

Only with AC on? I would say its the AC compressor when it kicks on. The pressures are neutral on the high and low sides when it kicks it. It will put a strain on the system for about a second when it kicks in. Very common to hear a strange "stress" rattle noise when the AC kicks in and a noticable loss of power (kind of a low power surge) too. To test the torque converter drive at highway speed with very little load, you can hit the brake pedal very lightly to disengage theTC. Let off the brake to reingage. This way you can "play" with it to see if it really is the TC. It is fairly common to get a TC shutter on the highway under light load but the flush will usually fix or at least change it. The shutter will feel like a slight miss under light load at high speed. If it is the TC look for a product called Transmission Doctor its in a red tube. I've had good luck with it.
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On Thu, 6 Dec 2007 14:20:11 -0800 (PST), snipped-for-privacy@sun.com wrote:

My guess is that you need a new serpentine belt. The added load of the compressor is causing the belt to slip for the first few seconds. It stops slipping after the start up load drops off and the belt heats up and gets stickier from the extra load. Then the compressor shuts off for 30 seconds and the cycle repeats. It's also possible that the belt tensioner is bad and not keeping the belt tight enough. I really doubt it is the transmission if it only happens at the same time as the compressor kicks in.
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Now this sounds like a reasonable explanation. The jaring vibration I feel does seem like something you could get from a belt repeatedly gripping and slipping for about a second. I don't believe the belt has ever been replaced. I had a tune-up (done at the Ford dealer) about 10k ago and they didn't replace it then (and no recommendation to replace it). I'm going to bring it to a mechanic I sometimes use rather than the Ford dealer and see what he says. Given the mileage on the belt, it's probalby worth replacing it, even if it ends up not being the cause.
Thanks.
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On Dec 10, 12:55 am, snipped-for-privacy@sun.com wrote:

Hi, My 2002 Windstar (91k mi) is doing the same thing, but I notice the tach rise about 100-200rpm during it's cycle. It goes like this... tach rises 100-200rpm wait about 5 seconds shudder as tach drops wait 15 seconds (almost exactly) repeat It does this any time I use the AC or Defrost (I live in Wisconsin, so this it's either of these almost all the time). I got really annoyed while driving the flat southern MN roads and having it do this 4 times per mile. No hills and if I shut off the AC it stops immediately. I do have a slight shudder sometimes when reaching overdrive, so I think mine might be transmission/tc...but I really hope it's a belt or tensioner. I haven't tried a trans flush yet...that's next on the list.
Thanks, Lyle
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snipped-for-privacy@gmail.com wrote:

I think I see the problem. Your engine is running way to slow. It should idle at about 500 to 700 RPM. Yours is idling at less than 100 RPM.
Jeff

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