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?

jeff

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

scottm

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.

ashton crusher

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.

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.

lyle.stoll

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

jeff

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