'89 Caravan Shimmy at 35 mph

I have a newly acquired 1989 Grand Caravan (106,000 miles, LE with 3.0 V6
and the A610 transmission) that often shimmies at 35 mph. It starts at
about 33 mph, peaks at 35, and tapers off at about 37 mph. It happens only
when accelerating or to a much lesser extent when holding a steady speed
at 35, but never any shimmy at all when coasting.

Sometimes, it doesn't shimmy even when accelerating. Whether the motor and
transmission is cold or hot seems to make no difference.

At 35 mph, when accelerating, it can be a very noticeable vibration. The
whole front of the vehicle shakes side to side and you feel it in the seat
even more than the steering wheel.

I'm thinking this is most likely a CV joint, am I probably right? Or is
this a symptom of "transmission shudder" and if so is it a symptom of an
imminent transmission failure?

I have a Haynes manual, in the troubleshooting section it suggests "CV
joint angle" as a possible cause, but after reading the CV chapter I see
no information about setting or even checking CV joint angle.

There is a procedure to check the length of the CV shaft (and adjust by
sots in the motor mounts) but after reading it a couple of times I see no
mention that this is an angle adjustment. Is setting the length the same
as setting the angle?

 

When one of the inner CV joints is worn, it will not rotate inline with the
axle at certain speed ranges and under load and will cause vibration at
those speeds. The severity of the wobble depends on how badly worn the
joint is.

The only thing you can do is rebuild the joint or replace the axle.

 

Thanks for the info. The CV joints was my best guess, but considering the
history of the A610 it is a relief to know that the shimmy probably isn't
the transmission.

Plus, I can replace the half shafts myself with relative ease, but I'd
rather not tackle a transmission swap - not to mention the cost.

 

I've done axle swaps on my '88 a few times and, frankly, it's an easy and
inexpensive job. When the inner cv joint on mine went out, I couldn't
figure out which axle it was. I guessed wrong and ended up with two new
axles.

Anyone know how to figure out which one it is?

 

I've only swapped axles once.

It turned out to be very unpleasant due to the parts company putting the
wrong part in the box. In a case of things coming together in a bad way, a
friend had stopped by and monkeyed with the old half shaft. He left the CV
joints with the tips barely touching (which was concealed by the boot)
instead of meshed, which caused the new (wrong) part to be the same length
as the old one. I just couldn't figure out why that new axle seemed an
inch too long to go in when it matched the old one perfectly. It
took me an entire day to figure it out.