automatic transmission: shuddering on downshift?

[BoneStock67]

Every once in a while, mostly I think when I've started going up moderate incline, the van seems to vibrate for about a second and makes a grinding or shuddering sound at the same time.

Not knowing anything (yet) about automatic transmissions, I checked the ATF, found it was a little low, and topped it off, but the problem persists.

My research here is veering towards the differential, which I haven't serviced or even thought about in the four years/~60K miles I've owned the van.

Any thoughts on what's going on?

Thanks.
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1986 Wolfsburg Westfalia Weekender 2.1L stock
1967 Beetle, now becoming my daily driver again, after a long rest in the garage

"There are two possible outcomes: if the result confirms the hypothesis, then you've made a measurement. If the result is contrary to the hypothesis, then you've made a discovery.” -Enrico Fermi

[GrindGarage]

How many miles are on your van?

Make shure the throttle is properly adjusted at the trans. Wouldn't hurt to change your diff oil.
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-cliff

91 Vanagon AUTO
97 Single Port EJ22 all smallcar.com

[BoneStock67]

Thanks for the reply, GrindGarage. With a name like that, you sound like the perfect guy to be giving advice on this topic.

There are around 250K miles on the van, but only about 120K on the engine, and the final drive was replaced with a good used one right after I bought the van about 60K miles ago.

I was definitely planning to change the differential oil, since I've never done it. And I will look into the throttle adjustment too, as soon as I learn how to do it (assuming it's in the Bentley, right?)
_________________
1986 Wolfsburg Westfalia Weekender 2.1L stock
1967 Beetle, now becoming my daily driver again, after a long rest in the garage

"There are two possible outcomes: if the result confirms the hypothesis, then you've made a measurement. If the result is contrary to the hypothesis, then you've made a discovery.” -Enrico Fermi

[GrindGarage]

I got my AT van with around 200,000 on the clock. The trans dropped 1st gear and totally wiped out reverse camping in PA. Drove it 200 miles back to NJ that way.

I think the trans went since the PO decided to do a filter and fluid change. It was probably never done before and something didn't agree with the new fluid. Got a rebuilt replacement from oregon transaxel for about $1,200 and swapped it out myself along with a smallcar cooler.

There is a thread on here about adjusting the bands in the trans for shift quality, but it's something I have never done myself.

The cable adjustment is pretty straightfoward. When you have the pedal to the floor you want to make sure the transmission is seeing enough movement that it is opening the kick-down. You can feel it on the trans it kind of is like pushing against a spring.
_________________
-cliff

91 Vanagon AUTO
97 Single Port EJ22 all smallcar.com