harsh downshifts in 535i auto trans appear fixed...

[Robert K]

Every so often, the auto trans in my 1991 535i with 149,000 miles downshifts hard when coming to a stop. The upshifts are usually fine, which lead me to believe it was an electrical gremlin. Anyway, I decided to drop the auto trans pan the other day to have a look around. I typically change the trans fluid about every 5,000 miles by draining the pan and adding approx. 3.5 quarts, so the fluid didn't look too bad. I went ahead and pulled the filter. It was slightly dirty and had a thin film of gray looking scum on it. I cleaned it with gas at first, then decided to go ahead and purchase a new filter and put it in.

When I removed the old filter, it didn't have an o-ring between the filter and the valve body. The valve body appeared to have a groove for one, so I checked the online ETK and my Bentley manual. Both stated that there was supposed to be an o-ring. The one I purchased from an auto parts store was the correct diameter, but was too thick in cross section. I occassionally rebuild Bostitch air staplers where I work, so I looked through our spare parts and found an o-ring that appeared the right size. Took it home and it turned out to be a perfect fit. So, I installed the new filter with the o-ring, refilled the trans with new fluid, and for the past couple of days, the trans has been shifting perfectly, both up and down the gears.

Thought I'd share this in the event that someone else was having similar problems.

Grace and peace,

Robert K
1991 535i

[Russell]

Great results. However, why not go to the dealer and buy the proper o-ring? Should not have cost much. Of course I am assuming it was available by itself.

[DABLACK]

I had the same situation as you Rob I got a complete kit at work and
no probs it shifts alot smoother now! But sometimes in alot of situations oem is the way to go if you can swing it dollar wise, even with my discount somethings lead me to go aft/mkt.

[Robert K]

Dealer number one...65 miles away
Dealer number two...75 miles away
Dealer number three...60 miles away

That's a lot of time and gas for a $.15 O-ring. Besides, the one I found worked.

Grace and peace,

Robert K
1991 535i

[Russell]

Makes sense. My dealer and independent are each 15 minutes away.

[gale]

Most common black o-rings are Nitrile or Buna-N, which is "o.k." for a non-moving part in the trans in the short term. Would be better to use a Viton o-ring. Viton o-rings are usually black but can also be brown or other colors. They're more impervious to hot ATF which can do a number on common rubber compounds. If you did use a Buna o-ring, you're probably good for many 10's of thousands of miles, but for peace of mind, I'd put a Viton o-ring in it the next time a fluid change is due.