[W126 Coupe] Re: Hard shift cause and cure

[Richard Hesek]

I haven't been following this thread closely, so forgive me if I am
repeating something already mentioned.
I had a problem with the transmission on my Euro 560 some years back. If I
did a 1st gear start or kickdown to 1st, it would not shift back into 2nd
under acceleration and would over rev and cut the engine with the over rev
circuit, then drop into 2nd (and no it wasn't the slider adjustment just
behind the air filter to adjust the shift points). I had the car at the
local MB service centre and they stripped it down, cleaned, tested,
adjusted, etc, etc. The result was exactly the same. Basically they
couldn't find the problem. At the time the fluid was fine and not milky or
anything. I had at home this package of engine and transmission additive
which I think a family member bought for me, but I never bothered with it
(until then). I thought why not, it can't make things any worse, so I
dumped in the bottle of the transmission additive in the tranny. Next few
days driving, suddenly the transmission started working perfectly and has
done ever since, but now the fluid looks milk
 y, cloudy and sometimes foamy too, but the transmission itself works 100%
and with great shift changes throughout the range.

So, after all that, it could be that sometime in the past (unless you have
had the fluid changed) that a previous owner has done the same thing as I
did and put some additive into the transmission. If so, I doubt that your
transmission fluid is causing your shifting problem. Have you tried
adjusting the modulator? Also, have you checked that your shift points are
correct? i.e..under full acceleration shifting just before, or dead on the
red line at 6000rpm and of course your limit marks per gear are also marked
on your speedo.

Just a thought.
Vance.

Thanks Vance,
 Modulator was good and functional. The fluid was, in fact, the cause of a
poor shift quality. Since I got the car back from that work it has
performed like new. Seriously, very smooth, quieter and tighter
acceleration. I believe the tranny was slipping under way. A little bit of
water, which disperses thru the fluid, will wreak havoc in the tranny valve
body and shift quality.
  In the case of your transmission behavior, I suspect there was a stuck
spool valve in the valve body and was not shuttling to relieve the pressure
in upshift position. This spool valve may have been gummed up with old
burnt fluid or maybe even dirt. The additive was solvent enough to "free"
up that spool valve and back to normal.
  That is a benefit of these additives in the short term. There are no long
term benefits of these additives since they can cause problems with rubber
seals.
I would drive your car spiritedly for a couple thousand miles then get the
fluid and filter changed. I'll bet you will never see that shift problem
again.

Rick Hesek