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Old 11-26-2000, 05:00 PM
Registered User
Join Date: Nov 2000
Location: west chicago,il,usa
Posts: 68
This is a great site!!!

My 1992 500sl transmission will sometimes "thump" when Im slowing down, not quite at a stop, and then accelerate hard. Any ideas?
The car has 60K.

thanks again for all the past help!!!
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Old 11-26-2000, 08:33 PM
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Join Date: Mar 1999
Location: Suwanee, GA, USA
Posts: 4,712
I have seen where the little cap on the modulator valve will crack and could cause this.

Donnie Drummonds
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Old 07-29-2001, 01:56 PM
Registered User
Join Date: Nov 2000
Location: west chicago,il,usa
Posts: 68 update. I've been casually looking into the
previous suggestion to check/replace the rubber cap that
covers the vacum adjustement mechanism. Although
my cap was "cracked" every mechanic I talked to said:
"its just a cover and doesn't affect the transmission".

Well two weeks ago I subcribed to and checked
out the tech bullitens and found one that talked specifcally
about this rubber cap and the affect it has on shifting.
I had it changed last week and what a difference!

This did not fix my 2-1 shift thump, but guess what, there
is a tech bulliten on that too!

Just though I would share this info.

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Old 07-29-2001, 08:39 PM
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Join Date: Jun 2001
Location: Omaha, NE
Posts: 962
Could you share any info you get about the service bulletin about the 2-1 shift? I get the same thing, coasting at about 20 mph, then a very firm 2-1 shift when I accelerate. If I "baby" it, no problem.
'93 400E
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Old 07-30-2001, 01:58 AM
Mervyn's Avatar
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Join Date: Jan 2001
Location: Singapore
Posts: 268

I have read that replacing the vacumm control valve (vacumm modulator) is a good idea especially for transmissions more than 10 years old. It's a cheap item, and besides, the newer modulators come with an improved design that is supposed to work better with the vacumm.

My autotrans currently shifts hard, with hard jerks during upshifting. I had my modulator tested with a gauge and the pressure turns out fine. I will do an overhaul next year, but meanwhile, as a temporary measure, I will flush and replace the ATF and filter. I am also going to replace this modulator with a new one. My question is, is replacing the modulator worth the while?


1983 W126 (280SEL)
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