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Old 02-13-2007, 09:36 AM
Registered User
Join Date: Aug 2006
Location: Houston, TX
Posts: 450
Question adjusting shift firmness

'92 300SE

I think my transmission shifts a little too firm on the 2-3 shift. If I'm VERY easy on the gas it shifts like silk - normal driving gets a rather stiff shift. This is more pronounced with the A/C on, and I'll assume that's because the motor is working harder and the vaccuum signal is lower. Oddly, when I'm pulling out from a corner it seems to shift better - maybe that's because my initial acceleration rate isn't the same as starting out straight from a light?

The arrows on the throttle linkage align just fine.

There is no slippage at any point - all shifts occur when they should, just a little more harshly than I personally care for.

Is there an adjustment to the bowden cable, or perhaps the modulator that I should be looking at?

Does it matter where on the intake manifold I pick up the signal for the transmission modulator?

Thanks in advance.

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Old 02-13-2007, 11:20 AM
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Join Date: Aug 2006
Posts: 141
Adjusting shift firmness

The vacuum modulator is on the drivers side of the transmission. It can be one of two versions. The early version has a black rubber cap that cracks and changes vacuum signal, Just replace the cap and roadtest for shift ouality. If the shift is still firm remove the cap and turn the adjuster counter clockwise in small increments until you get the shift you want.
The second version has a white plastic cap and has limited range of adjustability. Turn the plastic cap counter clockwise until you get the shift you want.
Good luck
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Old 02-13-2007, 12:13 PM
david s poole
Join Date: Mar 2005
Location: dallas
Posts: 1,822
check the vacuum line from the manifold to the modulator.i have found one or two that had a hole chewed in them by creatures.the other thing as mentioned is the cap over the modulator which cracks and loses vacuum that way.i doubt you will needto adjust it ,it sounds as though you lost all vacuum to the mod.
David S Poole
European Performance
Dallas, TX

"Fortune favors the prepared mind"
1987 Mercedes Benz 420SEL
1988 Mercedes Benz 300TE (With new evaporator)
2000 Mercedes Benz C280
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Old 02-13-2007, 01:51 PM
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Join Date: Aug 2006
Location: Houston, TX
Posts: 450
I know I've got the black cap - remember it from when the transmission was out to R&R the rear main seal. I'll check it for cracks.

The hose is good - I checked that at the same time.

I'm assuming a quickie test would be to use a mity-vac on the engine side and see if I can pull any vaccuum at all. I don't know if the modulator would ordinarily "hold" a vaccuum, but if the cap is cracked, I'd think I'd be unable to pull any vaccuum at all.

Thanks for the suggestions.
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Old 02-13-2007, 06:56 PM
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Join Date: Jan 2003
Location: Dallas
Posts: 790
Brian, just as a suggestion on something you might check.
My '91 300SE started shifting a little harsher than usual last month.
Then I noticed the engine temp was running about 70-75C, about ten degrees lower than normal. The bad thermostat was replaced, the temp went to 85C, and the tranny started shifting normal again.
But, just like yours, my little engine can't handle hot weather and AC, and it always shifts harder in the summer, just trying to move that big car.

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Old 02-13-2007, 09:00 PM
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Join Date: Aug 2006
Location: Houston, TX
Posts: 450
Thanks for the suggestion - I just replaced the thermostat - temp holds around 90c most of the time. Some times a little lower, some times a little higher.
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Old 02-15-2007, 11:53 PM
86560SEL's Avatar
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Join Date: Jan 2003
Location: east Tennessee (southeast USA)
Posts: 3,015
When warm, my car shifts very smoothly, with the 1-2 shift being more noticable, but not harsh by no means. The 2nd-3rd and 3rd-4th are very smooth, but not slipping. When cold, the 1-2 is really noticable, with others smoother, but harsher than when the car is hot.

I personally would like a smoother 1-2 shift.

Reverse? Huh... when hot, it harshly engages into reverse (after a hesitation), when cold, its smooth into reverse. Been like this since I have owned it... no changes, so I am assuming it is nothing to cause concerns.

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