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Old 11-08-2002, 05:35 AM
David C Klasse's Avatar
CheFrac is Back!
Join Date: Jan 2001
Location: Mission Hills in the City of San Diego
Posts: 2,352
Downshifting; Transmission Vacuum

It seems that there is a vacuum leak of some sort (I am pretty sure) with my transmission.

I have a 1995 C280 with the 4 speed auto. Car has 130,000 miles on it, and still drives like a dream. Trans. maintenances have been performed between 10-25k miles consistently. Everything is great with the transmission, albeit the progressing Reverse engagement delay.

When accelerating from a stop, all shifts are normal.
But then if I want to speed up, the car won't downshift unless I engage kick-down.
Let me give you an exaple:
If I'm cruising along at 40 mph, and I push the gas so that the trans will downshift to 3rd gear, the transmission won't downshift (will stay accelerating in 4th gear). I will push the gas to WOT, and it still won't downshift. But if I then engage kickdown, it will downshift to 2nd gear and take off.

The kick-down is fine, it's just the vacuum pressure in the trans. If I hit the gas all the way (no kick-down) down and do it fast, it will sometimes downshift to 3rd gear properly. However, if I push it slowly, it won't downshift.

I hope this was the least bit understandable!
2006 E350 w/ 155k miles (Daily Driver)

1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)
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Old 11-08-2002, 11:52 AM
goldstone's Avatar
Join Date: Oct 2002
Location: Dutchess County, New York
Posts: 154

I'm very new to this group, and you've been very active for awhile, so I know you're far more knowledgable about these things than I; however I have searched and read lots of previous postings about transmission problems such as the ones you're now experiencing. The advice that has been repeatedly given in response to these problems usually involves either adjustments to the Bowden Cable or adjusting/replacing a bad shift modulator.

Of course, your tranny could be dieing a slow death, but you don't have THAT many miles on her yet. (After all, the first 100,000 miles is considered the break-in period for these cars, isn't it ).

Good luck!

1991 560SEL-Euro (145,000 miles)
1989 420SEL (Retired at 325,000 miles)
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Old 11-08-2002, 12:36 PM
jsmith's Avatar
Join Date: Aug 2001
Location: At Sea
Posts: 1,729
I think David's already an expert on Bowden cable adjustment. I suppose you are going to need something like a MityVac - just saw it on sale for $42. Might pick one up myself since it can also do brake bleeding. You'll want the one with the pressure gauge like this one:
MityVac Pro
1993 300e-2.8
- gone now <sigh>
"Do not adjust your mind, it's reality that's malfunctioning"

Last edited by jsmith; 11-08-2002 at 12:56 PM.
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