Another weird trans issue- '85 300SD - PeachParts Mercedes-Benz Forum

ROLLGUY · #1 10-21-2013, 06:09 PM

I was working on a friends '85 300SD, and driving it, I noticed a weird transmission shifting issue. Starting out from a stop wit my foot to the floor (shifter in D), it seems to start out in second, and immediately shifts to third and then to fourth. This all happens before 20 MPH. When shifting it manually, it starts out in second (shifter in 2), and winds out with good power (can feel the turbo working), and shifts firm when shifter is moved to the next higher gear. It shifts very smooth, and almost feels like a CVT. The RPM is steady around 2,500 throughout all the shifts until fourth, at which time the car is accelerating along with the engine. I thought it might have a vacuum problem (after all it is a CA spec '85 with seemingly a mile of vac hoses and valves), so I hooked up just the modulator and VCV to vacuum. I also checked the bowden cable, and it is adjusted all the way tight (should not shift until throttle let off a little), still no change. I also checked (although not thoroughly) the kick-down switch and associated fuse. I don't know how to test the solenoid in the trans. Is there an easy test or fix without removing the trans? Any help would be appreciated. Thanks in advance, Rich

kerry · #2 10-21-2013, 06:14 PM

Search on 'stacked shifts'. I had a similar problem with a 116 I once owned. Adjusting the bowden cable helped somewhat but it didn't cure it. I think one of the available shift kits is designed to solve this problem.

JamesDean · #3 10-22-2013, 11:57 AM

Agreed. Sounds like stacked shifts.

Superior Transmission sells a kit with parts that should fix this.

Superior Transmission - Superior K722A-D

ROLLGUY · #4 10-22-2013, 12:12 PM

It seems that the Superior kit only fixes one "stack" at a time (2-3 or 3-4 etc). The problem with this trans is that ALL, I repeat ALL 4 gears are stacked, meaning the car is in fourth gear by 20 MPH shifting one after another in a smooth fashion. Again, it feels like a CVT, in that the RPM is nearly constant as the car increases in speed. Only until after the trans shifts to fourth, and above 20MPH, does the RPM and power increase because of turbo boost (positively felt).

BillGrissom · #5 10-22-2013, 02:56 PM

One tiny piece of info, my 1985 300D CA W123 uses the transmission of later 1986+ 300D's. I found that out when changing the front seal while I had the engine out. I vaguely recall it was a mid-year change, but maybe due to the unique exhaust w/ catalytic converter, in which case might be all 1985 CA. Don't know if 300SD were the same deal, and may not even matter in your problem.

ROLLGUY · #6 10-22-2013, 06:01 PM

Quote:

Originally Posted by BillGrissom

One tiny piece of info, my 1985 300D CA W123 uses the transmission of later 1986+ 300D's. I found that out when changing the front seal while I had the engine out. I vaguely recall it was a mid-year change, but maybe due to the unique exhaust w/ catalytic converter, in which case might be all 1985 CA. Don't know if 300SD were the same deal, and may not even matter in your problem.

I believe it is in fact the same trans.....Rich

vstech · #7 10-22-2013, 10:24 PM

My 87 does EXACTLY the same thing.
Roy seems to think it's the vacuum modulator.

test yours.
on my car, the vacuum line was not connected to the modulator, but reconnecting it did not solve the issue. I figure it may have ingested dirt/grime into the vacuum line plugging up the orifice...

JUST possibly, it's related, I TOTALLY trust Roy's advice.

Sugar Bear · #8 10-22-2013, 10:52 PM

Had an 81 380 SEL doing this years ago and IIRC correctly it was the modulator. It made a large difference in the performance once corrected.

ROLLGUY · #9 10-22-2013, 11:07 PM

Quote:

Originally Posted by vstech

My 87 does EXACTLY the same thing.
Roy seems to think it's the vacuum modulator.

test yours.
on my car, the vacuum line was not connected to the modulator, but reconnecting it did not solve the issue. I figure it may have ingested dirt/grime into the vacuum line plugging up the orifice...

JUST possibly, it's related, I TOTALLY trust Roy's advice.

I would think any dirt/grime would not get anywhere near the modulator, but rather the VCV, being that the path of the vacuum is from the mod. to the VCV.
As far as I know, the modulator only controls the shift "quality", not the timing. The bowden cable is responsible for the timing. I checked the modulator, and it holds vacuum. I too trust Roy's advice.

vstech · #10 10-22-2013, 11:14 PM

yeah, I know which way the vacuum flows, that's why I figured the line or the orifice restrictor likely has something in it.
I agree... I was always told that MB's transmissions the vacuum only controls the firmness... not the timing of the shift, but I figure a bad modulator could influence control pressure... perhaps it alters the shift pattern?

ROLLGUY · #11 10-22-2013, 11:30 PM

Quote:

Originally Posted by vstech

yeah, I know which way the vacuum flows, that's why I figured the line or the orifice restrictor likely has something in it.
I agree... I was always told that MB's transmissions the vacuum only controls the firmness... not the timing of the shift, but I figure a bad modulator could influence control pressure... perhaps it alters the shift pattern?

I don't know, as I have not heard from our resident trans expert (should not be much of a "stretch" to figure out who that is).