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  #1  
Old 08-02-2016, 09:58 PM
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Join Date: Jun 2006
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w123 dropping gear

1985 300D (W123) with bowden cable and VCV. Suddenly on the hway today transmission dropped from 4th gear to 3rd. Letting off the fuel I could coax it back to 4th then it would slip back down again. City driving after that it would get stuck in 2nd or 3rd. Sometimes the only way to get it back into the correct gear was to put it in neutral then back into drive. Then it would be fine for a short time.

The shifting itself seems smooth enough. Bowden cable seems fine. Transmission fluid level seems a bit high but as I haven't added fluid in over a year and this is a daily driver I don't think it's overfilled. I tried disconnecting the kickdown switch and no change. Shift linkage seems fine. Replaced B2 piston over a year ago but I don't think the symptoms are consistent with that anyway.

Whatever the problem is, it's inconsistent. Driving the car this eve it shifted fine for awhile, then the problem came back within 5 minutes. Even when the problem is present, it isn't consistent.

I don't think it can be VCV as this adjusts the shifting harshness but not when the actual shift occur. I know this car well, as it has been my main car for 8 years, something has drastically changed suddenly but I don't have a guess as to what.

I haven't crawled under the car yet but am not even sure what to look for. Speedo is fine so I know the tranny is seeing the right speed. Unless the kickdown solenoid is shorted (seems unlikely. No other electrical problems present in the car) I'm really in the dark as to what to check.

I'm not even sure if I should keep driving it, hoping it gets worse for a better diagnosis, or if I am doing damage or risking getting stranded.

Any thoughts/advice welcome.

-John

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  #2  
Old 08-02-2016, 10:43 PM
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Join Date: Sep 2001
Location: central Texas
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When was the last time you changed the fluid and filter ?
Have you checked the level according to the MB instructions ?
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  #3  
Old 08-02-2016, 11:17 PM
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Yes I believe I have checked the fluid correctly (engine at operating temp, idling, level ground). No I didn't use a leather cloth the way MB suggests but other than that I am think I checked it correctly. I have not changed the fluid/filter in a year or two but are the symptoms consistent with that? I think I would expect slipping if there was a fluid/filter problem?
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  #4  
Old 08-06-2016, 08:02 PM
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I drained the torque converter and transmission pan, replaced the transmission filter and fluid. I also added a bottle of Trans-X. Now it seems to have a lot of trouble engaging in gear. When I put it in Drive or Reverse it feels like it isn't in gear at all. If I give the engine a lot of fuel I can get it to move. Shifting is overall worse than before. It's flaring between shifts a lot now too.

I tried disconnecting the vacuum to the vacuum modulator and no change. So that makes me wonder if it could be the modulator itself. I don't really know the magic inside the modulator but I've imagined there to be a spring and a diaphragm inside (anybody have a link to a dissection of one?) I had a vacuum modulator fail on me on a previous car but the symptom was vacuum leak and harsh shifts. I would imagine that was the diaphragm leaking. I'm wondering though if this hypothetical modulator spring breaks if maybe the shifts could become so soft that this could explain the symptoms I am seeing?

When I had the transmission pan off, I had my wife press and release the pedal so I confirmed the bowden cable is moving smoothly. She tried pressing the kickdown switch and I couldn't hear the solenoid clicking nor did I see anything moving. But that seems unrelated to what I am experiencing now, and a relatively minor problem comparatively.

So...am I hosed?
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  #5  
Old 08-06-2016, 09:05 PM
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Location: central Texas
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Have you checked your vacuum ' condition' ? Any other symptoms that your vacuum might be less than it should be or is leaking anywhere ?
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  #6  
Old 08-06-2016, 09:46 PM
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Vacuum seems ok (door locks work, car shuts off, etc.) but if the vacuum had a leak I believe that would only affect the quality of the shifting, not when it shifts, and it would make the shifts more harsh. Right now the shifts are way too soft. And that's why I disconnected the vacuum from the vacuum modulator. I believe that should have made the shifts harsher, which it didn't. :-(
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  #7  
Old 08-06-2016, 10:29 PM
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Location: central Texas
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Seems like some auto trans specialists should have arrived by now... perhaps they are watching the Olympics...
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  #8  
Old 08-07-2016, 01:19 AM
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Location: Manteca, CA
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I had this problem on my 300SDL before the transmission completely died. The main cause of the transmission "dropping gears" was that the two screens on the top of the smaller valve body in the transmission had become clogged with peices from the clutches.

My transmission had been very neglected by the previous owner. They had only changed the transmission fluid once and they had put something like 100,000 miles on the car. The valve body was full of sludge. I ended up restoring fuction of the transmission for another 30,000 miles by just cleaning the valve body and replacing a few springs in it.

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1992 300D 2.5 TURBO

1991 560SEL M119.960

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