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  #1  
Old 11-14-2013, 03:00 PM
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85 300SD shifting hard and other issues

Greetings everyone. I have been browsing the forums for the past few days for a solution for an issue I've been having with my 85 300SD, but figured maybe I should post about it and see what others may think. A bit of background on what's been happening:

My car has been sitting for the past 8 months while I was overseas. I left with it my parents who were suppose to take care of it, and completely neglected it. I finally had the car towed (at the time it would not turn over, I later told them after finding this out that the system needed to be primed because there was probably air in the line) a few weeks ago prior to me getting back to be worked on. Only thing I had done was the belts and brakes. Since having the car back the shifting has been atrocious. It will only shift at 4500RPM at 25MPH and 45MPH when the peddle is all the way at the floor. I've tried to ease into the shift, but the car just doesn't want to shift unless the peddles is all the way down I can only feel a downshift when I brake below at 25MPH. While driving between shift points, it drives anywhere between 3k to 4k. I tried to take the car over 75MPH and the engine reved to 5k and would not go any faster. I tried playing with my Bowden Cable and haven't had much luck with that (maybe I should lessen the tension more?). I'm wondering if the car is even hitting the right gears.

I've been reading around and found posts that have pointed towards the vacuum lines, which would make sense since the engine will not cut off when I turn it off (rather have this issue than it not turning on), and driver side door will not lock/unlock at the key hole. So, I went out and bought and bought a mity vac to test the lines. While turned on, I hooked it up to the line going from the vacuum amplifier (I think that's what it's called) on the driver side near the brake fluid, and got around 20(don't remember the unit of measurement). It was the only line I could get a reading off of. When I tried pumping the mity vac on the others, they would not hold pressure. I'm hoping it isn't the vaccum pump itself that could be causing this and maybe it's gunked up somewhere or a line fell off from sitting for so long.

Hopefully I gave enough info on my issue. If any more is needed please let me know. I'm afraid to drive the car around, but unfortunately I have no choice since I have no other way to get to work. Any advice and help is greatly appreciated. I want to try to fix this myself before I drop the money on a mechanic.

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Old 11-14-2013, 03:44 PM
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Answer

My best guess:
* Vacuum leak.

A disconnected or broken vacuum line.

Transmission:

.
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Old 11-14-2013, 08:14 PM
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Here is a link, with some heavy reading, that goes into great detail about checking the vacuum for the transmission: It's CRITICAL... how you set your transmission's vacuum system on your diesel MBZ...

BTW, the vacuum system for the door locks is a separate system on the W126.

However, if things worked great before, the first, least expensive, fix would be to replace the flex lines including the 3-way vacuum hose connectors and the modulator of the transmission. They dry rot with time and get small cracks. I would do this before trying to make adjustments of any kind. We're talking about less than $10 in parts that eliminate a bunch of vacuum gremlins.
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Old 11-15-2013, 05:41 AM
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.

I would check also for sticky/stacked kick down switch under pedal.

low vacuum will give you shifting but with the bangs,as you have pointed to high rpm's as need for shift
the kick down switch would be my starting point,next is linkage. I drove mine without vacuum at all as a test and car
shifted fine but with clear bangs between shifts.

anyways,checking vacuum lines and T conectors is always advised...


cheers
ChO
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Last edited by cho; 11-15-2013 at 07:21 AM. Reason: explanation
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Old 11-15-2013, 06:44 AM
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I second the vacuum leak. A Google image search for "W126 vacuum diagram" yields a bunch of results. You can use golf tees to block off different parts of the system to help you isolate and find the leak.
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  #6  
Old 11-15-2013, 07:38 AM
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X3 on vacuum leak.

I was messing around near the driver side inner fender and knocked a vacuum line loose. Thing shifted like a truck until I put 2+2 together and reconnected.
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Old 11-16-2013, 05:10 PM
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Thanks for all the input. It ended up being the vacuum pump itself. There was 0 vacuum coming out of it. A bit of a pricy repair but at least I got over that hurdle. There is a noticeable difference in the braking, it responds a lot better. I'm still having the issue of the high shift RPM. It is driving at a much safer rpm when not trying to shift (about 2500) but it won't shift until that 4500 mark. I ended up taking it in to be diagnosed and was told my ignition switch was leaking a lot of vacuum. They did a smoke test to see where I was getting my leaks. Also my vacuum transducer/modulator/the blue thing below the master cylinder (yup, very technical there) has the connection broke off for the transmission modulator. Before this line broke I was still having problems, so I know that isn't the root cause there. Going back to the switch, when I disconnect the red line going into the vacuum pink and black thing under my dash (sorry some parts I don't know the proper names), my engine WILL turn off automatically.

I want to do the switch itself but am confused on how to take the thing out. I'm going to go into my disks and see where to go from here. Regarding the switch, do I just need to buy the rear portion of it or do I need to buy the whole thing, tumbler etc? Also, any one know the exact part number for the vacuum transducer? 123 300 1133 is not giving me any results.

Thank you once again for the help!!
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Old 11-18-2013, 07:04 PM
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Quick update: So it appears that my kickdown switch is also a culprit with this shifting issue. I went to press it down and noticed it stuck in the down position. I have a new one on order and should be available tomorrow. Now, how do I remove this switch? I've been browsing around the forums and haven't been able to find any threads that indicate how to actually remove it. Is it as simple as pulling it out? To see if there was a difference, I disconnected the kickdown switch and attempted to drive it around. I felt shifting at 15MPH and 20MPH, but wasn't able to push past 30 too well. It would rev to around 4200RPM and stay there. Maybe I need to readjust my Bowden Cable?

The vacuum transducer I ended up patching it up with krazy glue and heat shrink and am getting good vacuum going through the line. This is, of course, a temporary fix until I can find the actual part.
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  #9  
Old 11-20-2013, 08:00 AM
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hi mate


it could be several things,wire,ground,solenoid......

take a look at this link

300SD 84 Kickdown switch stopped working

cheers
ChO

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