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'85 300D Turbo
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Discussion Starter #1
Hi,

I was driving my 85 300 D Turbo today when I noticed the shifting was acting a little weird, so I pulled over. When I accelerated again, it wouldn't shift past second gear. It shifts into second fine, nice and smooth, but then it won't shift into third. I didn't push it past 4500 rpm, but it wouldn't shift into third for the whole 45 minute ride home at 20 mph. It's dark out now, so I can't check out the car until tomorrow afternoon, but if anyone has any ideas what might be causing this, or where to begin to look, I'd appreciate it. Thanks,

-Tim
 

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Everybody always says vacuum lines! Is there more vacuum lines than the one know about? As far as I know the only vacuum line to the transmission is for the vacuum modulator which controls shift timing and quality. The gradual loss of function sounds like a hydraulic problem; most likely a gummed up checkvalve within the valve body. Buy a can of "Seafoam" for transmissions and follow the instructions on the can, they should read; add so many onces to transmission then drive for a few minutes and change fluid/filter. If that doesn't work, find a shop that believes that the bands need to be adjusted. Transmission shops are a bit like Chiropractors, some will tell you like it is, while some will string you along to get the most money they can out of you.
 

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1980 300D
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There is one vacuum line that controls the transmission modulator, but there are other vacuum lines that determine the vacuum applied to the transmission modulator line. The other vacuum lines deal with valves that not only open and close but bleed specific vacuum to the line that goes to the transmission modulator. This rube goldberg setup need have only one leak and the transmission doesn't know where it is shift wise. I write from experience - check the vacuum before anything else.
 

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DOUGLAS MARTIN - 3/10/2005 3:20 PM

There is one vacuum line that controls the transmission modulator, but there are other vacuum lines that determine the vacuum applied to the transmission modulator line. The other vacuum lines deal with valves that not only open and close but bleed specific vacuum to the line that goes to the transmission modulator. This rube goldberg setup need have only one leak and the transmission doesn't know where it is shift wise. I write from experience - check the vacuum before anything else.
That's good to know because all I've read is that If the modulator doesn't have vacuum, or enough vacuum, you will just experience early extremely hard shifts but never a no shift condition. It's always better to hear it from somebody who has actually experienced the problem to know what could really happen.
 

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1991 300 SE
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Stevenpfaff - 3/10/2005 4:11 AM

Everybody always says vacuum lines!
No, everybody should start with the simple things first – in the case of a w123 diesel with shifting problems the lines of the integrated vacuum system must be checked for leaks before bounding off to more esoteric possibilities.
 

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1984 300D Turbo
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timbro - 3/9/2005 9:55 PM

Hi,

I was driving my 85 300 D Turbo today when I noticed the shifting was acting a little weird, so I pulled over. When I accelerated again, it wouldn't shift past second gear. It shifts into second fine, nice and smooth, but then it won't shift into third. I didn't push it past 4500 rpm, but it wouldn't shift into third for the whole 45 minute ride home at 20 mph. It's dark out now, so I can't check out the car until tomorrow afternoon, but if anyone has any ideas what might be causing this, or where to begin to look, I'd appreciate it. Thanks,

-Tim
try unplugging the kickdown switch behind the accelerator pedal. maybe it's stuck.
 

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'85 300D Turbo
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Discussion Starter #8
I looked around for loose, cracked, worn hoses, etc. I saw a few little cracks here and there, not sure if they're connected to the tranny or not (this is my first benz). I unplugged the kickdown switch, as this seemed to be the simplest thing to try. I took the car around the block, and she shifted smoothly into third. I pulled over, plugged the kickdown back in, and drove a little farther. Shifted smoothly into third. I'm not sure what to make of it. I was planning on taking the Benz on a 5 hour each way road trip this weekend, but now I'm leary. What do you guys think?

-Tim
 

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96 E300D
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timbro - 3/10/2005 9:13 PM

I looked around for loose, cracked, worn hoses, etc. I saw a few little cracks here and there, not sure if they're connected to the tranny or not (this is my first benz). I unplugged the kickdown switch, as this seemed to be the simplest thing to try. I took the car around the block, and she shifted smoothly into third. I pulled over, plugged the kickdown back in, and drove a little farther. Shifted smoothly into third. I'm not sure what to make of it. I was planning on taking the Benz on a 5 hour each way road trip this weekend, but now I'm leary. What do you guys think?

-Tim
I think you found the culprit. Doesn't your car have four speeds?
 

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Discussion Starter #10
You think it's the switch? I wasn't sure because when I plugged the switch back in, it still shifted into third. I live on a curvy road so I couldn't get up enough speed to get into 4th. I was so happy to get into third, I came back home.
 

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Ok here is what you should do. Check the throttle linkage. Look for something bent or hanging up. Next check the bowden cable. I know "What is that?" It is located on top of the valve cover and it hooks onto the trottle linkage. It will have a plastic adjustment. Pop the cable off of the linkage and check to see if it just barely goes back on without pulling on the cable. If not then adjust the plastic screw until it does. Making the cable tighter will give you later shifts.
 

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timbro - 3/11/2005 11:28 AM

You think it's the switch? I wasn't sure because when I plugged the switch back in, it still shifted into third. I live on a curvy road so I couldn't get up enough speed to get into 4th. I was so happy to get into third, I came back home.

does your tach + speedo work?
if you are at 50mph in 4th the engine should be turning 2400rpm's
 

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'85 300D Turbo
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Discussion Starter #13
Well, my wife took it for a test drive today, and it drove fine. Just like it always did. Shifted into 3rd, then 4th. The last thing I want to do is make the car shift later. After this learning experience, I'm going to adjust it to shift a little earlier. I'm just wondering, was it that little black switch under the gas pedal, or is there some other problem that isn't going to show until we get half way to Tahoe this weekend? If we go, I'll definitely be taking the whole tool box with me. Thanks everyone for your advice.

-Tim
 

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you're about the 4th guy in the past month that had a kickdown switch stuck and one guy even wanted to sell his car!
 

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'85 300D Turbo
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Discussion Starter #15
That's funny that he wanted to sell his car!! I verified that it's the kickdown switch when I took it for a ride yesterday. It stoped shifting past second after climbing a hill. I pulled over, unplugged it, and started driving again, shifted like butter. Thanks for your help everyone!!!!
 

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timbro - 3/13/2005 12:30 PM

That's funny that he wanted to sell his car!! I verified that it's the kickdown switch when I took it for a ride yesterday. It stoped shifting past second after climbing a hill. I pulled over, unplugged it, and started driving again, shifted like butter. Thanks for your help everyone!!!!
if we had waited a while maybe you would have put yours up for sale as well out of frustration[}:)][:D]
j/k
glad it worked out for u.
 

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Discussion Starter #17
You're right, I was so bummed when I considered the posibility that the tranny was shot. Thanks for your help!!

-Tim
 
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