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  #1  
Old 06-17-2009, 12:35 AM
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1983 300SD TD Trans downshifting

Guys, I could really use your help. The other day after the car was all warmed up, it suddenly downshifted while I was going about 45 mph. So I limped home and tried to think about what was causing it. The next day I started it up and it drove just fine until it was warm, so about 20 miles later the downshifting started again. Any idea as to why this is happening and what I need to do to correct it? Thanks Rick

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  #2  
Old 06-17-2009, 08:58 AM
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Check to see if you are low on transmission fluid.
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  #3  
Old 06-17-2009, 08:59 AM
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I'll add give the transmission a service.
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  #4  
Old 06-17-2009, 09:02 AM
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unplug switch at fuel pedal to see if its bad.
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  #5  
Old 06-17-2009, 09:11 AM
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Quote:
Originally Posted by oldsinner111 View Post
unplug switch at fuel pedal to see if its bad.
x2, or pop it a few times, make sure it isn't getting stuck down or hard to press.

Of course check fluid level, and replace the filter if you haven't in a while.
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  #6  
Old 06-17-2009, 10:06 PM
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I have an '81 240d, not sure if tranny is similar.
I had a leaky modulator, it slowly filled the vacuum line with tranny fluid after it was warm and behaved similar.
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  #7  
Old 06-17-2009, 10:36 PM
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Thanks guys I'm going to take a look at all of these things you have recommended and will let you know how it goes.
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  #8  
Old 06-18-2009, 10:51 PM
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Unplugged Switch/Too Much Fluid

I unplugged the kickdown switch at the pedal, praying that was it. It ran perfect for 20 minutes, but once again....as soon as it was nice and hot it started downshifting again. Very frustrating!

Could too much transmission fluid be a problem?

I will work with the fluid and getting the screen changed tomorrow.

What about that modulator, could that also be the problem? If so, where is that located.

Thanks again for all your help guys. You are helping me get to the bottom of it.

Rick
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  #9  
Old 06-20-2009, 12:00 PM
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Well I checked the fluid level again at operating temperature and they looked way too high on the dipstick. I probably overfilled it when I added some fluid a month ago. The only thing I can think of is that it is starting to get hot here in the Southwest and the temperatures are causing the fluid to expand a bit in the summer. So I got some 1/8 vinyl tubing and fed it down the dipstick tubing and began siphoning fluid out. I took out a little over a quart and went for a test ride of 40 miles, no problems. Brought it back home then went out again this morning for another 70 miles and no problems. Could that have been it? I had no idea a little too much fluid would create such an issue. Anyways, thanks for the help guys. Now on to the cruise control.
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  #10  
Old 06-20-2009, 12:12 PM
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Quote:
Originally Posted by RickR1983 View Post
Well I checked the fluid level again at operating temperature and they looked way too high on the dipstick. I probably overfilled it when I added some fluid a month ago. The only thing I can think of is that it is starting to get hot here in the Southwest and the temperatures are causing the fluid to expand a bit in the summer. So I got some 1/8 vinyl tubing and fed it down the dipstick tubing and began siphoning fluid out. I took out a little over a quart and went for a test ride of 40 miles, no problems. Brought it back home then went out again this morning for another 70 miles and no problems. Could that have been it? I had no idea a little too much fluid would create such an issue. Anyways, thanks for the help guys. Now on to the cruise control.
Yup that'll do it! Too much fluid is just as bad as too little fluid, and ATF is designed to expand quite a bit. Glad to hear its working ok again!
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  #11  
Old 06-20-2009, 08:04 PM
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Dang I thought it was fixed. It was hot this afternoon and I drove the car across Phoenix. Lot's of stop and go traffic and I had the A/C on. As the engine temperature gauge started going over 100 C it started happening again. I had a tranny shop take a little more fluid out because it still looked a little high, but the problem persisted. Would it make sense to have a transmission cooler here in Arizona?? Anyway I limped home 20 miles in 2nd gear going about 15 mph. Any other ideas???? Would changing the filter help at all or is that a long shot?
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  #12  
Old 06-20-2009, 09:29 PM
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The filter would probably help if it hasn't been done in a while, routine maintenance. If that doesn't help, you may have to take the valve body out and clean the little strainers in it, which could be gunked up.
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  #13  
Old 06-21-2009, 10:00 AM
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I took it out again this morning and it is shifting fine again. It has some flaring between 2-3 and 3-4, like always, until it warms up and then it runs fine. However it is early in the morning here and still fairly cool. I'm going to take it out now on a little bit longer of a ride......not too far from home......and see how it does. I find it strange that it seems to happen as the car really get hot in the afternoon. Also wondering if I may have the problem that greg described about with a leaky modulator. He had transmission fluid slowly seep in as he drove and then the transmission would begin to malfunction. After the car sat for awhile the fluid would seep out again and the car would shift fine for awhile. I'll let you know how this next trip goes.
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  #14  
Old 06-21-2009, 11:30 AM
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After the car was warm, I drove it until the temperature gauge hit about 100 C. I had the A/C on even when I really didn't need it, just to see what would happen. It started slipping again at that point. Not sure I want to mess with getting into the transmission. So I'll probably research to find someone around her who specializes in mercedes diesel transmissions. Either that or I'll just drive the car only 30 miles per day!!!!!
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  #15  
Old 06-24-2009, 11:38 PM
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Update and Information

I think I found my problem. My transmission shifts fine when the transmission fluid is cool, but when it gets hot it is no longer holding the necessary pressure to keep it in 4th gear. This happens because the fluid thins as it gets hotter. As this occurs the fluid is slipping by older worn seals and releasing pressure, causing the downshifting. Bottomline is that I need a rebuild. Just wanted to lay this one to rest and update all of you. I hope this may help others having similar problems.

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