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Old 10-17-2012, 08:35 AM   #1 (permalink)
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Transmission: Hard down shift

Hello,

I have a 1985 300D turbo diesel and am new to the w123 world but excited to finally have one.

It seems that after accelerating or when the car is slowing down there is a hard down shift and a clunk can be heard.
I've been looking through these forums and it seems that adjusting the bowden cable has had positive results for some, however I've also read that this could be a vacuum related issue.

The automatic door lock system is not working properly, after locking the driver door, the passenger door will slowly lock and then the driver side rear door will very slowly begin to lock with the passenger side rear door typically not locking.

Is the door lock vacuum system connected in any way to the transmission vacuum system? ie could the door lock leaks be causing the down shift clunks?

I had thought about adjusting the bowden cable but thought I should find out if the two issues are related first, if so I should get the door locks fixed and then proceed with trouble shooting the downshift. If they're not related then I'll adjust the bowden cable and go from there.

Thanks for any help and let me know if you have questions.
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Old 10-17-2012, 10:23 AM   #2 (permalink)
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Welcome to the forum Craig.

If the vacuum system leaks - and you have evidence of it as your door locks don't work - then you need to fix that first. Don't go adjusting Bowden cables until you are sure your vacuum system is working properly.

Do you have a vacuum gauge (and bits to connect it)?

There's loads of information here

http://www.benzworld.org/forums/w123...ent-722-a.html
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Last edited by Stretch; 10-17-2012 at 10:23 AM. Reason: Added a welcome
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Old 10-19-2012, 07:43 AM   #3 (permalink)
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A leak anywhere in your vacuum system can cause all sorts of gremlins to pop up in seemingly unrelated places, like your transmission.

Fortunately, there's an easy test: disconnect the "accessory" vacuum line feeder from your main vacuum line under the hood, and cap it off. There are plenty of threads to be found on where that line is - a picture is worth a thousand words!

Take the car for a drive and see if it shifts different - not necessarily perfect... just different. That way you'll know if even PART of your problem is your door locks. If it drives like a brand new car, you've solved your problem. Good luck!
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Old 10-19-2012, 07:52 AM   #4 (permalink)
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Thanks for the prompt responses. I'm going to look at it this weekend and I'll give you guys an update
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Old 10-25-2012, 03:21 PM   #5 (permalink)
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Hello,

I didn't get a chance last weekend to disconnect the "accessory" vacuum line feeder from the main vacuum line and cap it off.
However, what I have learned:
It seems that if I lock the drivers side lock while inside with the car running, they will all slowly lock and typically when I get out of the vehicle they will lock when the car is not running. They tend to work 70% of the time. I imagine there is a slow leak somewhere.

More importantly, with all of the doors locked the car runs much better. The accelerating shifting is smoother and the deceleration shifting is much improved.

Since the shifting issues seem to be related to whether the doors are locked or unlocked I'm assuming I should get a vacuum gauge and start testing the lock system. Let me know if this makes sense or both issues could be related to something else.

Also, any recommendations on what vacuum gauge and bits to get? I'm also looking for a service manual if anyone has a suggestion as to get one of those as well.

Thanks!
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Old 10-25-2012, 09:11 PM   #6 (permalink)
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Yes, by accident I disconnected the Vacuum Line to the Transmission Modulator and the Transmission up shifted about the same but down shifts were extremely harsh.

Vacuum leaks anywhere can effect the other items that use Vacuum; it all depends on how sever the leak is.
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Old 10-26-2012, 04:12 AM   #7 (permalink)
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...so in other words you need to make some measurements with a vacuum gauge.
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Old 10-26-2012, 12:11 PM   #8 (permalink)
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Looks like you were able to isolate the problem. Congrats! If you can't get to testing & replacing vacuum lines immediately (and don't mind you doors going full manual for a while) disconnecting & capping that "accessory" line should fix you transmission problems, insofar as the vaccuum is the problem... just to keep you from getting whiplash before you get around to vaccuum testing!
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Old 11-03-2012, 06:56 PM   #9 (permalink)
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If the status of the lock system affects shifting, you also need to re-check which systems are supplied by which vacuum feed off of the line from the vacuum pump to the brake booster. Each feed has an orifice, which makes them effectively isolated from one another. The transmission and the locks are not supposed to be on the same feed.

But you should still fix the lock system, because even if the plumbing is routed 100% correctly, a leaky lock system will still cause climate control gremlins.

You can never have enough golf tees on hand for testing of Mercedes vacuum system problems!
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