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  #1  
Old 01-11-2004, 04:30 PM
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85 300TD Vacuum Question

I received this via email and don't have time at the moment to formulate a response. Somebody please give it a go.

"I have a 85 300TDT that seems to have a vacuum issue. The trans thumps when I slow to a light, otherwise it shifts fairly normal. I noticed my brake pedldle is also a bit lower than normal. I have a mity vac and pulled 10 lbs on the vacuum line which held. I'm not sure where to start. How do you test the vacuum pump? What should it be. Thanks, Brian"

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2014 Tesla Model S
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2017 Nissan LEAF
Former MB: 99 E300, 86 190E 2.3, 87 300E, 80 240D, 82 204D Euro
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  #2  
Old 01-11-2004, 05:00 PM
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Yes, a thump downshifting is an indication of a vacuum problem. Push the far right button on the heating panel. This shuts down the heater and excludes any vacuum problems in the heating control system. Then plug the line going to the doors (yellow line just to the right of the brake booster I think). Then go for a drive. If it shifts better, you know the vacuum problem causing the harsh downshift is either in the heater or the door locks. Then turn on the heater. If the downshift is still smooth, you've traced the problem to the door system. If it is harsh again, it is either just in the heating system or in the doors and heating system.
If it is still harsh with the doors and heating system excluded the problem is in the brake booster, shut off valve system or transmission control system, or vacuum pump itself.
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  #3  
Old 01-12-2004, 10:04 AM
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Ok, now let's address these things one at a time.

I have no experience with W123 automatic transmissions, so will just go with what Kerry said.

Regarding the brakes, inadequte vacuum will make the brake pedal harder to push down - nothing else. If that's not the symptom, then something else is wrong with the brakes and you need to explain in more detail what you mean by the pedal is "lower."

Using a MityVac to pull a vacuum on the main line won't tell you very much because it displaces a relatively small volume of air, while the brake booster and the door lock vacuum reservoir hold a lot of air. So you'll have to pump a long time with the MityVac to get anywhere, and if something leaks you won't know what it is.

The first test I would do is pull both the small connections for vacuum consumers from the main line. Block one of the ports using a rubber connector and a golf tee. Then connect the MityVac to the other port and start the engine. The engine driven vacuum pump should easily produce 20" or more of vacuum. If it doesn't then one of three thing is wrong:
1. The vacuum pump itself is broken.
2. The main line has a leak.
3. The brake booster has a leak.

Then you start chasing from there. I haven't been down that road, so I can't tell you how to disconnect the main line and connect the MityVac to the pump. Maybe somebody else can help or it will be obvious when you get there.

However, if the vacuum tests good there then you can go on. Shut the engine down and use the MityVac to test each of the circuits separately. If you find one that doesn't hold vacuum, then you start chasing. Lots of information is available here by using the search feature. I wrote a complete description of how the locks work a few months ago. You could find it by searching for "vacuum and locks" in this forum under my name.

Post back here if you still need help.
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2014 Tesla Model S
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2017 Nissan LEAF
Former MB: 99 E300, 86 190E 2.3, 87 300E, 80 240D, 82 204D Euro
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  #4  
Old 01-12-2004, 10:24 AM
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And now a word about Mercedesshop philosophy. From time to time I get questions like this via PM or email, and I am very reluctant to answer them in private for two reasons.

1. Other people may be reading the board and have the exact same problem, but they will get no benefit from a private email conversation. Searching the archives is a very powerful thing, so it is best that everything is conducted here "in the open" for the benefit of all, now and in the future.

2. I'm not perfect. Everyone makes mistakes, and an incorrect answer here will almost certainly be challenged by someone else, while an incorrect answer in an email may lead you astray and cost you unnecessary time and money. The real power of Mercedesshop is the collective wisdom of all the members, not just a few or just one.

You may have noticed that in many of my posts, I provide links to other posts. This is because I tend to have a lot of free time at work, so I'm able to read everything in this forum, and I remember much of what is posted. I know which members have expertise in which areas, and what has been discussed over the past few months. I'm happy to hold someone's hand and help them find answers here, but they should at least try to search for it themselves first.

And a final note regarding the fear of being flamed. If you're scared to post here because you think someone might insult or ridicule (flame) you, please remember that you can always just delete your post. If yours is the first post in a thread, then deleting it will make the whole thread go away. So, get over your timidity, and post away!

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Rick Miley
2014 Tesla Model S
2018 Tesla Model 3
2017 Nissan LEAF
Former MB: 99 E300, 86 190E 2.3, 87 300E, 80 240D, 82 204D Euro
Chain Elongation References
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