300td no shifting, no vacuum -- pump diaphragm? - PeachParts Mercedes-Benz Forum

Zacharias · #1 08-16-2011, 12:12 PM

I've got a 1980 300td with the vacuum-control-only transmission. It stopped shifting, period -- when it tries to shift up out of first I lose drive completely.

I just did what I ought to have done first, checked the main vacuum feed to the booster. The needle is shaking/twitching between 2 and 10 units. What I find odd is that the car shuts down fine and the brakes feel fine.

Questions:

1. If I understand correctly, the vacuum reading should be 20+ and steady, going into the brake booster?

2. There is no noise from the vacuum pump. Can I assume this is a diaphragm issue only, or should I get a rebuilt pump and be done with it?

3. Anyone have a link to instructions on doing the diaphragm? I am not having much luck using the search engine.

In the past I've left work in the vacuum pump to the indy but I'd like to try this one myself, this time around....

Thanks

Biodiesel300TD · #2 08-16-2011, 12:24 PM

The vacuum only controls shift quality, that is, how firm or soft the shifts are. You should have a linkage that runs from the throttle linkages on the passenger side of the engine to trans. Make sure it's connected properly, and has all the bushings it needs.

Stretch · #3 08-16-2011, 12:36 PM

Whilst it is possible you've got a vacuum issue I don't think that that will stop it from shifting past first gear.

Saying that though I did buy a gearbox from a chap in a McDonald's car park (eeeek!) who was whittering on about a a dodgy T joint that caused shifting problems - I didn't believe him though - neither did I believe that the gearbox had previously been driven by a Nun and that she'd only driven 80,000km... but then you never know eh do you?

As far as I am aware the vacuum does the smoothness of the shifts.

1980 - would probably mean you've got a 722.1

In this gearbox the first gear is "held in place" with the B2 piston + brake band and the free wheel brake. If you are actually in first gear and the B2 parts aren't working you'd also loose drive in 2 and 3... but if you have first then it isn't that.

Before I drone on about other stuff - have you done the obvious and checked the fluid level / replaced with fresh fluid / filter etc?

Zacharias · #4 08-16-2011, 12:36 PM

As I said in the post, the 1980 has a vacuum-only transmission. It was a one-year-only deal. There is no connection to the throttle linkages.

It's been the subject of a few threads on here.

Stretch · #5 08-16-2011, 12:38 PM

Quote:

Originally Posted by Zacharias

As I said in the post, the 1980 has a vacuum-only transmission. It was a one-year-only deal. There is no connection to the throttle linkages.

It's been the subject of a few threads on here.

My 1981 722.118 doesn't have this whole bowden cable set up either - just the select lever and then the vacuum pod at the back. Is that the same as yours? Sounds like it...

kerry · #6 08-16-2011, 12:57 PM

It sounds like a bad pump to me if you are taking those readings directly off the main line to the pump. I think it could be causing your shifting problems. If you have a diaphragm pump, rebuilding it is pretty easy. Took me less than an hour and a half to do it. No need to pull the radiator or the whole pump. Just remove the front of the pump with the diaphragm and check valves. Do you have a vent line going from the pump to the either the intake manifold or air cleaner? If so, check to see if it has oil in it. If it does, diaphragm is torn.

Zacharias · #7 08-16-2011, 01:12 PM

Quote:

Originally Posted by Army

My 1981 722.118 doesn't have this whole bowden cable set up either - just the select lever and then the vacuum pod at the back. Is that the same as yours? Sounds like it...

Yes, no linkages connected to throttle input, no Bowden cable.

Yes, I drained the fluid and refilled it but have not yet pulled the pan.

Two observations

1. the fluid was overfull when drained it. By a lot. What came out was dark but not really burned.

2. when I got the car it was winding up way too much before it shifted into 3, then it stack-shifted into 4 right away. After putting some miles on it the shifting became perfect for a while, then suddenly it would not shift out of first.

EDIT: I have a spare, conventional (rods + vacuum) tranny on standby. But I read most or all of the threads on these vacuum-only things and I could swear I remember one guy saying that when the white vacuum switch (valvecover) failed on his car, he had the same symptoms as me. If there is one message I got from those threads it's not to give up on these things until there is no doubt the problem is mechanical....

Stretch · #8 08-17-2011, 03:44 AM

It sounds like you just have a 722.118

Take a look on the casting just above the transmission oil pan - there you'll see a row of numbers on the right hand side (taken as though you are sitting in the car).

In this picture you can see the physical differences between a 722.118 (background) and a 722.112 (foreground) - no lever for the linkages on the 722.118...

When you say overfull - did you physically measure it?

Are you 100% sure that you have the correct amount of fluid in there now?

(You know the dip stick should be checked with engine running etc on a warm gearbox but not too quickly after driving etc etc etc - it is a pain in the arse)

Someone posted some info for checking on a cold gearbox - I'll try and dig it out - If I remember correctly the tip was from the owner's manual - what a funny place to find information!

Zacharias · #9 08-17-2011, 10:36 AM

It's definitely a vacuum only transmission. The linkages/levers on the valve cover are set up as well for that unit (they are a bit different from the norm).

The fluid level was checked after the car was hot and had driven (such as you can in first gear).

Biodiesel300TD · #10 08-17-2011, 10:38 AM

If it's a fluid related problem. You might try a new filter a bottle of Trans-X and new fluid. I've had good success with Trans-X. I put it in my 82 wagon, and it helped out a bit.

Zacharias · #11 08-17-2011, 10:39 AM

Quote:

Originally Posted by kerry

Do you have a vent line going from the pump to the either the intake manifold or air cleaner? If so, check to see if it has oil in it. If it does, diaphragm is torn.

Yes, it's the old style. I have not pulled the tube running across to the air cleaner off to look, but I will.

Stretch · #12 08-17-2011, 10:48 AM

Linkage like this?

Zacharias · #13 08-17-2011, 11:04 AM

Yes, but I can't match you for cleanliness....

Stretch · #14 08-17-2011, 01:38 PM

Have a look at this thread - this has loads of good information in it...

http://www.peachparts.com/shopforum/showthread.php?t=158216

Hopefully it has the answer too!

Zacharias · #15 08-17-2011, 02:55 PM

That thread does contain some excellent general info, but when I did have the patience to plough through it last fall, it had little or nothing to say about the vacuum-only transmission.

It's 11 pages or so and is loaded with commentary, smart-arse observations, newbie inquiries and almost off-track questions and gets on my nerves something awful. However if you want to learn about what the various bits and bobs of the system do, it's the only resource.

Maybe some day I will edit it down to a short summary to be used as a sticky.

There are other posts that deal directly with this tranny, I will just have to search through my subscriptions. It's been a while.

Two things I do remember: that the function of the white valve on the VC is crucial. And that some cars benefit from enlargement of the opening in the T-fitting running from the main vacuum line.

I cannot, however, remember what the driving state of those cars' trannies was before the problem was fixed.