Calling transmission gurus: 1980 300td with 722.1 won't go into 2nd - PeachParts Mercedes-Benz Forum

Zacharias · #1 08-21-2011, 09:39 PM

I have posted some of this before in other threads, but the subject wasn't as focused as it might have been. So here is my last crack at it:

1. My 1980 has the "vacuum-only" transmission. No linkage connected to the throttle. When I got the car last fall it revved way to high to shift into 2nd, then stack-shifted into 3 immediately.

2. The car had sat for some time so I decided to drive it a bit before looking into the shifting. After a few days it suddenly started shifting much better when warmed up, just a bit of slip into 3.

3. Then without warning it stopped going into 2nd. Winds up in 1st and if you back off the throttle it just loses drive entirely until you slow down a bit. Then gives a slippy first. If you roll to a stop it goes back nicely into first.

4. I checked the main vacuum line and output was low. Changed the pump diaphragm this weekend, turned out the old one was probably fine but was installed wrong side in and the cover was barely locked in place. Road test, no difference. Tried swapping around vacuum lines in case they were misrouted (there is a post in this somewhere). Road test, no diff.

5. Then installed a new vacuum splitter valve (the white valve on the VC). Road test, no difference. At this point the predicted fine day turned into a nasty afternoon of thundersorms so that was it for the weekend.

6. The buddy who was helping me (he having small hands, me not) wants to try switching over the modulator valve from my parts tranny, next weekend. I figure at this point it's a waste of effort.

In need of ideas. Maybe I should just swap in the known good 'regular' used tranny from my old 240d, and install the linkages to make it work. This is getting very old and the car needs much other attention that I'm not getting to....

Thanks

kerry · #2 08-21-2011, 09:43 PM

I wouldn't do any parts swapping without monitoring the actual vacuum levels going to the transmission. T into the line going to the transmission, hook a mityvac into it and run the mityvac out the hood at the cowl and shut the hood, leaving the mity vac gauge visible thru the windshield. Drive around monitoring the vacuum level and report back the levels you are finding. Without knowing what amount of vacuum is going to the transmission you're just shooting in the dark.

ROLLGUY · #3 08-21-2011, 10:11 PM

First check to see if your modulator holds vacuum. If it does not leak, then do what Kerry suggests.

tangofox007 · #4 08-21-2011, 10:14 PM

Quote:

Originally Posted by ROLLGUY

First check to see if your modulator holds vacuum. If it does not leak, then do what Kerry suggests.

Vacuum to the modulator is a factor in how the transmission shifts, not whether it shifts.

Zacharias · #5 08-21-2011, 10:24 PM

What controls whether the transmission shifts?

Remember this trans has no Bowden cable or throttle linkage connection.

So what controls it?

I have asked this before, in relation to THIS particular trans, but got no response....

Tks

kerry · #6 08-21-2011, 10:25 PM

Quote:

Originally Posted by tangofox007

Vacuum to the modulator is a factor in how the transmission shifts, not whether it shifts.

Is that the case on the 'vacuum only' transmission?

Zacharias · #7 08-21-2011, 10:30 PM

Quote:

Originally Posted by kerry

Is that the case on the 'vacuum only' transmission?

Nail. Head. Bang. THANK YOU.

If vacuum isn't controlling it, what is, some sort of internal governor?

Stretch · #8 08-22-2011, 03:44 AM

Quote:

Originally Posted by Zacharias

Nail. Head. Bang. THANK YOU.

If vacuum isn't controlling it, what is, some sort of internal governor?

I'll try and answer this for you. I'm still leafing through the ATSG manual... unfortunately as I said before it is written for the transmissions that are fitted with the regulating pressure lever linkages that join to the throttle linkages.

My guess at the moment is that it is done by the "extra" valves in the valve body that I couldn't identify when I took it to bits...

I've nearly finished my write up - though I now feel obliged to write about how the sodding thing works!

Take a look at the other thread - could be B1...

mbdoc · #9 08-22-2011, 09:21 AM

When you accelerate hard in 1st gear, does the speedo register?
If NOT, possibly the nut on the output flange could be loose.
Then the governor will not work as well.

Zacharias · #10 08-22-2011, 09:37 AM

Quote:

Originally Posted by mbdoc

When you accelerate hard in 1st gear, does the speedo register?

Yes it's working normally.

Tks

Zacharias · #11 08-22-2011, 04:46 PM

Found this interesting thread....

http://www.peachparts.com/shopforum/showthread.php?t=193291&highlight=installing+modulator

This would suggest the modulator does have a bigger role on this model of transmission.

The last entry also mentions the governor.

Stretch · #12 08-23-2011, 03:19 AM

Please can you tell me what is going on?

In this thread you are saying that the transmission is stuck in first

but

In this thread you say it isn't

http://www.peachparts.com/shopforum/showpost.php?p=2775406&postcount=11

Quote:

Originally Posted by Zacharias

...

By the way, I probably not been describing the symptoms as well as I might. It isn't stuck in first... rather when it tries to go into second it loses drive. If you let it scrub off some speed it picks up first again, albeit in a slippy way unless you stop and start off again.

I am thinking of writing up a new thread on this, just in case someone hasn't picked up on my previous ones.

Thoughts?

I don't get it.

Is there any chance you can make a video?

By the way as for the last comment in this thread:-

http://www.peachparts.com/shopforum/showthread.php?t=193291&highlight=installing+modulator

I don't see how you could reach the governor from the outside of the transmission. You can pull out the secondary pump but that's as far as I can see anyone getting

http://www.peachparts.com/shopforum/showthread.php?t=301783

1980-300D(bio) · #13 08-23-2011, 04:04 AM

Condensed info:
http://www.benzworld.org/forums/w123-e-ce-d-cd-td/1597891-diy-w123-transmission-diagnose-adjustment-722-a.html

gregszustak · #14 08-23-2011, 05:35 AM

I have an '81 240d, vacuum only transmission.
My modulator was leaking, pulling up transmission fluid into the vacuum line preventing vacuum from reaching the modulator.

To shift properly it rev'ed up and I had to back off the pedal and it would shift hard into gears.

New modulator fixed it.

Zacharias · #15 08-23-2011, 11:16 AM

Quote:

Originally Posted by Army

Please can you tell me what is going on?

In this thread you are saying that the transmission is stuck in first

but

In this thread you say it isn't

Previous to this I managed to drive Mercedes for 15 years with my knowledge of their auto transmissions being an ability to distinguish which piece is the TC. I imagine most people on this list are similar.

My lack of precision in my language describing a problem no doubt reflects a pitiful lack of understanding of what goes on inside an auto trans.

So here goes:

When you take off it winds up in first. I have taken it to just before the warning dot on the speedo and it does not shift up.

If you take it to near the warning dot and back off the pedal it slips into neutral until the road speed decreases, then it engages first again.

I did say before it was going into second and slipping, but either I was mistaken or that is no longer the case. On the weekend I drove it further than I had attempted previously and I only had forward drive in first. However on previous occasions I was sure I had some very limited forward drive when I backed off the throttle, albeit with HUGE slippage.

By the way it engages both drive and reverse instantly from a stop. No hesitation.

If you want other info, please ask specific questions.