Sorry for the long-winded post. One question off the top: did your car sit for an extended period unused before you got it, by any chance?

No offense but both previous replies are wrong to some extent.

1. It's a 1980 300d, not SD.
2. The 1980 300d has no Bowden cable.
3. If it is set up like my 1980 300td, it has a vacuum-control-only transmission (no shift control rod or cable).
4. My car has more or less the same issue. I have worked through all manner of small fixes to try to correct it but no luck yet.

Unfortunately, these transmissions appear to be the least understood of all Mercedes transmissions in terms of technical troubleshooting info.

Basic info summary in simplified form: Most Mercedes of that vintage have a control linkage rod or a cable that runs to the transmission from the throttle linkage. That influences WHEN the transmission shifts in relation to throttle position. There is also a vacuum line running to a modulator valve mounted into the transmission body. This acts on operating pressure inside the transmission and affects shift timing and quality.

HOWEVER, assuming you do have the same trans as I have, the only control going from the motor to the trans is a vacuum line to a modulator. Based on (sometimes contradictory) info I have gleaned off this board, an internal governor (not accessible or adjustable) also plays a role in when/how this trans shifts.

Before you can proceed you need to dig the car out and look to see how the trans is set up: whether there is a linkage rod running to the trans, or alternatively if I am correct, just a vacuum line.

If you can get underneath, there is a number stamped into the side of the body (passenger side) just above the pan. You will probably have to clean off a lot of gunk to see it. That is the trans model number and the last three digits, after the DOT, will confirm what I have written here, one way or the other (if like mine will probably be .117 or .118).

In the meantime, there are seveal long threads on here with much info on Mercedes transmissions. You should do some reading to understand how the vacuum function relates these transmissions.

Regardless of which model of trans you have, as psaboic said, you need to start with vacuum troubleshooting:
- vacuum pump output,
- careful inspection of condition of vacuum lines and rubber junction fittings,
- checking the routing of the lines,
- condition/proper operation of the one or two white switchover valves you probably have on the top of the valve cover, and
- possibly driving with a vacuum gauge hooked into the line to assess vacuum levels going to the modulator valve.

If you have the model of trans I think you have, that is all you have to assess from engine controls. Next stop then is hooking up a gauge to measure transmission operating pressure while driving, which I have not yet attempted.

In my case I have systematically found problems and corrected them all the way along the line: replaced vacuum pump diaphragm, replaced broken switchover valve, replaced dried out vacuum line connectors. Replaced modulator. All this has done, in toto, is to move me from being stuck in first to shifting into second at unacceptably high road speed/rpms.

__________________


Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

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