Help with Mercedes 300d shifting problem - PeachParts Mercedes-Benz Forum

JohnnyDeeep · #1 03-20-2013, 03:49 PM

Hello, I have a 1980 Mercedes 300d non turbo. I purchased it via phone about a year ago in a different state and had family members test it and pick it up. When I went back home to Califonia to pick it up I noticed that it had trouble shifting from 1st to 2nd gear. Once it accelerates to about 20mph it either shifts normally, hard or doesn’t shift at all, all depending on how I step on the accelerator but more often than not it doesnt shift. It only has this problem from 1st to second gear and drives wonderfully after at that. I have it in storage at the moment and waiting for the snow to clear to drive it and Im hoping someone could give me advice on what the issue could be.

Thank you

psaboic · #2 03-21-2013, 10:16 AM

Could be a vacuum issue as the transmission in these cars depend on vacuum for shift quality (soft/firm), or a Bowden cable adjustment which controls shift timing (what RPM it shifts at). Or it could be a combination of both.

Try this link Diesel Vacuum system adjustments and do a search Bowden cable adjustment for info.

Hope this helps

vstech · #3 03-21-2013, 10:47 AM

the 80 116 SD has some issues with the transmission. it's possible you have a loose band, or a weak B2/B1... it's also possible you have a stuck on kick down switch under your gas pedal. if you hold the 1st until 4000rpm does it then shift to 2nd?

Zacharias · #4 03-21-2013, 11:40 AM

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.

Govert · #5 03-21-2013, 11:59 AM

The 1980 300D N/A did have a vacuum-only controlled transmission.

The vacuum controlls both when the transmission shifts and how it shifts.

Also 1980 transmission had a feature not found on earlier N/A diesels: first gear take-off. When it stands in idle, it is in second gear, but when you press the accelerator pedal it shifts back to first gear. This is also controlled by vacuum.

On top of the valve cover there is a switchover valve, also known as 3/2 valve. At idle, it provides high vacuum (22 inHg) to the transmission. When you press the accelerator pedal, the valve switches to the lower vacuum from the Vacuum Control Valve (VCV). The VCV starts out with about 10 inHg of vacuum and it further reduces it to 0 when you fully depress the accelerator pedal. The sudden switch from high to low vacuum causes the shift back to first gear.

The switchover valve (or the little flipper that actuates it) wears out over time, so that the valve leaks. This will cause 0 vacuum and the transmission "thinks" you are pressing the accelerator pedal fully for a hard acceleration and therefore late (and hard) shifts.

So a check of the vacuum valves would be the first step.

It could also be something more serious, but vacuum is more likely.

If the transmission oil and filter wasn't changed recently (within 35,000 miles), you should do that too.

wildhorsemfg · #6 03-21-2013, 12:22 PM

yea that vacume crap is usually the problem ..
80 must of been the first year they put that on the trans ...
my 79 had a control rod .

JohnnyDeeep · #7 03-24-2013, 09:15 PM

thank you for your suggestions. Hopefully it takes care of my issue once I fiddle with the vacuum.