I have a 1989 560 sel that, when cold, shifts as it should. I was on I-95 today, in Florida and I was doing 70 when all of the sudden, it shifted down to 3rd for no reason. I put it in neutral and reved the engine, then shifted it back into drive, and all was well for about 30 seconds, then the same thing happened. I got off on the exit that I needed to and continued to drive at about 50 mph and then it shifted on its own to 2nd. I put it in neutral and reved the engine and then put it back into drive and it went in to 4th again, but only for a short period and then shifted down to 3rd and then down to second. Mind you that when it did this, the economy guage stayed pegged to the left indicating that it was getting "great" fuel economy, but that is not true. Do you think that my problem could be as simple as a vaccum leak? Or is it something more serious? Any thoughts or opinions would be greatly appreciated! Thanks in advance!

You checked your fluid level? Is the stuff dark or milky rather than clear and pink? Does is smell like roasted, toasted clutch liner? -CTH

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Vacuum mainly controls shift quality, not actual gear position as far as I know.

Only thoughts are fluid, as mentioned, kick-down switch or funky Bowden cable. I put money on the kick-down switch first.

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

Okay, this morning my wife took the car up to our business and was there only about an hour. She came out to the car, started it, put it into reverse and... no reverse! Managed to push it out of the parking spot and put it into drive and immediatly brought it home. I drove my motorcycle up to the local MB dealer and picked up a filter and gasket ($59.12) and came home and drained the transmission and the torque converter.
Transmission fluid was very dirty and the small magnet in the bottom of the pan looked like a porcupine with all the metal shavings. My first thought was that I was screwed but continued by cleaning out the pan thouroughly making sure that all the metal shavings were cleaned out, pulled the filter, installed the new filter, re-installed the pan and filled the transmission with 8 quarts of valvoline high mileage transmission fluid and 1 quart of lucas transmission treatment. Took it out for a test spin around the block and a short ride on the main road and it shifted as it should and reverse worked as well. I then took the car to work, about 20 miles and the transmission shifted like it should both to work and then when I came home tonight. So, for the time being, I think I have solved my problem, but for how long, only god knows. All those metal shavings on that magnet can't be good so we'll see how long it hold out.

Did you check the level before doing the work?

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

Sounds like you have a worn-out B3 (reverse) clutch set, if not others. This causes the metal shavings. The B3 clutches are also affected by forward motion and gradually wear down. if you do some searching on this and other forums relating to B3 clutches on 722.3s, you'll quickly learn more about the issue.

Likely time for a rebuild. You may have bought yourself a little, but not much, time with the fluid. You may want to source a good used unit as the parts and labor for a good quality rebuild will run you $2,000-2,500 to have a reputable shop do the work. MB spec's 9+ hours for a shop to remove/install the transmission from the car. The parts cost for seals, clutches etc. is a small fraction of the overall rebuild cost, even for factory parts.

I'm facing the same issue right now with my E500 (variant of same transmission) although I've not had any of your problems yet. I'm going to proactively pull and rebuild/re-seal my own transmission this coming spring.

Cheers,
Gerry

B3 while a commonly repeated problem, and oddly never proven with pics, can cause no reverse of course. But there is also a bulletin posted about a broken actuator spring or seals that can cause the issue. It would be the first thing I would look at. $8 part, or about, and much easier for most to work on.

MERCEDES 722.3 - 722.4 SLIPPING AND/OR NO REVERSE
COMPLAINT: Before and/or after rebuild, any Mercedes equipped with the 722.3 - 722.4 transmission may exhibit a chattering in reverse, slipping in reverse, or no reverse.

CAUSE: The cause may be, a stuck or sticking Accumulator Switching On Control Valve, a collapsed or broken Accumulator Switching On Control Valve Spring, a missing, cut, or leaking seal ring on the accumulator piston, or a cracked Accumulator Switching On Piston (See Figure 1).

CORRECTION: Remove and disassemble the valve body. Inspect and replace as necessary, the parts and concerns listed above, as shown in Figure 1.

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

Looking for answer, 1990 420sel. 160,000 I was experiencing slight banging in 2 3 as it changed gears the same when slowing down, I found the vacume hose was disconected at the manifold. had new hose, new modulater installed, oil change and metal screen installed, was better but still feel it banging but not all the time, other than banging its great on the hyway and around town. My Mechanic said I need a new auto trans kick down cable, said its worn and sloppy, its a very expencive cable so im wondering if it could be anything ell's,
before I invest, hoping for an answer, thanks

By "banging" do you mean that shifts are harsh? If so, that is a function of modulator adjustment.

The "kickdown" cable is actually a throttle pressure cable, and it controls the point (speed) at which shifts occur. It also is involved in part-throttle downshifts, however, full-throttle kickdown is caused by an electrically actuated hydraulic valve, not the cable.

If the cable is not broken, it can probably be properly adjusted.
In the event that it is broken, prices for the cable alone range from ~$30. to $80.