My transmission has developed an odd downshift characteristic. While driving down the highway, and with no warning, the tranny will downshift from 4th to 3rd. It will not shift back on its own. If I come to a stop and accelerate back up to speed it will go back ito 4th but it's just a matter of time before it downshifts again. Sometimes when it has downshifted, I can put in in neutral and then back into gear and it will drop into 4th.

The first time this happened, I checked the fluid level. Itwas a bit low so I brought it up to full. The problem went away and I thought all was OK. Well, the downshift problem came back and the fluid level is correct.

Has anyone seen this before. And, should I cancel my vacation and use the money for a new transmission? Thanks

It sounds like your control pressure cable is out of whack. You will need to have that checked before you dive into the tranny itself. Sounds like it's too sensitive. There may be other things causing this problem, but that's where I would start..

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Jeff Lawrence
1989 300e
2000 Dodge Grand Caravan SE

You did not mention the year model of your car. Mine is an '87 300E. Several years ago it started down shifting just as you have described. Eventually, it would actually shift down to second or first gear on its own. Required complete overhaul. Hope this is not the case with your car, but would have it checked by a tech who does MB transmissions.

John,
Jeff makes a good point, it could be an adjustment problem. You might also try changing the transmission fluid and filter(don't forget to change the fluid in the torque converter also) Hope this helps.

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1987 300SDL
1989 300E
1986 300E
1981 300D
1983 240D

The last car I had shifting like yours had packed the filter so tight that it sucked a piece of the paper into the vavle body.

Your experience is usually do to one of two things. Pressure problems do to filter or fluid level problems (including overfilling which foams the oil causing exactly your condition and worse) and governor problems. The first step would be to service the tranny and see whats in the pan. If that solves the problem you can avoid testing the governor pressure under road conditions (including the actual event).

As to the control pressure cable or valve. They will cause it to be near impossible to get 4th gear, but very unlikely in initiate a downshift without stepping on the gas hard. In other words they stick but they don't initiate.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

[This message has been edited by stevebfl (edited 09-15-2000).]

Steve, help me out, My 300 was struggling to stay in 4th gear. Every time I tried to accelerate, it would downshift to 3rd and would stay there until I totally got off the gas or put it in neutral. Tried all the usual stuff, modulator, change fluid, filter (not blocked), shift linkage. Had the control pressure cable end adjusted and "BINGO" problem solved. I'm confused...
:confused: :confused: :confused:

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Jeff Lawrence
1989 300e
2000 Dodge Grand Caravan SE

Just wanted to give an update. The car is an '86 300E 135K Miles.

Changed the fluid and filter. The old stuff didn't look bad and didn't smell burnt. Very little debris on the bottom of the pan. However, no help - tranny still doesn't like to stay in 4th gear.

Then I tried adjusting the pressure valve. I bought the factory manuals ($140) and guess what - tells everything about the car except for transmissions. Read some old posts and started turning that little "T" in the pressure valve behind the nickel size rubber plug (is the plug the only thing that keeps it fromfalling out?). With one turn it was working better. Two better yet. Now I'm at four turns and it's pretty good. 1-2 is harder than before but 3-4 shift is not quite as good.

Total cost $30. Total time 3hrs (with test drives between adjustments). Outside temp 105° (I would do this on the hottest day of the year). I'm probably running on borrowed time with this tranny but I'll take every minute. Thanks to every one who replied.

John H

I am having the same phantom downshift syndrome on my 92 300CE, 114k miles as John H. noted I can drive for several hundred miles and then bang, it will suddenly downshift to 3rd gear. Once it ever freewheeled before downshifting but only once. It will downshift into 2nd gear on rare occasion.
I have serviced the transmission, no garbage was found in pan and, after breaking the metal case open, the filter was clean. I checked the detent / pressure cable for a sticking cable and found it to be free and it returned well.I adjusted the accelerator cable so that most of the slack was gone.
On Wednesday the car drove just fine and on Thursday the car acted up both in the morning and the evening commutes. I stuck my foot into the gas and when I let off it would upshift just to return to phantom downshifting within a mile or so. after 3-4 miles of this it was fine for the rest of the ride home.
I am going to back off the pressure adjustment and see it that does any good.
What is the"T" behind the nickel plug that John H noted is this the modulator pressure adjustment?
I understand that there may be a vacuum switch on the side of the transmission that can go bad but have not found any info either ya or nay on this.

Thanks for your help in advance

P.S. I sold the 93 Sportline as all three were a drain on the insurance budget.

I am having the same phantom downshift syndrome on my 92 300CE, 114k miles as John H. noted I can drive for several hundred miles and then bang, it will suddenly downshift to 3rd gear. Once it ever freewheeled before downshifting but only once. It will downshift into 2nd gear on rare occasion.
I have serviced the transmission, no garbage was found in pan and, after breaking the metal case open, the filter was clean. I checked the detent / pressure cable for a sticking cable and found it to be free and it returned well.I adjusted the accelerator cable so that most of the slack was gone.
On Wednesday the car drove just fine and on Thursday the car acted up both in the morning and the evening commutes. I stuck my foot into the gas and when I let off it would upshift just to return to phantom downshifting within a mile or so. after 3-4 miles of this it was fine for the rest of the ride home.
I am going to back off the pressure adjustment and see it that does any good.
What is the"T" behind the nickel plug that John H noted is this the modulator pressure adjustment?
I understand that there may be a vacuum switch on the side of the transmission that can go bad but have not found any info either ya or nay on this.

Thanks for your help in advance

P.S. I sold the 92 Sportline as all three were a drain on the insurance budget.

Hey Jeff!:)

I see you live in Grandview...I used to live in KC (moved to TX in 98), but I still have in-laws in midtown KC (the Waldo/Brookside area south of the Plaza). Would love to see your 73 (and your 500E) if I get up there for Thanksgiving...

Check the vaccuum modulator line for leaking or broken rubber connectors, and check to see that the vacuum modulator itself holds vaccuum. If the line is leaking or broken, the modulator gets the "wide open throttle" signal all the time, and will downshift very easily. The plastic connetor rod behind the modulator can also be a problem -- modulator isn't too hard to replace, and isn't vastly expensive.

Also, I think the cable is supposed to be just fully pulled out at full throttle -- if it is too tight, it will cause late shifts and/or easy downshifts.

The other options aren't any where near as cheap -- broken governor spring, broken downshift spring in valve body, etc.

Peter

300e upshift
 

Techsters, New at this but think it's great idea. Hope someone can help. I have a 1990 300e, (3liter-6) 145K miles, I am second owner and had it for 125k of it's miles.

Prob is the trans wont "upshift" to highest gear any longer. shifting has also become slightly "clunky" but car runs great and performs well otherwise. I had my local MB guru look at it and he couldn't nail the problem. I have pulled the pan off the tranny, replaced oil w/ new synthetic, no major debris noted in the pan, cleaned everything up an put new trans oil filter, gasket, etc. but still no fix.

car will start fine, sometimes idles a little high but shifts at normal points till high gear, then stays in 3rd. It should shift up to "overdrive" or highway gear and let rpms fall down to about 2000 rpms when hittin' bout 60 or so, right? it just refuses to do this any longer. whazzup? thanks!

(ps, I also have an 84 928 and a bored out, screamin 81 CJ-7, 304 cu in V8)

1993 400 SEL
 

155 K on this Car, I have a similar problem with Tranny but it is an up-shift problem not a down-shift.

Occasionally the car will take a few seconds to go from first to second gear after standing at a stop light or something like that. I know that this car will not go to second as long as it is cold to pre-heat the CC. But this happens after I drove for 30 miles on the E-way.

I start the car in the morning warm up for 10 minutes (Chicago is cold this time of year) and then hit the E-way, I go for about 30 miles non-stop 60mph to 70mph straight though until I hit down town area and then it is stop and go, then this problem occurs. It happens not everyday and once it does it only happens once or twice and then I can do stop and go for the rest of the day and no problem.

Any ideas will be appreciated.

Check and/or replace all the rubber ends on the vacuum lines in the engine compartment. They either get soft and sticky or rock hard, and leak in either condition. This is the cheap fix if you find a bad one.

Also, check the idle valve hoses if you have CIS injection (I don't know what year it changed to LH) -- if these hoses are bad, you will have a serious vacuum leak.

This will also fix idle speed variations and rough idle problems.

If all the rubber bits are in good shape, connect a hand vacuum pump to the transmission modulator line and see if it holds vacuum. If it doesn't, you need to replace the modulator.

Very late shifts and light throttle downshifts are an indication that modulator isn't getting a good vacuum signal from the manifold.

Also check the electric kickdown switch under the accelerator pedal -- if it is stuck or shorted, the tranny is getting the kickdown signal all the time, and will downshift at speed.

It is also possible that the solenoid spring is broken, but that won't be intermittant.

Mostly likely you have some bad rubber hose ends -- when they get hard, they can seal sometimes and not other times. Don't forget to check the rubber hose end on the modulator, too!

Peter

I had this in my 190E, quite simply the transmission was shot and I needed a recondition.
At first it would lose drive in 4th, so it would rev freely.. then if you revved it high enough (to the right revs for 3rd at that speed) it would engage 3rd. After a while it hated 4th and 3rd, only 2nd would stick.

It was actually because water was getting into the transmision oil - it's cooled by a radiator up front, but the box was already shot anyway.

Try a specialist automatic shop - I did and they recognised the problem immediately. By numbers, they are sure to have seen most of the problems that can occur on Mercedes automatics, more likely than a dealer. They'll hopefully be quite friendly too and will drive the car and tell you the likely problem.



later,

Russ