For a few months, my 1991 420SEL (W126) with 171,000 miles has been getting slower to engage into reverse, and now it has gotten to the point where I need to blip the gas a couple of times before it engages. Everything seems fine in forward gears - the fluid was changed at 150,000 miles (every 30K), and is at the correct level.

I have done a fair amount of searching, and found multiple references to an external adjustment of the reverse (B3) band, but apparently this doesn't apply to my transmission, which I believe is a 722.3 (??).

So, I am hoping you all have some suggestions:

- Is it really true that there is no external reverse band adjustment for this model?

- Is there any other form of adjustment that would help? I saw a post or two that talked about adusting the vacuum modulator, but isn't this mostly for forward gear shifting? If adjusting the modulator would help, how is this done? I see the modulator on the side of the transmission - is it basically a loosen and rotate? If so, which direction, and how sensitive is it?

- If none of the above are likely to help, what should I expect from a dealer repair - are there relatively minor adjustments that they can make, internal to the transmission, that are short of a rebuild?

Thanks for your advice and help.

rschleicher
1991 420SEL, 171,000 miles

any chance - doubtful perhaps - that this delay is related to a linkage condition? as in worn bushings. will the bugger engage in a timely fashion if you giggle the shifter a bit? again, longshot.

janko,

I don't think it is a case of bad or worn bushings, based on "feel", but I will try to wiggle things around a bit.

Thanks for your reply.

rschleicher

sorry to bring bad news, but: either the reverse clutch is worn out, and/ or, the oil pump is worn, and/or internal seal leakage.

It will have to come apart probably to fit it.

all of above presuming the filter is clean and fluid in good condition.

Its sounds like your Lb3 clutches (reverse clutches) are worn causing your complaint. There is no external adjustment for that.
The tranny is gonna have to come out, sorry and good luck

Sounds like the clutches are not receiving the fluid as fast as they should. I would suspect a valve is not opening all the way. I would try some trans fix or similar from the local auto store. If that doesn't work you can pull the pan and poke around and try to fix it. Or just live with it, after all how much as a faster reverse worth to you?

Usually if the clutches are worn out you would see their remains in the fluid and bottom of the pan. Worn clutches do take longer to engage, but shouldn't be very noticable (varies with vehicle). Most importantly; worn out clutches usually show themselves by slipping. A good solid engagement is a good sign, do you have it?

As a follow-up to my original post:

- As multiple people advised, the problem was indeed a worn-out B3 reverse clutch. Earlier 722.0 to 722.2 transmissions use a B3 reverse band, with an external adjustment. No such luck on the 722.3 transmission in my 1991 420SEL (W126). It has a B3 clutch mechanism for reverse engagement, with no external adjustment.

Repair of the B3 clutch requires removal and opening of the transmission, and so the question becomes: just repair the B3 clutch, or do a full rebuild, or even consider swapping in a replacement rebuilt transmission? Prices for either a full rebuild of my transmission, or a new rebuilt replacement transmission, were in the $2600 to $3000 range (from multiple shops), at high SF Bay Area labor rates. I got a quote for just the B3 repair of $1300 from Walnut Creek Transmission (Walnut Creek, CA), who also priced a full rebuild at $2600.

I basically told them: Look, I was already intending to sell the car in about 6 months (which is true), so I would prefer to jsut do the B3 repair. But, if you see signs of other imminent problems, or generally worn parts in other areas of the transmission, I will do a full rebuild, so as not to leave any "ticking time bombs" in the car.

After opening up the transmission, the mechanic said that the B3 clutch as toast, but that everything else seemed in pretty good shape. His advice was that just doing a B3 repair was fine, but if I was going to keep the car for a long time, it might be worth doing a full rebuild. I opted for just the B3 repair, which also included new seals.

In looking at the problem myself, before taking the car in, I had tried to adjust the vacuum modulator a bit, to see if it helped reverse engagement at all. Adjusting the vacuum modulator produced the expected changes in the firmness of forward-gear shifts, but didn't seem to impact reverse engagement at all (more or less as expected). But, I noticed that there was a small amount of ATF under the rubber dust cap of the modulator adjustment "T-bar". Mechanic confirmed that the modulator was leaking slightly, so it was also replaced.

Car now engages reverse right away, and foward-gear shifting is good (as it was before) - nice firm shifts at full/heavy throttle, and softer shifts at low/partial throttle. If anything the low-throttle shifts are a little smoother than before - this is presumably due to the new vacuum modulator "side issue" (slight leak of old modulator probably lowered the vacuum applied to the transmission during low-throttle shifts).

Price was as quoted $1315 including sales tax. $700 was for removal, diagnosis, and replacement of the tranny, $350 was the labor for the repair itself, and the parts (B3 clutch pieces, modulator, seals, etc.), new ATF and filter, and sales tax came to $265.

Bob Schleicher

Ok, the guy said reverse was toast, (not sure how he knows that) but you're saying reverse engages normally now??

Well, in the few hundred we have done that were toast, it was easy to tell.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Chevota,

Well, I didn't see the removed B3 clutch parts, but the worn-out nature of them was apparently obvious to the mechanic.

To reiterate what my original symptoms were:

- for the past year, the time it takes to engage reverse had slowly gotten longer, until it was up to 3-4 seconds. But, up until recently, it had still always engaged.

- recently, if the transmission was warm/hot, it had started to not engage at all. (If cold, it would still engage after 4+ seconds). To be more precise, after selecting reverse, I'd see the idle RPM drop a little, but there was no positive engagement of reverse. If I blipped the gas I'd get a slight amount of rear-ward motion, if on perfectly level ground (like my garage), but not enough to overcome any sort of slope (like a very loose fluid coupling). Sometimes, after a long while, and more throttle-blipping, it would finally engage (typically with a lurch, from the blipping of the throttle). Once engaged, it didn't seem to slip. But, if I put it out of gear, and then back into reverse, it would start the whole "not engaging" process all over again.

Following the repair, reverse engages positively within a second, as with a new/normal transmission. Engagement into Drive, as well as shifting in forward gears, was always fine, and was not affected by either the problem, nor its repair. (It is my understanding that there is another failure mode that can cause slow engagement into BOTH reverse and drive, but this is not the B3 clutch issue, and is far less frequent on these transmissions.)

BTW, in doing a Google on the key words 722.3, Mercedes, and transmission, I stumbled across a .pdf file that describes repair and diagnosis procedures for 722.3-722.6 transmissions. The link is http://w126.pp.ru/akp722.pdf This document specifically covers 1996 transmissions, but the 722.3xx versions described for the 1996 SL are basically pretty close to the transmission in W126's (probably due to the fact that this version of SL came out when the W126's were still in production).

It contains a short section on symptoms and probably causes, as well as the removal/repair/replace procedures. I wasn't trying to figure out how to do the work, but I did want to know about the most likely causes of my problem, in order to be able to have a partially-knowledgable conversation with the mechanic.



Bob Schleicher

Oic, I misunderstood. I thought you had just taken it in to be looked at, I didn’t realize the guy had pulled the thing out already.

I too have a 1991 420SEL, with a bashful reverse. For the last few months, this has been an issue. It is really bad when i park in a down hill direction, i dont do that any more. And, when shopping or parking, i either find a way to park to the side or pull through the parking space so i can drive straight out. A time or two, i have had to open the door and push backwards with one leg.

I called my mechanic and he suggested the linkage or the bushings.

Now i know what i did not want to know, but its 2009, and my little girl only has 109,000 on it, so i guess the $1300 is the right repair to attempt too.

Fortunately, i live in Alabama and have a great transmission man who replaced the transmission in my 88 300E several years ago, who, i think should be able to take these notes and be able to beat that price.

Today, as i was about to head out on a quick jump to Huntsville,and as the window switch had been going down good, raising that sucker was getting intermittent and i pressed hard on the drivers side window button and pushed it into the console, fortunately, i was able to pull the console cover, swap out the drivers side rear seat switch with the front switch so this wont be happening again. Then, i thought about the transmission problem and a quick google search found this forum.

This forum has been terrific. Thanks Guys.