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1991 350SD shifting trouble

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mbaird's picture
Santa Cruz, CA
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1991 350SD

Hey everyone,
As my post count shows, I'm brand new to these parts, but hopefully that'll change soon. :)

My brother gifted me with a '91 350SD about two and a half years ago. It's had no trouble outside of routine stuff (like shock replacement...they were 10 years old!). Over the last few weeks, though, I've been feeling more and more worried over this weird problem with shifting. Going from second to third, or third to fourth, the transmission clunks and the shift is hard and can be felt inside the cabin.

It's definitely a major change from when I first had the car, and so I took it to Aamco for an external diagnostic. They test-drove the car with me, and I pointed out when it was happening and the tech agreed it was strange but he did not think it was something to be concerned about. He did say, and I checked and do agree, that the transmission fluid is a little old, but not real bad yet (just a faint hint of char to the smell).

So anyway, Aamco wants $200 for a fluid purge, but I can probably do that myself, yet I only want to if that's likely to solve the problem.

That aside, I am particularly concerned because last night, for the first time, I put the gearshifter into reverse and the car did not follow. After restoring the shift lever to park, and then once again shifting into reverse, the transmission followed and performed the shift. I am concerned that if I do not get this repaired soon, this could become much worse.

A colleague also pointed out that if the transmission is electronically controlled, it's possible the system could be out-of-sync as the gears are a little worn, etc. I don't believe this model has that kind of transmission, but I could be wrong.

I've searched your forums here and others on the Web, to no avail. So that's my story so far! If you have any advice, it will be much appreciated. Thanks in advance!

-Michael

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rogo's picture
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Re: 1991 350SD shifting trouble

Sounds like a classic vacuum problem.

Crawl under the car and locate the vacuum modulator on the left side of the transmission. It is the only thing down there with a plastic vacuum line on it. Pull it off the modulator and check for the presence of transmission fluid. If present, replace the modulator.

If not present, check for vacuum. If not present, follow the line up into the engine compartment and check at the vacuum amplifier and keep going all the way back to the vacuum pump if neccessary, until you find vacuum then fix the leak.

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Virginia Beach VA

87 300SDL
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Former Member
mbaird's picture
Santa Cruz, CA
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1991 350SD
Re: 1991 350SD shifting trouble

Thanks for the tips, Robert. What can I do to check for vacuum, if the modulator is okay?

Edit: Should I check the modulator itself or the vacuum line for transmission fluid (I've never seen the modulator, so I'm not sure if I can see "inside" it)?

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mbaird's picture
Santa Cruz, CA
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1991 350SD
Re: 1991 350SD shifting trouble

I understand that changing the fluid would be a good thing to do; I just don't want to spend $200 to do it, which is what I would need to do if it were something that had to be done yesterday. Since it's not definitively the cause of my problem, I can put it off a week or two longer, eh?

I don't believe the shocks I replaced were the originals, but they were definitely worn out. They were also fairly salted from New England weather (I had to use a hacksaw to get one of the nuts in the engine compartment off the shock rod :eek:), though the car is now in California. A Rolls-Royce Phantom has nothing on the new ride :P

I figured the transmission was not governed by the chip. '91 wasn't exactly the peak of automotive computer tech. Thank you for confirming that.

I have the service manual on CD, bought from someplace I can't remember anymore. I will try to find the transmission manual, and I'll do some research in my area to see if I can find a decent shop to rebuild the transmission in the coming weeks.

Thanks for the tips; I'll keep at it :)

Edit: I had another question come to mind: when checking for fluid/vacuum pressure in the modulator and/or rest of the vacuum system, should I be doing it with the vehicle running? If so, engaged in which gear?

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Re: 1991 350SD shifting trouble

Just get up in the middle of the night and pull off the vacuum line. If fluid is present you will know it.

If you already have the CD, you should download Steve Nervigs Index for the Service CDs. It allows you to use the CDs easily without having to learn what Mercedes meant when they structured their proprietary product. You can download the index at: http://web.mac.com/dakota/iWeb/Mercedes/Indexes.html

Nervigs Index also includes a transmission manual.

Additional resources are available by joining the Mercedes Benz Club of America. MBCA membership application is at: https://www.mbca.org/mbca/index.htm

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mbaird's picture
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1991 350SD
Re: 1991 350SD shifting trouble

Oh, that's a handy link, Robert. Thanks :)

I will check out the modulator line when I get home, after things have cooled off, and let ya know how it goes.

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Re: 1991 350SD shifting trouble

Changing AT fluid & filter a must before rebuilding - drastic. Harshness of shift on my 722.5 trans. is controlled by trans. modulator which is located below driver's side on tranny. If you check w/ vacuum pressure tester and no vacuum, it needs replacement. Also make sure no cracks in the plastic line running to it. The timing of the shifts is also controlled by the throttle linkage cable running to trans. Sounds like you have a vacuum modulator problem (if you're lucky). Otherwise problems on the trans. shifting & reverse problem might be needs new fluid. Worse case scenario check George Murphy our moderator & tech. rep. on what he recommends & who he recommends to do the rebuild if it's gone that far. George can also supply you the parts at pretty good prices. Flaring in transmission has also been associated with B2 piston problem w/ metal sleeve. My trans. had upgrade to plastic sleeve to prevent sticking.

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mbaird's picture
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Re: 1991 350SD shifting trouble

I will definitely take it step by step, changing the transmission fluid and filter before a complete rebuild, further diagnostics, etc.

For now, Ill follow Robert's and your own advice and inspect the vacuum line around the modulator. If there's no fluid leaking into the modulator area, Ill check the pressure through the system as Robert suggested.

If I run into bad luck, who is George Murphy and how do I contact him? :)

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Re: 1991 350SD shifting trouble

Get under the car and check the shift lever bushings - sounds like it is not going entirely into gear when shifted - a sign of worn bushings - very common and easy to fix - 2 new bushings cost less than $5 - heat them in boiling water to make them easier to insert into the shift levers . . .

But do replace the modulator if over 100,000 miles - they do get tired even if they are not leaking ATF . . .and don't fall for a "transmission flush" . Drain the transmission AND torque converter and replace the filter for proper fluid change. It will take 6.5 quarts of Dexron III ATF . . .email me for more details . . .

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Re: 1991 350SD shifting trouble

I echo looking at the shifter bushings! *#*#*#*# just replace them. If nothing else you will get back the spring tension feeeling when you shift from park. You can also just use a nut washer and bolt technique to push them in.
Bruce
Good luck

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Bruce
BENZITCH

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Re: 1991 350SD shifting trouble

$200 for a transmission fluid change--which is what you need-- is a bit expensive.

I say "what you need" to contrast that Mercedes transmissions do not require a "flush" which is probably what AAMCO has in mind. The reason for "flushing" is to pay for the shop's expensive flushing machine.

http://www.thecarconnection.com/Enthusiasts/Mechanics_Tale/Mechanics_Tale_The_Flushing_of_America.S281.A10297.html

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Re: 1991 350SD shifting trouble

I might consider having my transmission flushed if I were going to remove it and tear it down right away.............but not for $200.

When one flushes the tarnsmission, how does one replace the filter????

I do my own for less than $50.

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Re: 1991 350SD shifting trouble

I do all my own wrenching (head swap, AC service, clutch, etc) except for alignments and tranny service.

Certainly there's nothing magical about pulling the pan, bathing in ATF, removing a few bolts to change the filter, and reassembling. However, I like the experienced ASE transmission-specific mechanic to read the "tea leaves" of the dregs in the bottom of the pan to get a read on the internal condition.

Specifically my guy warns against any "violent" activity on an older transmission. Specifically he has gained MANY customers who brought the car into Aattco or QwikGrease yesterday/last week for a flush and now need a rebuild. His claim is that the internals are covered with detritus (a la the dirty-tank filter-clogging problem when switching to biodiesel) that is best left alone. If it has accumulated over years, leave it be to slowly disipate into now-proper fluid flushes.

On my 2x123 and 126 he wouldn't even pull the converter plug. He didn't want to change "too much" at once and shock the system. He also suggests that Benz trannys are happier on petro ATF rather than synth (I use synth for every other fluid on every vehicle). Certainly if the trans has been regularly serviced there shouldn't be too much crud, so a full change would likely be OK; on these service-unknown cars that many of us are now acquiring there seems to be such a thing as too much love.

Naturally your mileage may vary.

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James Marriott 1987 300SDL (155k, mein Auto, 100% homebio 3 years) 1983 300D (221k, frau Auto, 100% homebio 3 years) 1997 Suburban (149k, gas/4WD towmaster 6000) www.engineeringworks.biz 1982 300SD junker, 265k

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Re: 1991 350SD shifting trouble

I've replaced the shift bushings on a manual '89 Jetta I sold a couple months ago, so it should be similar, right? I pulled up some parts lists and took a look at the pictures, and the bushings look pretty much the same as the Jetta ones I replaced. I should have time for that this weekend.

I also have a complete record on this car - all the way back to the day it was bought, including the dealer's window sticker. Rare find these days :)

I'll review the record and see what I can find. I've done it a couple times now and don't recall much about transmission maintenance, but maybe I overlooked something.

Thanks for all the tips and advice! I'm looking forward to finally having a real resolution to this problem!

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Re: 1991 350SD shifting trouble

FYI for anyone else with this problem in the future...
There was actually a vaccuum line disconnected at the engine block (driver's side near the manual stop lever). Reconnecting it restored vaccuum and completely resolved the shift problem.

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My shifting problem exactly

I too have experienced the same issues. The slow (very slow, when hot) reverse shifting is the most troublesome. My shop found a vacuum leak and fixed it some months ago. But the problems started again and i expected the worse. After reading this thread, i went out and checked the vacuum connections on the engine block. Sure enough, the little rubber elbow connector on the engine that connects one of the hoses was cracked. I disconnected the hose and shoe-gooped the elbow together again. Wa-la, the tranny shifts a lot smoother, and reverse engages promptly.

For such an otherwise beautifully engineered and built car to depend on a bunch of brittle plastic tubes connected with soft rubber couplings to operate properly doesn't make much sense to me. What were they thinking?

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Dr. J.
Annapolis, MD
1991 350 SD

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