Please help. Transmission occasionally going out of gear - speedo bounces

Started by jbrockminer, 13 September 2015, 07:22 PM

jbrockminer

Hello.  As of yesterday I am a 1979 300sd owner.  My wife and I have been looking for years for "the one".  Now we have it, and I fear the transmission is done-for.

When I drove it from the PO's house to mine (40 miles), about halfway home, the trans temporarily went out of gear (the speedometer started dancing around a bit).  I was in 4th going 70mph or so (felt like 170), let of the gas and returned to 4th.  The rest of the drive home went without incident.

Today, my wife and I went for a little ride, and the same "incident" happened several times (speedometer goes crazy / trans. out of gear for a second or two), but at speeds never exceeding 45mph.  In fact, I think some unnecessary downshifting and up-shifting also occurred.  Dejected, we headed for home.

I adjusted the kickdown cable (there was plenty of slack in it), and checked the fluid (looked healthy).

We went for another ride, and while there was no unusual shifting, you could tell that an "out of gear episode" was coming; preceded by the speedometer's "convulsions".

Anybody seen this one before?  Any advice?

By the way, between the out-of- gear experiences, I looked over at my wife and I knew that no matter what, I've got to fix this car!
WE LOVE IT!





daantjie

Always start simple especially with transmission issue. Suggest to drop the pan and also drain the torque converter. Check pan for sludge and/or metal flakes. Lots of mean trouble.  Replace filter and top up with good quality DEX III fluid. Make sure the torque converter gets filled by shifting through gears with park brake on. Make double sure level is correct and go for a spin.  If it still slips and flares check your vacuum modulator. Might need a turn or 2 clockwise to tighten up the shift.
Daniel
1977 450 SEL 6.9 - Astralsilber

Diesel 617

What color is the transmission fluid? Is it at the correct level?

use the pdf to check the fluid on your trans.

If there is an advanced auto near you they will rent you a vacuum pump for $75 deposit, be sure to ask fora  fresh one, as people tend to use them to flush their brake fluid. Use it to check the vacuum modulator as daantjie said.

By the way our 1978-1980 300SD have a 722.120 Transmission. It is the only transmission that will function properly for our cars. A trans from a 300D will fit and even work, for a little while, then the speedometer sensor will burn out in the tail of the transmission and the eventually the car will refuse to shift past 2nd gear.
X3 1980 300SD Blown Heads, trans -Retired
1980 450SEL Parted out/Scrap
1980 450SEL to Diesel Parted Out/Scrap
1979 280SE Euro Spec Cloth Seats Sold
1979 300CD Daily Driver - Sold


Inventory of w116 Parts
[url="https://www.ebay.com/usr/vintagepartsandstuff"]https://www.ebay.com/usr/vintagepartsandstuff[/url]

jbrockminer

Thank you replying.

As soon as I get home,  I will first make sure the correct transmission is in the vehicle.
I know the fluid level is high and looks/smells new.

Then, i'll order the filter kit and get that done.

By the way, I just remembered that in park, when you give the vehicle some gas, the car lists foreword ever so slightly.   I could understand the car "twisting" under this scenario,  but it really moves foreword a little then rests back into its original position.

Just compiling clues here.

Thanks again. ..  I will report my findings.

jbrockminer

Well, it's definitely a 722.120
Hope it's not too big a hassle getting a filter kit.

jbrockminer

Does flaring only occur between shifts? In other words, is it still considered flaring when you are alteady in gear, and it "slips" out of gear momentarily? 

daantjie

If you get good shifts at low revs/idle throttle but rapid revving with no or little forward movement at bigger throttle opening this is what I consider flaring. This can be attributed to control pressure too low or incorrectly adjusted vacuum modulator. Adjust your control pressure up by the bowden cable. Then you can adjust the firmness of the shift by turning the key inside the vacuum modulator. It is a bit of a marriage between the control pressure and modulator pressure. Many times folks used to a soft shift found in US cars try to mimic it by monkeying with the pressures. Older Benzes are known for a firm shift especially the 1-2 shift. This means virtually no slip thus much higher longevity if properly maintained.
Daniel
1977 450 SEL 6.9 - Astralsilber

jbrockminer

Thank you daantjie and Assault

I used the proper procedure (thanks again) to check the transmission fluid level.  Spot on.

I took her for a spin at normal operating temp. and the shifting was really all over the place in  city(ish) driving conditions.

I went home and searched around for the bowden cable which I learned I do not (exactly) have.  I found the pressure control rod and noticed that once removed from the linkage it seemed that in the position it was in (on the linkage) had what I am going to call a door (out of complete ignorance) in an open position (at the transmission).  I turned (two complete revolutions out toward the front bumper) it in order to elongate the linkage, believing that I had shut "the door" a little.

I drove it through my city test course and except for a late (probably too late) 2-3 shift (I had to let of the gas and resume to go to 3rd), the vehicle behaved superbly.  With each minute I grew more and more excited believing that my new purchase was a gift and it had been to easy - then, venturing out in to more of a country road course, the trans. flared again.  I was contemplating going home, then decided to keep the test going.

I would say it flared four times in 15 minutes.  I feel like it would flare right before an unnecessary downshift, which is interesting because if I'm in 4th and "punch it", it never downshifts. I also had the feeling that there was a certain throttle position / rpm range it disliked.  I would be holding a steady pace, say 50mph, and whammo - a flare.   

Too soon for me to understand this, but I will resume testing tomorrow.

I thank you both sincerely for your input.

By the way, do you recommend any certain company to buy your parts from, or constantly shop around?



     

daantjie

Well if a little is good then more must be better right  8) maybe try a little more on the adjustment. Not quite sure what you are adjusting down at the trans side. Maybe post a pic of what you are seeing.
AutohausAZ is good if you are in the US. Mercedessource is also great for tips and tons on videos on YouTube featuring Kent Bergsma. He is a treat to watch. Also if you cannot effect a downshift by flooring it it tells me your control pressure is still too low.
Daniel
1977 450 SEL 6.9 - Astralsilber

Diesel 617

Have you had to opportunity to check the vacuum lines? Especially the line going to the transmission.
X3 1980 300SD Blown Heads, trans -Retired
1980 450SEL Parted out/Scrap
1980 450SEL to Diesel Parted Out/Scrap
1979 280SE Euro Spec Cloth Seats Sold
1979 300CD Daily Driver - Sold


Inventory of w116 Parts
[url="https://www.ebay.com/usr/vintagepartsandstuff"]https://www.ebay.com/usr/vintagepartsandstuff[/url]

thegreg92

Another thing to check is the bushing on the rod going from the throttle linkages down to the transmission. The bushing is on a lever attached to the right side of the transmission and most likely long gone. I believe this adjusts shift timing but its been a while...
1982 W123 240D - sold
1979 W116 450SEL
2007 W164 ML350
2011 W212 E350 Bluetec - lease expired
1978 W116 300SD - sold

daantjie

Correct, there is a plastic bushing in the coupling between the control pressure rod and the lever going down to transmission. You have to remove the assembly  then press in a new bushing. This removes slop from the rod. No one likes a sloppy rod eh ;D it is really amazing how precise you can adjust these old Benz transmissions by what appears to be relatively crude adjustments. They really are well built units and if maintained well they last a very long time
Daniel
1977 450 SEL 6.9 - Astralsilber

jbrockminer

I didn't mean to suggest that I was manipulating anything on the transmission itself.   The control rod (when disconnected from the linkage) and actuated freely feels and sounds like it is opening and closing a "door".  I'll have to get under there tonight and study what's going on down there (at the transmission).

If interesting,  I'll shoot a video.

I also read on another post that it's possible that the kickdown switch is stuck.  All I know for sure is that upon my initial examination of the kickdown swich it appeared to me that in order to make the switch operate, I would have to push the accelerator pedal very, very deep.  I never considered that it might have been stuck.


daantjie

To test the switch turn ignition to "ON" engine off. Push the switch by hand then you  should hear the solenoid clicking on the transmission.
Daniel
1977 450 SEL 6.9 - Astralsilber

daantjie

Also a quick note on the control pressure rod (some guys call this the kick down rod).  It works directly on the control pressure valve piston inside the valve body.  If you move the rod forward (my 6.9 has a clip on the throttle linkage) towards you as you are standing in front of the car, it has the effect of raising the control pressure.  Moving it back towards the firewall reduces control pressure.  The literature shows the rod must be pushed back towards the firewall, but I have found this to be an incorrect position for my 6.9 3 speed at least.  I get much crisper and better shifts with the rod forward.  Also you have to adjust your modulator to get a firmer or shofter shift.  As I said before firmer is better ;D
Daniel
1977 450 SEL 6.9 - Astralsilber