1987 300SDL Not Shifting into Fourth Gear - PeachParts Mercedes-Benz Forum

Hexaflouride · #1 09-06-2015, 05:49 PM

A few weeks ago I was half way through a 2 hour drive to our vacation campground when, after getting caught in stop and go traffic on the interstate, my car dropped out of fourth gear into third while I was going 65 mph. It's terrifying to see the tachometer suddenly jump to over 4000 rpms. I took the first exit and began making my way on the side roads. I found that by mashing the accelerator and then letting off, I could get it to shift back into fourth gear, but if I ever let off the throttle most or all the way, it shifts back down to third gear.

After driving it a few more times, I found that when the car is driven when it's cold and if I don't stop for more than a minute or two, the car shifts into fourth gear properly. If the car is warm and it's sat for more than a few minutes, it won't shift into fourth without excessive coaxing (mashing the accelerator and letting off, then keeping the pedal in a fixed position).

I changed the transmission fluid and disconnected the kickdown switch to no avail. I inspected the vacuum lines and I don't see anything out of place, but I easily could have missed something. Is the transmission really on its way out or could there be some easier fix?

leathermang · #2 09-06-2015, 05:51 PM

look up Trans-x and read the threads...
cheap try... sometimes really fixes the situation.... but no harm if it does not...

mannys9130 · #3 09-06-2015, 06:11 PM

Is it only 4th gear that's misbehaving? Do all the other gears work as they used to and do their shifts occur at the same point in the rev range?

Hexaflouride · #4 09-06-2015, 06:11 PM

All other gears shift like normal, even when it won't shift into fourth. I have one can of Trans-X in there that I added a week and a half ago. I've driven 20 miles on it. Perhaps I should drain a little more trannie fluid and add another can?

leathermang · #5 09-06-2015, 06:44 PM

The instructions on the can are for two stages last time I read them... initial cleanup.. at a certain percentage of the total fluid in the trans....so you have to know how many oz your trans holds.... it took two cans for my initial cleanout... and one to be left in...metal almost qt size cans...
then change out fluid and filter and put in less percentage....
and twenty miles is not very far for something which may have been in there building up for decades.... also... did you do the ' running it through the gears ' sitting still ?
Patience is a real help on this deal... but I would suggest following can instructions.. they have been doing this a long time...

mechmagcn · #6 09-07-2015, 08:29 AM

Sorta sounds like the problem I had recently with my trans. Check the adjustment of your Bowden cable. Mine had broken at the threaded adjuster and I guess a piece fell out allowing excessive play. After adjusting it all of my shifts are crisper and fourth gear now works again.

cho · #7 09-09-2015, 03:56 AM

Quote:

Originally Posted by Hexaflouride

A few weeks ago I was half way through a 2 hour drive to our vacation campground when, after getting caught in stop and go traffic on the interstate, my car dropped out of fourth gear into third while I was going 65 mph. It's terrifying to see the tachometer suddenly jump to over 4000 rpms.

transmission downshifts to neutral/3rd at high rpm

.

Mxfrank · #8 09-09-2015, 09:17 AM

A good transmission shop would use a pressure gauge to diagnose this, but a few suggestions. Fourth places the most stress on the hydraulic system, because both clutch packs are engaged. By all means, do the usual incantations: adjust the cable, replace the K2 spring, K1 also if you're there. Change the fluid, add magic jamba juice. But the most likely cause is a worn clutch pack or seal causing an internal leak and drop in working pressure. Typically, it starts when the fluid is warm and viscosity is lower. But this will get worse in time, eventually it won't shift into 4th at all. And this can only be fixed the hard way...

leathermang · #9 09-09-2015, 09:51 AM

Quote:

Originally Posted by Mxfrank

...... add magic jamba juice....

Properly referred to as "built up varnish deposit dissolver " from companies who make things like Trans x, Seafoam, etc...
No one claims magic for these fluids...

Maxbumpo · #10 09-09-2015, 09:58 AM

Quote:

Originally Posted by Hexaflouride

I changed the transmission fluid and disconnected the kickdown switch to no avail.

Did you also change the transmission filter? I've heard that any brand other than Mann filters are prone to collapse and cause all sorts of shifting problems.

babymog · #11 09-09-2015, 04:16 PM

The real magic comes from solvents (acetone et al) in these products. The seals in your transmission get hard and wear over time, solvents can soften and swell the seals.

The effect from this swelling is that the hydraulic fluid that is currently leaking past seals instead of applying pressure where it should go to tighten up bands and clutches, now doesn't leak so pressure builds and moves things properly (or at least close enough to make it work for a while).

Mit freundlichen Grüßen

Quote:

Originally Posted by leathermang

Properly referred to as "built up varnish deposit dissolver " from companies who make things like Trans x, Seafoam, etc...
No one claims magic for these fluids...

Ceristimo · #12 09-09-2015, 04:30 PM

I second the changing of the filter.

I have had a very similar issue with losing 4th gear on my W123 a few months ago:
Lost 4th gear.....
Symptoms were pretty much the same: Sometimes 4th would work, and when it didn't I could coax it into 4th gear, but I couldn't keep momentum going though. It would slip like crazy.

The transmission fluid itself looked fine; I am pretty sure the PO had changed it not too long before I bought the car. I thought for sure my transmission was toast, but based on the advise here (thanks guys!) decided to just change fluid and filter and pour in a bottle of Trans-X, just for the hell of it.

I didn't think it would help much, as my tranny fluid looked fine...However, after dropping the pan I realized the filter looked in rough shape.

Changed the fluid and the filter (drained torque converter as well), threw in a bottle of Trans-X, and it's all OK again. Pretty sure that it was the restricted filter not allowing enough pressure to build up to engage 4th gear.

It would really suck to change the fluid again if you just did it, but if you didn't change the filter than it's worth a try. Don't forget to change the pan gasket as well.

babymog · #13 09-10-2015, 02:32 PM

I did have a 124 that lost 4th occasionally, the longer I drove it (freeway), the more likely it would go away. shutting the car off for a while gave me back 4th for a limited time.

The pan was full of friction material, which likely was clogging the filter, after sitting it possibly dripped into the pan enough to build pressure. Changing the filter and fluid did help some, but it came back in a couple of thousand miles, ... the amount of friction material in the pan told me that the transmission was well-worn and probably had been slipping for a while, low control pressures will do that (from a clogged filter?).

Hexaflouride · #14 09-26-2015, 04:09 PM

I have good news everyone! A complete draining of the transmission and torque converter and filter replacement with a Mann filter seem to have done the job! So far, the car is consistently shifting into 4th even when I drive it after the engine is hot and has been sitting for a few minutes. I put another bottle of Trans-X in with the new fluid for good measure. The old filter looked pretty dirty even though it only had about 25000 miles on it.