Manual transmission grinding going into 3rd gear - PeachParts Mercedes-Benz Forum

JimmyL · #1 02-14-2008, 07:52 PM

The car is China, 1980 240D.
History: she would occasionally grind shifting into 3rd on the first shift of the day when the weather was cold. Not having a baseline, I changed the tranny fluid. Used Redline MTL filled to proper level.

Gradually over the last couple of months the occasions of grinding going into 3rd {upshift or downshift} are getting more numerous, and not only when cold now. I would say that in the course of a normal day it will grind 5-8 times total. It doesn't matter if clutch pedal is buried to the bottom of the floorboard, or how precise I shift {as in up from second, over, then up to 3rd exaggerated}.
Car has 143K. Shifter linkages are tight to tranny levers {this style tranny levers/linkages attach with collar type fitting}
What causes this, and what repairs this?

Stevo · #2 02-14-2008, 08:10 PM

[QUOTE=JimmyL;1764124]The car is China, 1980 240D.
History: she would occasionally grind shifting into 3rd on the first shift of the day when the weather was cold. Not having a baseline, I changed the tranny fluid. Used Redline MTL filled to proper level.

Wow, 143K is nothing for one of those trannys. A friend had one die with over 250K on it and we found another at Benz Friends in Seattle for $200. I have never rebuilt one, and that might be something I would have done. Guess it depends on how you fell about the job, I think the "normal
wear parts" are fairly cheap. Good luck

pj67coll · #3 02-14-2008, 08:17 PM

Quote:

Gradually over the last couple of months the occasions of grinding going into 3rd {upshift or downshift} are getting more numerous, and not only when cold now. I would say that in the course of a normal day it will grind 5-8 times total.
Car has 143K. Shifter linkages are tight to tranny levers {this style tranny levers/linkages attach with collar type fitting}
What causes this, and what repairs this?

My recently departed 81 240D displayed exactly the same symptoms. Started a few months after I bought it and only improved slightly with new shift linkage bushings. However it had around 380K miles when this showed up. I'm surprised to hear of it a only 143K. Can't answer the question as to what causes it but a suspicion is it might be clutch wear.

- Peter.

rs899 · #4 02-14-2008, 08:25 PM

Are you sure your clutch hydraulics are not an issue? A cheap sanity check might be to bleed 'em and see if that improves anything (I know, its a PITA)

barry123400 · #5 02-14-2008, 08:45 PM

Damaged or worn syncro ring on third is a possibility. I have changed them on other brands. Usually pretty inexpensive.
I would very carefully inspect my linkages for wear or binding first though.

JimmyL · #6 02-14-2008, 09:16 PM

Clutch never has felt very strong, and if I don't shift her smooth she loves to chug-a-lug shifting from 1st to 2nd. A real neck snapping type of chugging.
But would a clutch issue only cause a grinding into one particular gear?

barry123400 · #7 02-14-2008, 09:37 PM

Try double clutching to help diagnose. Thats where you press the clutch. Move out of gear into neutral. Release clutch then depress clutch again and move into gear. If grind is eliminated it is usually the syncro ring.
There is a possibility that third gear is more fussy to a not totally disengaging clutch as well I guess.
Jack up one rear wheel. Have someone depress the clutch. Does rear wheel turn fairly easily. Or when stopped at an intersection is clutch totally releasing or does car want to creep with brake off?

300SDog · #8 02-14-2008, 10:24 PM

What wgt rating is Redline MTL manual tranny & transaxle lube?? 4-spd MB should have 30wt engine oil or regular ATF...... never 60-80wt.

JimmyL · #9 02-14-2008, 10:36 PM

Quote:

Originally Posted by 300SDog

What wgt rating is Redline MTL manual tranny & transaxle lube?? 4-spd MB should have 30wt engine oil or regular ATF...... never 60-80wt.

Manual Transmission Lube
http://www.racepartsusa.com/shop/customer/product.php?productid=16502&partner=froogle

JimSmith · #10 02-14-2008, 10:59 PM

I would also suspect the clutch. Air in the system and the clutch probably doesn't disengage fully when you press the pedal to the floor. Thus, there is a load on the dog clutch when it is pushed to engage. No synchros can deal with the load that creates. They are intended for no more than the inertia of the spinning parts when completely disconnected from the wheels.

Not clear why you only get this on one gear, but when a clutch isn't disengaging completely I think it is actually common to have this problem with lower gears more so than higher ones. In any case, I would examine the clutch operation before I went much further. It is possible the linkage is maladjusted as well, which could feasibly be stopping the dog clutch and the associated shaft from spinning, which will also put an unusual load on the synchro, which is outside its design capacity.

Good luck, Jim

PS: How long has the RedLine been in there?

300SDog · #11 02-14-2008, 11:06 PM

I'm confused - they call it 70W80 yet claim 10W30 "engine oil viscosity."

If it was me I'd run Ford type 2 ATF, same as MB's been using in manual trannies since about 1959. Question of needle bearings requiring thinner oil you understand. And whats the brown stuff left from previous owner that's been purged? If thats 60-80 wt stuff then hate to say it but damage coulda already been done.

JimmyL · #12 02-15-2008, 12:12 AM

Quote:

Originally Posted by JimSmith

I would also suspect the clutch. Air in the system and the clutch probably doesn't disengage fully when you press the pedal to the floor. Thus, there is a load on the dog clutch when it is pushed to engage. No synchros can deal with the load that creates. They are intended for no more than the inertia of the spinning parts when completely disconnected from the wheels.

Not clear why you only get this on one gear, but when a clutch isn't disengaging completely I think it is actually common to have this problem with lower gears more so than higher ones. In any case, I would examine the clutch operation before I went much further. It is possible the linkage is maladjusted as well, which could feasibly be stopping the dog clutch and the associated shaft from spinning, which will also put an unusual load on the synchro, which is outside its design capacity.

Good luck, Jim

PS: How long has the RedLine been in there?

I changed the fluid 1/15/08, almost a month ago. As stated, had the occasional 3rd gear grind prior to the change. Actually, I was hoping the fluid change might help.
When I swapped in the engine from Scar I didn't do anything to the clutch because at the time the car only had 136K. Throughout bearing looked pretty crappy, but who knows by looks....

Maybe a mis-adjusted shifter linkage for 3rd gear? Since tranny stuff isn't in the shop manual does anybody have a pdf of the linkage adjustment procedure, or is it just a case of moving it a tad either way?

Cervan · #13 02-15-2008, 12:14 AM

Try adding some ford synchromesh manual tranny fuild. I used this in my mercedes along with some synthetic lucas oil stabilizer with the rest as synthetic ATF fluid. ran great until i hit a tree.

JimSmith · #14 02-15-2008, 01:19 AM

I would try the previous suggestion of bleeding the hydraulic lines of the clutch. The synchros for first and second gear may just be a bit stronger because they typically have to deal with higher loads (the ratio difference from 1st to 2nd is higher than 3rd and 4th, so the synchro has to do more work to speed up or slow down the free spinning transmission parts before the dog clutch engages) and they actually can handle the added load of a clutch dragging ever so slightly. Otherwise, I would presume the problem is either linkage or something bad has happened to the guts of your transmission. I have never, ever seen on of these go bad though.

Jim

R Leo · #15 02-15-2008, 12:16 PM

A driver inexperienced with the 'stately pace' of 240D acceleration can force or rush a shift which can bend/tweak the synchronizer springs and cause gear grinding during shifts.

Thing one whacked Nell's synchros in 2nd the first time he drove her. It's ground intermittently ever since.