Gearbox grind

[Woodstock]

The four speed gearbox in my '71 280SL has an occasional grind when shifting from 1st to 2nd.  Is there any additive to the transmission fluid which will lessen the grind?  I have been told that to properly repair the syncro the gearbox needs to be removed and disassembled.  It has been recommended that the gearbox be completely rebuilt at that time at a cost of around $5K.  Does this seem reasonable?  Car has 100K miles on it.

[rjmarco]

I had this done on my 1965 230 SL a few years ago. The car had just over 50k miles.  I had grinding going from first to second gear and had to double clutch while shifting to change gears.  Yes, I had to have the transmission dropped and the synchros changes.  It was a little less than the $5k level but I think it was close to this when all was said and done.  Good luck!

[kampala]

For those in the USA - Sun Valley Mercedes Transmissions, in California would be the place I would go for Mercedes Transmissions.    Many shops in LA area that work on Mercedes buy their rebuilds from these guys.  You will need to check with them for Manual Trans as they specialize in Autos.   

http://www.mercedesdismantlers.com


Or - I would be tempted to get the car to Joe A. in Ohio and have him handle the rebuild etc.   

[Woodstock]

Sounds like the $5K is in the ballpark.  Thanks for the feedback

[ja17]

Hello Woodstock,

Watch your speedometer needle when  you accelerate and de-celebrate. If the needle drops under acceleration before it moves up, then you may have a loose slotted nut on the transmission output shaft.

[Woodstock]

JA17, funny you should mention a speedometer fluctuation because the car's speedometer reads real low (about half the speed you are traveling) and then sometimes it will read correct.  I will have to check that out.  Thanks for the comment.

[Benz Dr.]

I had a car in this spring that was doing the same thing. Easy test is to remove the speedo cable at the trans and use a small screw driver to hold the drive steady that spins the cable. Turn the drive shaft while trying to keep your screw driver from spinning. If you can hold the screw driver steady the rear flange nut is loose. Normally, the screw driver would get twisted right out of your hand if the drive is tight.
This quick and simple test will save you a lot of time trying to see if the flange nut is loose.

If the rear flange nut, or a similar nut is loose inside of the trans, it will cause the snycros to be loose inside the trans due to too much end play resulting in possible grinding.   

[Woodstock]

Dr. Benz-

Since I have just purchased this car and plan to fly in and drive it home 700 miles, if this nut or nuts are loose could this create a problem on the trip?  If so it might be wiser to trailer it home.

Also can the "rear flange nut" be tightened without removal of the transmission?  I would assume "a similar nut loose inside the transmission" would obviously require disassembly of the transmission.

Thanks for the information.  This forum is quite similar to the 356 Registry site where experts are always eager to assist diagnosing other members problems!  Great to have a site like this to help with these type issues.

[Benz Dr.]

I think you could drive the car but I would try to fix it if possible. The rear nut can be tightened after removing the trans mount plate, trans mount, and drive shaft. It's a very good idea to have the correct pin socket so that you can torque the nut down properly.

[Woodstock]

Dr. Benz:

Followed your testing procedure with a screwdriver in the speedometer cable connection and rotated the axle and it could be held.  We are locating the proper tool to tighten the nut and are keeping our fingers crossed that this cures the fourth gear pop out problem and the speedometer problem both.  Thanks for the tip!!

[Benz Dr.]

Popping out of high gear could by a worn input shaft at the pilot bearing end. I've seen where the pilot bearing gets seized up and then the shaft spins inside of the bearing causing wear. This bit of extra radial play can make the input shaft deflect and cause it to pop out of gear.

[Woodstock]

Dr. Benz:

You were correct about the loose nut causing the speedometer to read inconsistently and I was hopeful this would cure the popping out of fourth problem.  This nut was found to be only finger tight.  Unfortunately the car may have been driven too long with this loose nut and created some wear problem in the transmission.  Since I have a synchro problem shifting from 1st to second and a popping out of fourth gear problem, the transmission will need to be repaired.  On a car with close to 100K miles what would you recommend be replaced when the transmission is apart.  Obviously bearing and synchros would be replaced but I suspect there are more items needing replacing.

Thanks,
Randy

[Benz Dr.]

Well, it depends on what you find inside the trans once it's apart but basically you would be looking at four bearings, pilot bearing, throw out bearing, syncro rings if needed, gasket set, shims, and any internal parts that may be damaged. The last one I did was quite a bit less than 5K USD.

 Having used parts and the tools to work on these old units sure helps. I can take one apart in about an hour ( if everything goes well ) but it can take more than a day to put it all back together. I've been thinking about keeping rebuilt units for 230 & 280 in stock on an exchange basis to cut down on turn around times. Shipping is a bit of a factor because the bell housing has to be on the box or it can't be done.

[ja17]

Just tightening the slotted nut does not always work. When the slotted nut  is loose, a gear cluster in the transmission loosens up and some keys get damaged. Parts for these boxes are getting very pricey. Besides the gaskets and seals, you will need a set of synchros, the upper and lower shaft keys, all four main ball bearings, a pilot bearing in the flywheel and possibly an input shaft/gear. Consider clutch replacement, re-surfacing the flywheel, and closely check the driveshaft flex-disc while all is apart.