rear diff woes - PeachParts Mercedes-Benz Forum

dieselnut14 · #1 08-23-2004, 07:29 PM

I want to replace the rear diff. in my 1984 300d, it is wining and I like to take this opportunity to change it and lower the ratio for better mpg and alittle less noise at highway speed. I talked to the local mb dealer to see what my options are. They say I currently have 3.06 gears and that I have to order a ring and pinion for a 1985 which would turn out to be 2.88 ratio.
My problem is, my shop manual says I have 3.46 ratio and and it says the same thing for the 1985. Who or what is right. I took these #'s off my rear end this afternoon. can anybody tell which ratio I have with these #'s
115 351 32 08

3826

6N 84

+6f+m
any help would be appreciated

whunter · #2 08-23-2004, 08:03 PM

Hello dieselnut14

The 1985 300SD has 2.88 ratio.

If you can find one.

I looked under a 1985 300SD at a junk yard today, someone put an 1984 or earlier differential in it.

Old300D · #3 08-23-2004, 08:06 PM

The diff should have the ratio stamped in the case, to left and below the cover, looking at it from the rear. A '84 300D turbo should have the 3.06, the 3.46 came in the non-turbo versions.

I just pulled a 2.88 from an '85 300D at a local u-pull-it, for $60 including the axle shafts. That is the way to go instead of ordering new parts and having the unit rebuilt, IMHO. Properly maintained, a used unit should last a good long while. How much are you quoted to price new gears and have them installed?

oldnavy · #4 08-23-2004, 09:00 PM

Rear gear ratio's listed in 123 cars by service manual 1985 are: 3:58, 2:47, 2:24, 3.69, 3:92, 3:46 and 3:07.

84300DT · #5 08-23-2004, 09:36 PM

i replaced mine over 2 years ago with a used diff with 132k on it that was pulled out of an '81 300D.still going strong and running like a scalded dog

rg2098 · #6 08-23-2004, 10:02 PM

I have the 1985 Turbo Diesel brochure from Mercedes and it state a 2.88:1 rear gear ratio for the complete line-up.

oldnavy · #7 08-23-2004, 10:20 PM

Quote:

Originally Posted by rg2098

I have the 1985 Turbo Diesel brochure from Mercedes and it state a 2.88:1 rear gear ratio for the complete line-up.

I was quoting all W123 car's and they might all be interchangable.

dieselnut14 · #8 08-24-2004, 07:17 AM

A new ring and pinion would be $600 from the dealer. my friend that runs a shop said that we would install it together, no charge.
also if I go by the way of a used diff from an 1985, do I also have to swap the speedo drive at the transmission? I would think so since it would keep everything in calibrated as it was before.
thanks for your help guys

TonyFromWestOz · #9 08-24-2004, 10:51 AM

Quote:

Originally Posted by dieselnut14

A new ring and pinion would be $600 from the dealer. my friend that runs a shop said that we would install it together, no charge.
also if I go by the way of a used diff from an 1985, do I also have to swap the speedo drive at the transmission? I would think so since it would keep everything in calibrated as it was before.
thanks for your help guys

If you change the diff ratio you will need to either change the speedo drive ratio, have a modifier gearset (like on rallye cars) or have the speedo calibrated for the new diff ratio.
Or, you could just have a few speeds checked against a car you trust.

Gil · #10 07-03-2005, 07:56 AM

you could use a dash- mounted GPS to get the most accurate MPH times, then mark off your speedo. Not the best looking result, but at least you have accuracy in your favor.

t walgamuth · #11 07-05-2005, 09:21 PM

first, the diffs are notoriously noisy according to the mb buyers guide. personally i dont think so. are you sure yours is unusually noisy? these diffs are extremely durable.

if you take the decision to change it note that the 2.88 diffs from 85 and up are matched with an electronic speedo gizmo and wont work in your 84. a 380 gas car comes with a 2.65 i think and they have the mechanical speedo that will work for your car. this will give mileage improvments and less engine noise at road speed but will make your takeoff more doggy.

lots of recent posts regarding these subjects.

dieseldiehard · #12 07-05-2005, 11:11 PM

I believe dieselnut was considering swappin a diff into a 300D. 1985 300D speedometers are conventional meaning a diff from an 85 will work all the way down to a 79, further back perhaps.
I have them in my 123's. I love that 2.88 diff! They are NOT noisy!
Definitely worth looking for one if you have a turbo engine that is strong. The SD is another story though but I thought the thread was about a 300D?

boneheaddoctor · #13 07-06-2005, 08:20 AM

Quote:

Originally Posted by oldnavy

Rear gear ratio's listed in 123 cars by service manual 1985 are: 3:58, 2:47, 2:24, 3.69, 3:92, 3:46 and 3:07.

I agree....look on the case I bet you its stamped 3.07 my '83 300D is.

t walgamuth · #14 07-06-2005, 08:39 AM

first i have been lead to believe that the 85 has the electronic gizmo for the speedo. is this wrong?

second, what 123 body cars have a 2.47 and 2.25 ratio? i would have thought that those were ratios for the s class. with v8's.

R Leo · #15 07-06-2005, 09:19 AM

Quote:

Originally Posted by t walgamuth

first i have been lead to believe that the 85 has the electronic gizmo for the speedo. is this wrong?

second, what 123 body cars have a 2.47 and 2.25 ratio? i would have thought that those were ratios for the s class. with v8's.

'85 W123s all have mechanical speedos and 2.88 diffys. At least, all three of mine do.