Differential identification? - PeachParts Mercedes-Benz Forum

Stretch · #1 10-12-2009, 03:22 PM

Dear All,

I have searched and searched through all of the old threads and I can't find anything that shows how to identify which differential is installed on my 1981 w123 300D:-

I am looking for an easy way to do this - without removing it from the vehicle!

{Background information:- I've owned the car for 2 months and it is my first Mercedes. I live in the Netherlands where there are lots of information boards that tell you how fast you are driving (Radar). Every time I go past these things they tell me I am traveling about 7% slower than the speed limit. I've checked the back tires and they are not the recommended size. I have 195/65 R14 fitted where I believe they should be 195/70 R14 => calculations of the corresponding circumferences for these tires suggest that the ones fitted are about 3% smaller - but that doesn't quite match up with the difference in speed that I am seeing - SO I GUESS the differential is not the correct one for the car...?}

Any help will be greatly appreciated

MBeige · #2 10-12-2009, 03:26 PM

Your speedometer could be slightly off.

Differential idenfitication can be seen on the lower left corner of the housing. There's a number stamped onto the metal. 2.xx or 3.xx

Mine is 3.07:

I think the number you're looking for on the differential should be 3.46.

rrgrassi · #3 10-12-2009, 03:27 PM

Lower left hand side of the differential, not the rear cover should have the ratio stamped in it. You will need to clean the old oil and dirt build up off of that area. MBeige posted great pictures of the marking.

Sev · #4 10-12-2009, 09:39 PM

Quote:

Originally Posted by MBeige

Your speedometer could be slightly off.

Differential idenfitication can be seen on the lower left corner of the housing. There's a number stamped onto the metal. 2.xx or 3.xx

Mine is 3.07:

I think the number you're looking for on the differential should be 3.46.

weird. 3.07 is also the gear ratio of a euro 190e 2.3-16 diff. you sure that's original w123? actually on my 16v diff the number's stamped on the back cover. so, who knows maybe MB used the same guts for the 16v diff as on the w123 with a different housing. is yours an LSD?

rs899 · #5 10-13-2009, 07:17 AM

3.07 is the normal factory ratio for all the OM617 turbo cars from 1978 to 1984.

Stretch · #6 10-14-2009, 09:46 AM

Thank you all for responding.

As it has just stopped raining I've been crawling about under the car trying to find some numbers printed on the outside of the differential casing.

The only ones I can find are on the cover. I have made a photograph of my findings but they don't seem to quite fit the format you are all suggesting.

The numbers are 115 351 32 08

Does this mean I have a 2:08 differential fitted to the car?

The car is a 1981 5 cylinder 300D European specification vehicle that was originally sold in Switzerland. It does have a four speed automatic gear box but it is not a turbo model.

Can anyone confirm that I've got the correct set of numbers?

Or do I need to keep on looking?

UriahT · #7 10-14-2009, 10:16 AM

Keep on looking.
There was never a 2.08 that fit these cars.
I really imagine your vehicle has a standard 3.46, unless changed by a previous owner to 3.07. Any other size and you'd feel real off driving it.
Speedometer is more than likely at fault.

Stretch · #8 10-14-2009, 10:19 AM

I've just had a nasty thought that I was being daft - looking at the casing shape on the photograph kindly supplied by MBeige it looks a little bit like the left hand side of the differential casing in my photograph. So I have just checked my car again.

This bit of the differential does have a machined face but there are no numbers stamped on it.

I've also rather hopefully put the number I have found into the peachparts part finder - but got no joy. My trusty Haynes manual (sorry I hope that isn't a swear word here) has also proved itself to be nothing more than emergency toilet paper for help in this matter...

Stretch · #9 10-14-2009, 01:45 PM

I've been doing some more investigative work. According to Wikipedia the W115 series made before the W123 series also had a 5 cylinder engine. So I now have a slightly modified question:- would the numbers that I found be a part number for a W115? => 115 351 32 08

It seems quite likely to me as the "friendly dealer", from whom I bought the car, showed me all the new pipes he had fitted at the back of the car! Are the drive train parts for a W115 easy to stick onto a W123?

If does indeed turn out to be a W115 part would I be experiencing the 7% discrepancy in speed that I am experiencing or should I indeed be looking at the speedometer?

As a point of interest at highway / motorway speeds I'm scared to go any faster than an indicated 85mph (about 140 kph). I don't know what the engine revs are at this speed as I have no tachometer but it just feels like that is the safest speed to expect from the car although I have a feeling I could "do a little more"...

DeliveryValve · #10 10-14-2009, 02:04 PM

The casted number "115 351 32 08" is the part number of the rear cover of the diff.
The casing of the differential will also have a casted part number that will begin with 115. These parts are interchangeable with various MB models. What is important is that the ratio and speedometer is matched. For your vehicle the correct ratio should 3.46.

As MBiege posted earlier on the close up, here is a pic a bit further back of where the ratio will be.

It will be stamped, not a raised cast marking. It will also be covered in paint, grease, dirt or rust. So you'll need to clean the area to see it properly.

.

MBeige · #11 10-14-2009, 03:29 PM

In your image, right below the differential mount, beside the last bottom screw is where the stamped number should be. Looks like it's all crudded up.

snookwhaler · #12 10-14-2009, 05:26 PM

Dude....... The numbers are on there in your photo. Look where everyone is telling you.

I can see the numbers....I can almost make it out. I see a "2".

Get the picture from the other side of the diff. You took the picture from the passengers side. Get a closer shot of the diff. "from the drivers side" and I can probably tell you what it is.

Wipe the tab off with a wire brush and a rag before you take the pic.

Stretch · #13 10-15-2009, 05:59 AM

Thank you all very much for your help.
After some scrubbing there is indeed a 3.46 marking where you all said it would be.
My faith is restored in the "friendly dealer"!

(Thanks also to Snookwhaler for calling me "Dude" - it makes me feel young again!)

Here is the proof:-