Differential Detonation - 1984 300D - PeachParts Mercedes-Benz Forum

Bengoshi2000 · #1 02-08-2013, 11:19 AM

Greetings all,
Ok... It didn't "detonate" but I was just being dramatic to get your attention.

I Haven't been around in a while as "Khan" has been rolling with nary a problem for well over a year (now has 209k miles). This morning I started getting an odd knock from the rear on deceleration. Took it over to my indy, they put it up on the lift and we started listening around. Knock is clearly coming from the diffy. They drained the diffy oil and it was bubbly, milky, and contained plenty of metal shavings.

So I'm going to need a replacement differential (and I've confirmed I'm running the correct 3.07 gears). The gears had me concerned as the speedometer reads approx. 5 mph too fast.

My question: Would I see any benefit at all if I installed a 2.88 diffy?

Khan is my daily driver. My daily commute is approximately 60 miles round trip on country roads that are of average hilly-ness. (Is that even a word?) This constitutes 85% of my driving. The rest is a mix of around town and highway. Over the last 6 fill-ups, he's getting a solid 27 mpg.

I've searched here on the forum and have read several threads with hot'n-heavy debate over changing the diffy ratio. I think the conclusion has largely been that if you have a good differential, the cost of swapping isn't worth it.

But since I have to replace it anyway...

I don't want to kick off another debate thread. I just want honest, well-reasoned opinions.

And of course, if any of you have a good differential, I'd like to talk!

Peace,
B2k

charmalu · #2 02-13-2013, 12:14 AM

I was searching back a few pages and noticed no one answered you.

If you decide to up grade to a 2:88 Diff from a 3:07, then be sure to install the Spedo from the 85 300D. the Spedo and Diff are geared together.

There is about a 6% difference between the two Differentials.
There are more 3:07 Diff compared to the 2:88 as they were used for only one year. the 85 300SD also used that Diff, but the spedo will not fit/work in a W123. the W123 is mechanical driven and the w126 is Electric.

Charlie

t walgamuth · #3 02-13-2013, 06:00 AM

Changing to the taller 288 gears should offer a slight advantage in fuel economy as well as reducing engine noise on the highway. It is not enough that I would ever make the change unless I had a bad differential.

since yours is bad it would be a perfect time to do the change if you can find a diff and the parts. If change the gears but you don't change the speedo whatever speedo error you already have will be compounded by the additional 6% so your error would be more like 11% total, or 6 mph at 60 mph. So if your speedo reads 60 you would be doing 66.

cho · #4 02-13-2013, 07:23 AM

Quote:

Originally Posted by t walgamuth

Changing to the taller 288 gears should offer a slight advantage in fuel economy as well as reducing engine noise on the highway. It is not enough that I would ever make the change unless I had a bad differential.

since yours is bad it would be a perfect time to do the change if you can find a diff and the parts. If change the gears but you don't change the speedo whatever speedo error you already have will be compounded by the additional 6% so your error would be more like 11% total, or 6 mph at 60 mph. So if your speedo reads 60 you would be doing 66.

thats my math as I have 2.88 diff and 3.07 instrument cluster.
what about mileage (ODO) ?

.

t walgamuth · #5 02-13-2013, 08:50 AM

I believe the odo and speedo are tied together. That does not always mean they have the same error. One could be optomistic the other pessemistic etc. but if you change the diff it would affect the error of both the same, ie if it got a taller gear without matching the speedo, the speedo and odo would both become more pessemistic on their readings the same percentage.

vstech · #6 02-13-2013, 08:57 AM

well... if you keep your current speedometer and get the 2.88 diff, the 5mph too fast will balance out.
post the part number of your current speedometer... it may already be one from an 85...

Silber Adler · #7 02-13-2013, 09:45 AM

I assume that you had the differential refilled with new oil to get you by.
Does it sound any better? Water doesn't make for a good lubricant there.

andrewjtx · #8 02-13-2013, 10:19 AM

If you need a 3.04 from an '84, let me know. I have one sitting with 184k on it.

vstech · #9 02-13-2013, 10:23 AM

Quote:

Originally Posted by andrewjtx

If you need a 3.07 from an '84, let me know. I have one sitting with 184k on it.

I fixed it for you...

t walgamuth · #10 02-13-2013, 12:00 PM

Quote:

Originally Posted by vstech

well... if you keep your current speedometer and get the 2.88 diff, the 5mph too fast will balance out.
post the part number of your current speedometer... it may already be one from an 85...

You are right.

andrewjtx · #11 02-13-2013, 12:13 PM

Quote:

Originally Posted by vstech

I fixed it for you...

10-key epic fail... thanks.

cho · #12 02-13-2013, 12:44 PM

Quote:

Originally Posted by vstech

the 5mph too fast will balance out.

if I get this right
it can be compensated with proper tires
say ... 195/55/15 (5.3% on stock tire) ?

.

Bengoshi2000 · #13 02-13-2013, 04:53 PM

Thanks for the replies gents. The car is still at my indy. I'm not comfortable driving it. My indy spent well over an hour with it on the lift and he's convinced the diffy is bad, but he said in nearly 40 years of working on MB he's never heard one knock like this one. Refilling it with fresh oil made no difference in the noise.

He agreed that a 2.88 would be an improvement for the type/style of driving that I do. He also opined it would be a bit more difficult to find. So, he's looking for one, I'm looking for one... hopefully, one of us will locate a 2.88 before too much longer (because I hate being 43 and having to borrow my Mom's car...).

Thank you andrewjtx, I'll keep your offer on the short list if the 2.88 search drags out.

t walgamuth · #14 02-13-2013, 05:20 PM

Quote:

Originally Posted by cho

if I get this right
it can be compensated with proper tires
say ... 195/55/15 (5.3% on stock tire) ?

.

I have not done the math on your example but yes that is the concept.

Actually I think that would be smaller diameter than the stock tire, thus it would make the speedometer read more faster than it does with stock tires, increasing the error.

vstech · #15 02-14-2013, 09:51 AM

Quote:

Originally Posted by Bengoshi2000

Thanks for the replies gents. The car is still at my indy. I'm not comfortable driving it. My indy spent well over an hour with it on the lift and he's convinced the diffy is bad, but he said in nearly 40 years of working on MB he's never heard one knock like this one. Refilling it with fresh oil made no difference in the noise.

He agreed that a 2.88 would be an improvement for the type/style of driving that I do. He also opined it would be a bit more difficult to find. So, he's looking for one, I'm looking for one... hopefully, one of us will locate a 2.88 before too much longer (because I hate being 43 and having to borrow my Mom's car...).

Thank you andrewjtx, I'll keep your offer on the short list if the 2.88 search drags out.

um... I have two available...