Gonna change my dif fluid today, question?

[JCSC2]

Hey guys, I am going to pop the top bolt of my differential and check it and change it today. I have a lot of "whine" sound coming from the rear. I have had it on jackstands and have gotten reasurace from several people that it is not wheel bearings. The sound persisit whether in drive or in neutral. It gets louder with speed. Anyway, me not having a lot of money has decided I should change the fluid first all. I know the fluid was replaced about 5 years ago. Anyway my question is, should I use synthetic. I know everyone has gone over this a lot. But I have a boot that is cracked pretty bad but not leaking. Basically I would hate for synthetic in some way to cause the boot to fail quicker b/c of it's cleaning properties. I basically just want to avoid any problems that I could prevent. I am okay with putting in regular gear oil but if the general consensus is I should use synthetic than that's what I want to do. I do not run any other synthetic fluids by the way.
Thank-you

[RUN-EM]

Stays in the differential. It does not migrate into the constant velocity joints. There are seals on the extremeties of the differential to hold in the oil, yet allow the half shaft to enter & engage the gears within. The boot crack, if it does fail, should only affect the c/v joint that it is around.

Regards

Run-Em

The boots are not part of the differential, they have their own oil.

Synthetic is great and might reduce that whine.

[JCSC2]

Thank-you for the replies, I really appreciate it.
One more question. I see a lot of members on here recomending 75w 90. However I notice the manual says to use 85w 90 or SAE90. Which should I use? Also, the manual I have is for an 84 300SD and mine is an 85. Probably no difference but thought I should add that. Thanks!

[Bio300TDTdriver]

Remove the filler bolt before you remove the drain bolt. Mine came out easy, but you don't want to drain out all the oil and then not get the new oil in.

[JCSC2]

Originally Posted by Bio300TDTdriver

Remove the filler bolt before you remove the drain bolt. Mine came out easy, but you don't want to drain out all the oil and then not get the new oil in.

Thanks for the tip. Certainly don't wanna drain it all out and then can't refill.

[bgkast]

Originally Posted by JCSC2

Hey guys, I am going to pop the top bolt of my differential and check it and change it today. I have a lot of "whine" sound coming from the rear. I have had it on jackstands and have gotten reasurace from several people that it is not wheel bearings. The sound persisit whether in drive or in neutral. It gets louder with speed. Anyway, me not having a lot of money has decided I should change the fluid first all. I know the fluid was replaced about 5 years ago. Anyway my question is, should I use synthetic. I know everyone has gone over this a lot. But I have a boot that is cracked pretty bad but not leaking. Basically I would hate for synthetic in some way to cause the boot to fail quicker b/c of it's cleaning properties. I basically just want to avoid any problems that I could prevent. I am okay with putting in regular gear oil but if the general consensus is I should use synthetic than that's what I want to do. I do not run any other synthetic fluids by the way.
Thank-you

I hate to break it to you, but if it is whining in neutral the diff fluid change is not going to do squat because the differential is not doing anything if the car is not moving. My '82's auto transmission makes a whining noise that sounds sort of like a supercharger. It is loudest when the engine is revved in N or P, but can be heard when accelerating and driving too. I believe that it is the hydraulic pump, but it drives fine so I am just leaving it alone for now.

[nhdoc]

I've always used Mobil 1 synthetic which I think is 75W90 and it has been fine in all my M-B's. Like another post said, just remove the fill plug first to make sure you can refill after draining it and you will be good to go.

[nhdoc]

Originally Posted by bgkast

I hate to break it to you, but if it is whining in neutral the diff fluid change is not going to do squat because the differential is not doing anything if the car is not moving. My '82's auto transmission makes a whining noise that sounds sort of like a supercharger. It is loudest when the engine is revved in N or P, but can be heard when accelerating and driving too. I believe that it is the hydraulic pump, but it drives fine so I am just leaving it alone for now.

I think he means it whines when moving but in Neutral which could be the diff. There's no guarantee that a fluid change will help but it's like chicken soup...it couldn't hurt.

[Lexxani]

When I did mine I used 90 weight Lucas, worked like a charm, kept most of the whine down (there wasent a lot to begin with). There is still a slight sound if you listen carefully, mabye im just a perfectionist.

Make sure you got the right allen sockets to do the job and I also found using an oil transfer pump (looks kinda like a syringe or a grease gun with the plunger on the end) helped me immensly as you can not get a bottle of oil up in that location. You might also want to clean the dirt that accumulates on the diff, thoes cooling fins are suppose to serve a puropose.

Fill it up till it spills out, cap it up, go for a drive.

[bgkast]

Originally Posted by nhdoc

I think he means it whines when moving but in Neutral which could be the diff. There's no guarantee that a fluid change will help but it's like chicken soup...it couldn't hurt.

True. Mine could use a change, but I am going to swap a different ratio in soon so I am putting it off.

My new diff has an inch or so tall bolt on the top to the driver's side of the mount. Anybody know what that is? I have not looked at the top on many MB diffs .

[JCSC2]

Originally Posted by bgkast

True. Mine could use a change, but I am going to swap a different ratio in soon so I am putting it off.

My new diff has an inch or so tall bolt on the top to the driver's side of the mount. Anybody know what that is? I have not looked at the top on many MB diffs .

That is correct. It makes noise while the car is moving down the road in drive or in neutral. When sitting still all you hear is the sweet sound of "clack, clack, clack" from the motor!(LOL)

[bgkast]

That's good. Good luck with the fluid change.

[vstech]

... if the oil change does not quiet it down, try putting a cup or two of sawdust in there... if it then quiets up, you have some serious wear on the gears and you will be needing a new pumpkin... they are plentiful, so not to worry. I hate to break it to you, but if your oil was changed 5 years ago, and it has not leaked out, changing it again probly ain't gonna fix it...

[bgkast]

Sawdust?!