the mysterious freezing MB diff plug - PeachParts Mercedes-Benz Forum

JHZR2 · #1 11-07-2003, 03:57 PM

Hi,

I have noticed an odd phenomenon which I do not see on the othewr differentials I work on, on my other cars. What I notice is that when I change the diff fluid in the 300D, afterwards I tighten both plugs to 44 ft-lbs. However, when I try to get the fill plug out the next time, it takes considerable effort, sometimes me pressing on my breaker bar with my legs to get it loose. I put my torque wrench on it today when trying to loosen it, and it takes way over 100 ft-lbs to get it loose, compared to the 44 that I tightened it to. What gives? Does diff oil (M1 is what I use) serve like a sort of threadlocker when it dries out?

Also, How sensitive is the diff to fluid level? I drained and filled it over 20k ago, so since I was doing the diff on my truck with m1, I figured Id check the MB. As soon as I got the plug out, fluid started gushing out. In actuality, my rear was slightly raised, so the plugs were at the highest point, yet a lot came out before I decided to not add and quickly plug it back before it looses more.
The thing is, I filled it correctly last time.

I had driven the car on the interstate about 45 minutes before I checked it, and it was about a 30mi drive. The diff was warm to the touch, but I figured it couldnt be warmer than if I did the job on a summer day, so it was no big deal. I cant believe the diff fluid expanded that much though.
Since I suppose the level didnt fall below the fill plug, am I still OK with regards to level? Does it make too much of a difference, even if the actual resultant level is slightly below the fill plug?

Can damage be done by the diff being filled slightly high?

Thanks,

JMH

#2 11-07-2003, 11:17 PM

i can't think of a reason why being slightly high would hurt anything. As for the plug, the same has happened to me both times I changed mine. Both times it took a 2ft breaker bar with a block of wood and a hammer to break it loose. Why? Friction, Duh!

(The real reason, I have no clue)

Ken300D · #3 11-08-2003, 10:24 AM

Welcome to the properties of aluminium (spelling is correct?)

There's a corrosion effect that I think causes this to happen. Should not cause any damage to do what is necesary to break it loose. I would hesitate to use anti-sieze because I don't want it to contaminate the gear oil.

The one thing you have to be VERY careful about is to make sure the 14mm hex is FULLY seated all the way down in the plug. If not fully seated you can easily strip the hex fitting, because the plug is pretty soft.

Ken300D

samiam4 · #4 11-08-2003, 12:49 PM

Guys,

It's static vs dynamic coeffiecent of friction!!! You're installing a steel plug into aluminum. Washer is AL which gives you a high friction-which keeps it from loosening.
Most bolts we use on aluminum parts (ie engine) have a steel washer. Steel on steel is much lower friction coeff.
Normal force X friction coeff= force required to overcome friction.

Michael

clarknova · #5 11-08-2003, 01:25 PM

we see this problem in the harbor all the time.
dissimilar metals.
steel and aluminium don't like each other and will corrode together if given a chance.
the salt air will also accelerate this process (galvanic action).
whenever we rivet or bolt anything to aluminium we have to use aluminium fasteners or stainless steel fasteners with an anti-corrosive barrier paste (some anti-seize compounds will help with this, but we usually use a product called Alum-Elast, which prevents the dissimilar metals from corroding together).

I probably wouldn't recommend the use of this stuff on the diff plug (fear of contamination), but that's my $.02 on the topic.

(and yes, we have a mouse in our pocket

)

samiam4 · #6 11-10-2003, 12:57 PM

Clarknova,

Now sure how much saltwater your diff is running through-but it should be zero! Yep galvanic corriosion is a problem- I'm an aircraft guy and we proseal all rivets.

To say it's aluminum on steel in the threads would be incorrect. The plug is coated with *something* so galvanic corrosion. Plus all of the plugs I've ever reinstalled had plenty of gear lube on the threads shouldn't be the issue- but fighting friction is still there!

Michael

adamb · #7 11-10-2003, 08:14 PM

Would using teflon tape be of any application here?

#8 11-10-2003, 09:19 PM

Some anti-seize might help?????

Louis.

samiam4 · #9 11-10-2003, 11:36 PM

Not a siezing issue-

If you want it to come off easier next time. Use a copper washer!

But really- MB didn't want it to work loose, so it was a choice. My edified guess.

Michael

Nate Stanley · #10 11-11-2003, 07:12 PM

FWIW, the best way I found to get the plug out is to:

1: back the car onto ramps.

2: use a standard l-shaped Allen wrench, stick the short end in ths plug with the long end about 30 degrees below horizontal, and use a small floor jack to exert force on the end of the wrench. At that point you have the weight of the back end of the car working in your favor, and it WILL come loose, corrosion or no corrosion.

Enjoy!