On my 83 300SD. What is the best way to proceed when both the drain and fill plug are seized? It sounds like the cheapest by many hundreds of dollars would be to just drive until the diff fails and replace it with a pick and pull one (and repeat when needed), but I would prefer to do things properly if it doesn't require huge amounts of money.

Start with Aero Kroil on the fill plug. Does no good to drain it if you cant refill it.

Often times, a very slight tightening of such bolts/plugs helps break them loose.

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1985 300D 198K sold
1982 300D 202K
1989 300E 125K
1992 940T

"If you dont have time to do it safely, you dont have time to do it"

"The democracy will cease to exist when you take away from those who are willing to work and give to those who would not."

PB Blaster works pretty well too.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.

Here's the DIY from DieselGiant
Mercedes Rear Differential Fluid Change

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1984 300SD 300,000 miles--two tank WVO setup
2.88 diff & 500SEL anti-squat rear end

A little heat may break it loose..One of my cars had a stripped plug on the fill from a po.I ended up welding a large nut to it and it backed out like butter after that...

in addition to penetrating oil, I found a little heat from a propane torch & a few taps on the socket with a 2lb hammer before cranking on the breaker bar made the plugs come out without too much fuss.

good luck

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1991 300D

step 1. get a 14mm allen wrench... the bent steel type, not the socket type.

step 2. clean the seat of the drain plug really really well. surgically clean if possible. employ pics, brake cleaner spray, and rags... get it clean.

step 3. insert wrench and position it so it's nearly horizontal. HAMMER it deep into place.

step 4. set floor jack, or cylinder jack to lift the wrench and apply enough pressure to lift the car slightly off the ground, then jump on the bumper, or with the trunk open, plop you butt on the trunk edge.

if that does not do it, heat is the next best choice. pb blaster, and other similar penetrants are useless without heat, because it's a tapered thread that is impenetrable. just get a standard propane torch, and put heat on the plug. get it very hot, then let it cool. after it's cool spray some aero Kroil or PB Blaster on the plug, and let it soak into the space created by the heat. then removal of the fill plug should be a snap.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

I just did mine....got a stubby 14mm 1/2in socket....drain plug was a bit stubborn....I inserted the socket and put my legs on the wrench from behind the car and pushed the wrench....came loose very easily...

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A friendly amendment... make sure that your effort is going to open the drain plug and not make it tighter. If I recall correctly, on my w115 at least, that would mean that the allen wrench handle, for step 3, would be pointing forwards from the diff. Or if you prefer, the floor jack described in step 4 would have to be located in front on the diff, not behind it (where it would be wrongly located but most accessible).

Edit: Vstech pointed out below where the drain plug is on a w126. It is different than my w115. Sorry for any confusion.

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).

the fill plug on the 126 is in the rear cover plate... so the wrench handle should be pointing towards the passenger's tire.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

I use an air impact gun with a socket on it. Seems to work fairly well for me.

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1991 F250 super-cab 7.3 IDI. (rebuilt by me) Banks Sidewinder turbo, hydroboost brakes, new IP and injectors.
2003 S430 - 107K
1983 300SD - Tanoshii - mostly restored ~400K+.
1983 300SD - Good interior. Engine finally tamed ~250K.
Monark Nozzle Install Video - http://tinyurl.com/ptd2tge

Since the fill plug is on the rear cover plate, wouldn't an option be to remove the cover plate to change the oil. Kind of messy and you probably have to remove the diff mount, but less work than swapping out an entire diff. So if there was simply no way to get that plug removed, this is probably what I would do before I simply chose to drive the diff to the point of destruction. Not sure how feasible it would be on a w126 though.

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).

My guess is that you did not have enough of a lever. On these beauties I would never attempt loosening the differential plugs with anything less than a four foot bar. Usually a power bar with a piece of pipe slid over it.

A little heat is probably advisable. The good thing is we never have had a member come on site and say a segment of the rear cover broke out with the plug. These plugs can be extremely tight as the seal is metal to metal. A little anti seize on reinstall may not hurt for the next time. Or even Teflon tape.

While you are in there I would recommend removing the vent and cleaning it. It is screwed into the top of the differential.



-J

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.