1987 300 Turbo Diesel rear differential question.

[littlerobot]

I want to change the oil in the rear diff. I have done this in my 1984 300d, but that is w123, and this is W124, right?

The back of the rear differential has two allen head bolts, the same size and level to each other. The side has a smaller drain plug. Step one is to drain, so no problem there, but, step two: which of those two allen head bolts on the back do I remove to refill? Is that rear diff "compartmentalized" or something? Why are there two bolts instead of one?

I'm asking because I want to make sure those two bolts are indeed fill ports, that way the entire back end doesn't fall on me because I have released some crucial bolts that aren't drains ports at all, but structure bearing elements!

[babymog]

The fill/level plug is on the left side of the differential.

The common wisdom here is to loosen the fill plug before draining the fluid, to be sure it isn't seized.

[Jeremy5848]

I suspect the two rear Allen bolts that the OP sees are two of the three bolts that hold the differential to the rear subframe. [Do not remove these bolts.] The drain bolt is low on the right side of the diff and as Jeff says, the fill plug is high on the left side.

Jeremy

[jbrianfoto]

Yep - The fill is on the top. Take it our before you open the lowerone, it helps the draining process (like pokng two holes in an old oil can, one to let the air in, the other to let the oil out). I recently replced my rear diff in my 80' 300td. The original unit had a siezed fill plug. My replacment unit had a siezed drain plug. An 18" breaker bar fixed both in a jiffy. When I replaced the plugs I wrapped the threads with a few turns of teflon tape, to help prevent siezing.

[littlerobot]

Right you are, guys. The fill plug is on the driver's side of the rear differential, and the drain plug is on the passenger side. I'm glad I didn't attempt to remove the other two allen head bolts! Yet another reason this forum is great: safety.

Thanks everyone: the bolts came out easy with a breaker bar and changing the rear diff oil was a snap.

[scottmcphee]

So complete the story, how much oil came out?
And how much (of what) did you put in?

(I need to do mine, it's probably original!)

[littlerobot]

I put in Valvoline 75W-90 gear oil: about a quart of it, which is standard with most cars, but while doing so I noticed this differential is larger than my other 84' 300 TD. It may need a little more.

I let the old oil drain out slowly for a few hours just to get the most old oil out I could, so I'm not sure how much came out. I'd guess a little over a quart?

Important: make sure you put anti-seize on the allen bolt threads before you put them back in. This will make sure it's possible to get the bolts off the next time. I put anti-seize on every large bolt I take off and put back on the car.

Make sure you have a breaker bar, too. You'll have a hard time breaking them loose with just a socket wrench. Breaker bars are invaluable for working on these old cars....

[ashedd]

I have a large allen key wrench that was put on a bench grinder to fit the plugs. I have had to smack the thing with a small sledge hammer to get them off the first time. I imagine heat would get it off too.

[babymog]

I've never needed anything other than a 1/2" ratchet with a hex bit, but then I've never owned a 124 from north of KY.

[ashedd]

Originally Posted by babymog

I've never owned a 124 from north of KY.

Watch it.... mine is from Va.

[littlerobot]

I've worked on these old Mercs for a couple of years now, but the breaker bar has saved me some real headaches when removing caliper bolts, banjo bolts, and the large allen bolts. They are less than 20 dollars at Harbor Freight:

http://www.harborfreight.com/1-2-half-inch-drive-25-inch-breaker-bar-30395.html

I'm not a bodybuilder, so leverage is a good thing. I learned this from the "grizzled" veterans on the salvage yard. They have a lot of knowledge to purvey. Here's a few things I've learned:

1. Plan most of your work for the fall and spring, if you can.

2. Bring the right tools (the breaker bar falls into this one).

3. If you are going to be under a car at the yard, bring safety jacks. They prop the cars up so you can get under them, but don't trust that someone you've never met is going to do a good job. Bring your safety jacks: go home alive.

4. Penetrating oil ACTUALLY WORKS...

I live in Memphis. I love these old cars, and these knowledgeable "grizzled" guys rule (just like the folks on this forum): They'll make it easier for you, safer for you, and solve problems faster than I can.

Love it. Long live DIY, long live peachparts!

[Jeremy5848]

"Merz" not "Merc."