Hi.
Does it make sense that I have an M46 with a differential of 3.91? I'm wanting to change the fluid before winter and I don't know if it is a 1030 or 1031, open or LSD. The tag on the rear axle says 3.91, 1216144.
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__Stef; 81 DL wagon B21A SU carb M46 __313k

This may sound silly, but I thought the easiest way to tell if it's a LSD is if you spin the driveshaft while the car is up on a hoist, both rear wheels will turn the same way, if it's not an LSD, one will spin one way, and the other opposite....??

-Phil
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Phil Sinnott '81 245 630,000kms

Thats not true.

If you spin one wheel on a standard diff, the other wheel should counter rotate, if the driveshaft is held still.

As far as how a limited slip changes how the diffy acts up in the air, I am not sure.

that is what I thought too, but I guess I'll have to wait until snow to see for sure...
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__Stef; 81 DL wagon B21A SU carb M46 __313k

on a hoist if I spin one rear wheel the other spins too... does this carry meaning?
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__Stef; 81 DL wagon B21A SU carb M46 __313k

It should spin. It doesn't indcate a problem. With an open diff it will spin in the opposite direction. With the LS I believe it should spin in the same direction but don't quote me on that.
One warning I've seen here more than once (if you do have a LS) is not to spin one rear wheel while the other is still on the ground. But you're not doing that anyway.
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'80 DL 2 dr

It makes perfect sense. With a low rear axle ratio you really need the o/d on the highway. '79 & '80 U.S. models all the manual trans cars came with a 3.91 axle. I'd go out on a limb and guess the same is true for '81.
Your differential is most likely not LS. I believe you can tell at a glance (well, maybe those who have seen a lot of both kinds can). The LSD's case is more boxy and the open diff's is more globular. If it is a limited slip there should be a tag identifying it as such.
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'80 DL 2 dr

Our 81 244GL has a 3.73 with the M46. Maybe wagons had the lower 3.91.
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Bob (son's 81-244GL B21F, dtr's 83-244DL B23F, 'my' 94-944 B230FD; plus grocery-getter Dodge minivan, hobbycar MGB, and numerous old motorcycles)

Okay. I know that by '84 they were using several different ratios depending on engine and trans. I wasn't sure when that started. Sounds like it was '81.
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'80 DL 2 dr

Thanks for confirming the limited slip diff. identification - it has a boxy housing. I had one on our first 240 - didn't know I did. Had the shop swap it out for another diff. when it got noisy due to problems with the lube; it wasn't maintained and became polluted. After the swap-out, I suspected the old had been lim. slip since the new one slipped much more easily in snow.

I've been sorry ever since. The job cost about $500, so even if I find another L.S. diff somewhere cheap, I'm not likely to pay or do all the work to swap it into my current car. They should have told me what I had.

Boy, it WAS nice in snow!

Wish I could see a picture of them. Mine is round at the back and boxy towards the front (boxy part has fins along it).


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diff od/gearbox
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__Stef; 81 DL wagon B21A SU carb M46 __313k

As I recall, I only looked at mine (limited slip) from the rear. It looked like a square or rectangle with slightly rounded corners.

I'd guess that 75% or more of the 240's & 740's on the road have standard differentials. You could look at a few. If yours looks the same, then I think it's not limited slip. I've been noticing the shape of differentials ever since, and there was a clear difference looking from the rear. Also I haven't seen any more of the boxy ones!

My manual ('86) stated that you need special lube for the limited slip version. Don't know if that would be different from regular rear end oil, or what. Be sure toet the details if you have or get one.

Hope this helps.

If you do indeed have a LS then you must add "friction modifier" to the rear-end. Otherwise, it uses regular hypoid differential oil. An exception is if you have a torsen or quaife torque biasing LS in which case no additive is required at all.
The appropriate "friction modifier" is available at most auto parts places in little tubes.

Mike!

My description may not be the best thing to go by. What you describe sounds like a standard diff. The LS ones are actually pretty rare (although aftermarket are available if you want to spend some bucks). That doesn't mean one couldn't have been installed by a previous owner. But it is most likely a standard diff.


From the archives-
http://brickboard.com/ARCHIVES/2001JUL/20168596.shtml
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'80 DL 2 dr