Limited Slip or Open Diff

Whether or not a BMW diff is a Limited Slip Differential (LSD) is typically indicated in one of three ways. It may have:
1) a large blue "S" stencilled on the top of the diff case, or
2) an S preceding the ratio printed on a sticker on the diff case, or
3) an S preceding the ratio stamped into a stainless steel tag atached to one of the upper rear cover bolts.

If none of those is present, turn the input flange and watch the axle flanges... if they turn in the same direction it's an LSD... if they turn in opposite directions it's an Open diff.

What ratio?

The diff ratio is typically stamped on a stainless steel tag attached to one of the upper rear cover bolts. If not, it's still pretty easy to determine.

If the diff is out of the car, place a mark at the 12 o'clock position on the input flange and on one of the axle flanges. Turn the axle flange in the clockwise direction ­ counting the revolutions is makes ­ until the input flange completes one full revolution. When the input flange is back at the 12 o'clock position the axle flange should have made MORE than three (3) full revolutions and the original 12 o'clock mark should now be at one of the positions shown in the drawing below, indicating the ratio.

If the diff is still in the car, it may be easier to mark the driveshaft at the 6 o'clock position and rotate one of the tires clockwise, counting the revolutions it makes, until the driveshaft makes one full revolution and returns to the 6 o'clock mark. The resulting mark on the tire will indicate the diff ratio.

WARNING: If performing this test while the diff is in the car, be sure both rear tires are off the ground, the car is supported properly, and the transmission is in neutral.

NOTE: If the driveshaft makes one full revolution before the axle flange (or tire) makes LESS than three full revolutions, the diff ratio is LOWER than 3.00, e.g. a 2.91.

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