BMW E30 limited-slip differential and friction lock
How do I tell whether my E30 has a limited-slip differential?
The centre of a limited-slip differential is closed, whereas in a standard differential the differential gears are visible. You can also lift the rear off the ground and turn one wheel: if the wheels turn in opposite directions, there is no lock. The S and number at the end of the case serial number give the lock-up rate, e.g. S25 is 25 %.
What oil goes into the E30 friction limited-slip differential?
The recommended oil is Castrol SAF-XJ 75W-140, which is now sold as Syntrax LS 75W-140. BMW's original was Mobil VS500. The standard lock-up rate is 25 %. Friction-increasing additives such as Redline should be avoided, as they are reported to wear the friction plates.
How can I tell the friction lock is worn?
The friction plates wear and begin to slip: if the rear drives with only one wheel, the friction surfaces are the most likely cause. If the lock bangs and only one wheel drives, the differential is removed and opened. If the rear bangs and neither wheel turns, the fault is in the driveline and not in the lock or the oil.
What is a viscous lock and is it the same as a friction lock?
A viscous lock is a sealed housing containing plates and fluid. The fluid stiffens as it heats up, so the lock only grips once a wheel starts to slip, and more gently than a friction lock. The viscous lock is in the 325ix rear; in other E30 models the lock is a friction lock.