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Dear all

How do I know if the limited-slip differential in my car is working properly ?

I placed the car on an elevator and move (manually) the wheels. If I move the left hand wheel to the front direction, the right hand should move in the same direction or not ??

Thanks in advance

Gonzalo
 

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Interested too.
As far as I know: should be a 25% LSD
Means the difference of the turns of left and right need to differ by more than 25% until it locks and both weels turn same speed. So I don‘t think it will work to test by hand
 

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I watched a Wheeler Dealer episode on an 88 Mustang 5.0 and that is the technique that they used to determines that the LSD was not working.
 

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The basic 105/115 chassis drive train is very robust. My wife and I have actively autocrossed our '76 Spider since we bought it in 1983 and probably averaged 10 event days or more each year. Every autocross start is essentially a drag race launch. And, with our car we have always used the stickiest autocross tires available; those with wear ratings of 50 or Zero (Currently Hoosier A7 275/35x15). These tires provide sufficient grip to have broken my aftermarket panhard bar mounts and T-Arm substitute 3 times, yet I have had zero problems with the clutch, Giubo, or differential (which has the two additional friction plates for increased grip). I did replace the drive shaft support bearing and U-joints once during our 37 years of ownership, but these are known wear items.

So, again, to check your LSD:
Car in 1st gear
Foot on clutch
4000 RPM
Foot off of clutch quickly then WOT (Wide Open Throttle)
Check for two black stripes on the road (This also satisfies your inner teenager)

.seems kinda hard on the drivetrain, but effective;);)
 
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There are three ways to test the functioning of the LSD. The first, and easiest, is to jack the rear of the car. With the transmission in neutral, turn one of the rear wheels by hand. If the other wheel turns in the same direction (with respect to the road surface), it indicates that you have a LSD that is probably working to some degree. This test is not 100%, and a very worn LSD will test like an open differential in that the wheels will turn in opposite directions: lack of break-away torque.

The second test requires removing the LSD from the rear end (or the rear end from the car), clamping one output shaft / flange, and measuring the torque needed to turn the other output. This is the break-away torque. I cannot find a spec in the Alfa literature I have. For other ZF LSDs, the spec ranges from 10 to 60 Nm, for example a spec of 10 to 35 Nm would be considered healthy in some versions. With Alfa's rear end design, this is a harder test to set up than in cars with simpler rear end designs. (Sorry that the following images are not more helpful. I took them for my own records, not for show-and-tell.)

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1630773


The third, and best test is to remove the LSD from the rear end, disassemble it, inspect condition of all parts especially clutch discs, rebuild, and measure break-away torque. This is really the only thorough way to test and ensure proper function of the differential. Good luck!
 

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Richard Jemison
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You do not have to do any of the above.

Simply jack up the right rear wheel. Put the trans in any gear to keep the drive shaft from turning.
First turn the wheel by hand slowly. With no torque load the wheel should turn fairly easily.
Then do a hard/fast "jerk" attempt to turn the wheel. If it locks up your LSD is working.

Clearances change over time and use. In normal driving if you hear a clunk, it probably needs shimming to the proper clearances. These are mechanical devices and even well worn it will have some degree of torque transfer, but rarely will a factory 2 disk unit "lock-up".
Redline 75-90 NS is the lubricant to use in these differentials (and the Alfa transmissions).
 

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Richard is correct. The LSD doesn't activate until the "snap" he mentioned causes the little axle shafts on the spider gears to move up inclined - angled ramps and compress the fiction plates. This may sound like gobbledygook but is easier to understand when you disassemble
Blowup.JPG
the LSD unit.

You do not have to do any of the above.

Simply jack up the right rear wheel. Put the trans in any gear to keep the drive shaft from turning.
First turn the wheel by hand slowly. With no torque load the wheel should turn fairly easily.
Then do a hard/fast "jerk" attempt to turn the wheel. If it locks up your LSD is working.

Clearances change over time and use. In normal driving if you hear a clunk, it probably needs shimming to the proper clearances. These are mechanical devices and even well worn it will have some degree of torque transfer, but rarely will a factory 2 disk unit "lock-up".
Redline 75-90 NS is the lubricant to use in these differentials (and the Alfa transmissions).
 

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The LSD is always active. Its ability to transfer torque from the input (to the rear diff) shaft to the two axles is modulated by the clutch discs. The position of the cross-shafts "up" the ramps controls how much pressure is applied to the clutch discs by the pressure rings. It's not an on/off except when break-away occurs, that is, the limit of the locking is reached. When that happens, the unloaded side (wheel) will spin.
 

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But Mad North-Northwest
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The LSD is always active.
Not really. There is no clutch preload in a stock Alfa LSD. It acts as an open diff until different speeds on the outside wheels generate enough force to drive the clutches into contact.

If you stick the rear end in the air and spin one wheel the other side will spin in the opposite direction just like an open diff. You can't really tell anything about the LSD condition this way because you can't manually apply enough differential force by hand to get the clutches to engage.
 
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