[Cloud9...68]

My car has a Guard G50 torque biasing differential, which was purchased by the previous owner on 6/19/2003. In a recent inspection of my car, the mechanic (it was done at a highly reputable shop that works on nothing but Porsches) said it appears weak. So, I have a few questions:



1) What's the best way to test this particular unit? I know the traditional way to test for the operation on a limited slip diff is to jack the back of the car up, put it in neutral, make sure the hand brake is off, and turn one of the wheels. If the other wheel spins in the same direction, you have a correctly functioning LSD. If it spins in the opposite direction, you don't. Is this the best method to test my particular unit? The mechanic who ran the test said my diff appears weak - it didn't take too much effort for somebody to stop the opposite to wheel from spinning forward when he spun the other wheel. Is this a valid conclusion for this particular type of diff?



2) I don't know a whole lot about how LSD's operate, but from reading through a few threads here, I got the impression the Guard does not use clutches. And yet, the mechanic who did the test said that most likely the clutches were worn (and could potentially be replaced). So, I'm wondering if he was mistaken about the type of diff I have, and that (fingers crossed here) maybe the test he ran wasn't valid. So, for my differential's particular design, what's the best way to test if it's operating correctly?



3) I know I'll have to discuss this with Guard, but once I determine what, if anything, is wrong with mine, what's the moist likely course of action? Is it likely to be repairable, or would it have to be replaced?



One thing I do know - I can give it a fair amount of gas just past mid-corner, and the car really hunkers down and generates gobs of grip - it's one of my favorite things about the way the car drives. So, needles to say, I was shocked to hear the torque-biasing differential may not be working correctly.



Thanks.

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[Scott Collins]

Cloud,



The Guard GT torque-biasing diff (TBD) has no clutches or plates that need to be serviced. It is not a "LSD", known as limited slip differential. That's about another $1,000, depending of the make and model, and it will provide lockup in more situations, for instance, some will lock on acceleration and deceleration. Some can be adjusted to regulate the amount of slip that is limited, and how much lockup you will get on accel or decel.



Anyway, the Guard unit is very good at what it is designed to do, in my opinion, and that is why you might not even notice it. It is not adjustable, the way some LSD units are. The only way to find out if it is not functioning as designed, unfortunately, is to pull it out and look at the internals.



Here are some points:



1. In a straight line, The Guard TBD will provide power to both wheels, 50/50. (Do a burnout and go back and check the pavement - if you have the Guard TBD, and it is working properly, each rear tire will lay rubber, if you can break them loose.... An open differential such as what comes standard in all 968's will provide power to only one wheel, and you will see only one patch of rubber. Mine was the right rear, before I had the Guard torque biasing unit installed.)



2. In a zero traction situation, where one wheel is completely off the ground, it will perform like an open differential, and the wheel that is up will spin, while the wheel that is on the ground will have no traction. A LSD will limit the slip of the wheel that is in the air, and provide power to the wheel that is on the ground.



In a partial traction situation, like moving through a turn, the Guard unit will "sense" that the inside tire is getting loose and bias power between both wheels.



Here's a link to an article that compares two torque biasing differentials:



http://www.planet-9.com/reviews/cayman-p...rison.html



What I have stated above is all from my experience from having the Guard GT ATB installed in my car a few years ago. Any Forums members are invited to modify or correct any info above that is incorrect. I hope that this helps you sort this all out. Best of luck!



Scott

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[Cloud9...68]

Scott,



Thanks for all the info - you sound like you've really done your homework. But based on your understanding, do you think the Guard unit should be able to pass the classical test for an LSD, namely having both wheels spin in the same direction when the tester spins one of the wheels, when both rear tires are lifted off the ground, and the car is in neutral? When the mechanic did this test, both wheels did spin in the same direction, but when somebody else applied resistance to the other wheel, it stopped spinning, if I remember what he said correctly. He interpreted this results as a weak LSD. So my ultimate question is: does this really indicate my unit is bad? It sure seems to work well when I stomp on it coming out of a corner...



Another question: The mechanic also said that whoever installed the Guard may not have used the correct type of fluid for an LSD, and that this could have contributed to its premature demise. But again, he was thinking it was a clutch type design. Does the Guard type diff require a special type of fluid?



I guess I'll jack the rear of my car up and run my own test, and share the results here, and with Guard. It just seems very odd that my unit, which has such a reputation for trouble-free operation, should fail, though of course that is certainly a possibility. Thanks.

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[flash]

the guards unit will spin tires the same direction when the car is in the air - it slips a bit, but it does turn them both the same way - indeed, it takes extremely little resistance to stop that - that is NORMAL - mine is and has been the same way since it was brand new, works perfectly, and if somebody told me it was slipping, i would slap them

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[Cloud9...68]

Flash,



You've just made my day; this is what I had suspected. I think the guy who inspected my car, while he seemed very knowledgeable about it overall, missed the boat on this one. He obviously didn't know what type of differential is was, since he thought it was a clutch-type unit. And his conclusion that it's weak is completely contrary to the way it performs under power coming out of a turn. Plus my rear tires have always exhibited utterly identical wear patterns; none of the issue of the right one wearing much faster than the left that I've heard is common with these cars without LSD. OK, it looks like I have one less thing to worry about; things are looking up.

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[flash]

no worries

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[firefish]

The same goes for the stock torsen lsd which to my knowledge was the only available option in 93, 95 and 95 (in 92 you could get both). A knowledgeable Porsche mechanic should know this (unless he mistakingly thought that your retrofit was a clutch type). A m220 torsen has, when jacked, the opposite wheel turning the same way OR standing still, just like your guard.

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[Cloud9...68]

I contacted Guard yesterday, and they confirmed that the behavior of my car is normal. Spinning one wheel does cause the other wheel to spin in the same direction, just like an LSD, but it takes little effort to stop the other wheel from spinning.

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