Curtis - '97 Targa

This may be a rehash, but I searched and didn't see a definitve answer....

I've read several descriptions of the 993 that made reference to a locking differential. Usually the description includes something like "25% acceleration, 40% decelleration". Is this the same thing as a Limited Slip(LSD)?

I'm pretty sure my '97 US model has the G50/20 transmission. Does this transmission have any "lock" or is it full open?

Do any 993's have LSD's without ABD?

When my car is raised, each rear wheel will turn independantly and I'm trying to figure out if I have a broken LSD or never had one...

Thanks

Steve Weiner-Rennsport Systems

Good morning Curtis:

FWIW,...a locking differential, at least in Porsche parlance, means the same thing as an LSD (limited slip differential).

You may confirm whether your transmission has that option by watching which direction the rear wheels turn when you rotate one of them with the rear wheels off the ground.

If they turn the same direction, you have an LSD, if they rotate the opposite direction, then you have an open differential.

The only real way to know if an LSD is broken or not is by inspection. You can test its functionality by torque testing between the flanges with the axles dropped.

Hope this helps,

Curtis - '97 Targa

Thanks, Steve.

When my car is raised and spin one wheel, the other doesn't move at all.

I guess my question was more oriented as to whether I ever had LSD or not. Several of the model descriptions that I read indicated that it was on ALL 993's.

I don't have a specific option code on my car that specifies LSD so I just have whatever came on the base car.

So, do I have something broken/worn out or is it behaving correctly?

Curtis

tom_993

Steve,
Far be it from me to question you, but here goes. I hope I’m not digging my own grave. If I’m wrong, someone can take a star away from me.

I’m not sure the statement “If they turn the same direction, you have an LSD, if they rotate the opposite direction, then you have an open differential” is correct. I thought that under these conditions (car jacked up, trans in neutral) an LSD will behave identically to an open diff. You need to do that torque testing you mention, to put some forces on the LSD, to cause it to begin locking up. Correct me if I’m wrong, but I tried this test on my car (with LSD) and the wheels spun in the opposite direction. Later, when I had my clutch replaced, I had my mechanic check the LSD (as that would have been the time to fix it!). The torque test (described in the service manual) on the flanges showed it was working fine.

Curtis,
I don’t know why one wheel doesn’t spin at all. Sounds like something is broken.

All 993’s did not have LSD’s. You can determine if your car has an LSD two ways. One, check the option codes. Two, crawl under the trans and look for the number stamped on it. If I’m not mistaken, a G50/20 has an open diff, while a G50/21 has an LSD. Unless a previous owner installed an LSD into your G50/20, this stamped number will be the final answer.

Tom

Viken

Not quite. The G50/20 is a US gearbox while a G50/21 is a ROW gearbox to 1997. Here's how to tell by looking at the gearbox:

G50/20-1 No LSD
G50/20-2 Yes LSD

On the option sticker, you should look for both 220 (LSD) and 224 (ABD). They were only available together.

Curtis - '97 Targa

Viken,
Thanks for the info. So, No ABD=No Factory LSD. That is simple enough.

Do you know what SHOULD happen when you spin a rear wheel with no LSD? Should the other wheel spin either direction or not spin at all?

Curtis

tom_993

Thanks for the correction, Viken. My bad. My memory did not serve me well today!

Curtis, on an open diff, spinning one wheel will cause the other to spin in the opposite direction. If one wheel is on the ground, and you spin the raised wheel, the input shaft will spin while the wheel on the ground remains stationary. Since this is what is happening to you with both wheels raised, I'd guess that something is holding that wheel in place, or at least providing more friction than the input shaft. The obvious thing would be sticking brakes (either disc or e-brake), but it could be other stuff as well. Try putting the trans in gear (rather than neutral). This will hold the input shaft still. See what happens when you spin one wheel in this case.

c4-toy

Ok, now to throw a monkey wrench into this post - I am still not clear as to the difference between PSM on 996's and ABD/LSD that came standard on my 96 993 C4. The only thing that i can see being different is the ABD only works at under 44mph, which to me is useless at that low speed. Seems to me you would need it most at higher speeds. And by the way, i have not seen it (ABD) activate on my 96 C4 Cab (there is supposed to be a warning light that lights up when ABD is activated).
Thx for any info. Joe B 96 C4 Cab

tom_993

Joe,
There’s a big, no make that huge, difference between PSM and LSD/ABD. PSM is a complicated computer program that takes as input multiple sensors from the car. The sensors include each individual wheel speed, yaw angle of the car, steering wheel, throttle and brake positions, etc. The program can sense if the car is understeering or oversteering. The computer then controls engine power and braking on an individual wheel basis. It can apply brakes to just one wheel to correct for the problem, and it can do it much quicker and more accurate than a human (at least us mortal ones) can.

LSD is a limited slip differential. It’s entirely passive. You know how when you go around a turn, the outer wheel goes further? Well, the differential is what accommodates this. A limited slip differential is a special kind of differential that limits this difference between the two wheels. If one wheel starts spinning too much (more than can be accounted for from a turn), the differential starts locking up, so that both wheels get torque, rather than the one spinning. There are plenty of discussions about differentials out there on the net, here’s one: Discussion on Differentials

ABD is “automatic braking differential.” It’s a bit more active than LSD, but not nearly so as PSM. It has a sensor that detects rear wheel speed. It detects when one wheel is spinning too fast and applies brake to that one rear wheel.

Hope this helps.

Tom

c4-toy

Tom, that was an excellent explanation on PSM vs ABD/LSD. You answered my question, Thx very much. Joe B

Steve Weiner-Rennsport Systems

Hi Tom:

Good point,...I too, have heard of this in some cars with asymetrical or steep lockup ramps and clutch pack preloads that were not very high. All of these Salisbury-type LSD's that I've tested personally have performed as stated.

There really is quite a range of torque-specs when setting these things up and checking them either in the car, or on the bench does show major variations as to what constitutes "in-spec" even at the same set of locking factors.

Although you have pointed out that this test might be not be absolute and the "last word", I'd surely recommend doing that along with confirming the tranny numbers.

If there is any doubt, the only sure way is to pull the transmission side cover and have a looksee,... Its not hard to do at all.

phil@tech9

Guys,

Just to correct an earlier post. If your car has an LSD (as viken pointed out-denoted by the 2 after the gearbox type number), and you raise both rear wheels off the ground, then they should both spin in the same direction. If the other wheel spins in the opposite direction, then the plates are worn out.

The diff is a plate differential and has preload, therefore both rear wheels are 'connected'.

tom_993

Glad I could help, Joe.

I thought of another way to sum this up. All the ABD or LSD can do is limit the spinning of one rear wheel while under power, kind of like all ABS can do is control wheel lockup while under braking. PSM can control the whole vehicle under all circumstances.