After reading many, many archived posts about LSDs and how the stock Porsche units are prone to wearing out, I wanted to evaluate mine before changing the gear oil in them. I have the G6420 2 transmission (this denotes AWD and LSD). This is the behaviour I have with all 4 wheels in the air:

1. In neutral, if I spin a rear tire, the other one doesn't turn.

2. In gear, if I spin a rear tire, the other turns the opposite way.

3. When I spin the rear neither of the front wheels turn.

4. When I spin a front wheel, the other front wheels turns opposite direction.

5. When I run the car in gear all four wheels turn, although left side turns faster than right side.

My conclusions are that numbers 1 and 2 indicate that I do have LSD but it may be at least somewhat worn because in number 1 the other wheel really should turn the same way. However, number 5 gives me hope that all is OK. Number 3 gave me concern that the VC wasn't working, but number 5 again gave me hope that all is well with the VC. Number three was as expected for an open front diff.

What does this say to you about the state of my AWD system. To test the rear LSD, do I REALLY need to remove the shafts, (or the unit) and torque test it.

thanks,
Paul

 

Paul,

I believe all the AWD cars (C4, C4S, and Turbo) came with LSD standard.

I can't remember exactly, but believe that if I turned a front wheel it should turn a rear wheel(s) too. I can't remember, but was playing with this in the past and thought that was what I remembered.

I'm sure other's will post more accurate information for you.

-Skip

 

Skip,

My recollection is about the same as yours albeit reversed--turn a rear wheel and the front should turn.

 

Hi Paul
my way of testing is probably less technical, but try it in the snow; you'll immediatly see if both rear wheels are sliping. Thenwhen the front will start sliping too, then the ABD will come in action. It worked for me and I deducted that all was ok...

 

ABD only comes into action when one rear wheel slips at low speed. Best way to test ABD is to start from a stop in the rain with the wheel turned, so one wheel spins first. You'll hear the high-pitched whine in the rear from the ABS cycling.

 

Interesting idea. Shouldn't be too long before I have snow. I was hoping for something I could do in the garage though since I have taken driving insurance off the car for winter storage.

 

Yes, if any turn, they should all turn to some degree. The LSD should turn the wheels in the same, not opposite direction. If you put the car on jack stands on all 4 corners, start the car, put it in gear and see what happens. You can get the ABD to kick in as well and check everything.

 

How do you get ABD to kick in on jackstands?

 

Give the wheel a nice bear hug?

 

John, see point #5 in my original post. I tried that and all 4 wheels spin, albeit, the left side faster than the right. What does that tell me?

A buddy of mine suggested that with all 4 turning under idle power like that I should be able to stop either front wheel by hand since the front is an open diff, but I should not be able to do that in the rear if the LSD is working. Thoughts?

EDIT: Or is this idea just going to invoke ABD and not really test the LSD?

 

Paul,

With my car up and all four wheels free in neutral, turning the rear wheels would cause the fronts to turn. At idle in first all wheels would spin. If I stopped on of the fronts the other front would spin faster.

 

Minor brake stand (eccentially)
gas and brake at the same time.

 

I think you're only looking to reach a spinning torque spec of 15 - 37 ft-lbs and trying to measure that at the wheels (that are offset from the halfshafts) will totally throw your readings off. My 140k'ish (mostly highway miles) 964 LSD measured on a bench in the high teens to low twenties (i forget the exact number) using a Snap-On digital torque wrench.

from the workshop manuals,

"If a driven wheel starts to spin when the vehicle accelerates, this is sensed by speed sensors in the control unit and ABS control is started. The wheel speeds of both driven wheels are compared with each other and with the speeds of the non-driven wheels. If a specified wheel difference is exceeded, a brake pressure is built up at the spinning wheel. Adding residual drive torque and braking torque thus allows the wheel that does not spin to transmit a higher drive torque.

To ensure driving stability, the ABD system is switched off if any wheel spin tendency is sensed at the second driven wheel and the ABD control pressure in the brake is reduced. A wheel spin tendency of the second driven wheels means: Both driven wheels (rear wheels) rotate at a higher speed than the non-driven wheels (front wheels)."

 

Update:

After letting wheels spin in the air for a while and everything got warmed up I found that when I put it in neutral and turmed a rear wheel the other would turn slightly the opposite way. Its like it was grabbing just a bit, but this rotation is the wrong direction. I also found that with it in gear if I could force a rear rotor (with wheel removed) to stop. The other side kept spinning and also the ABD activation light would come on. If I continued to hold it the ABD malfunction and ABS lights came on. By these tests methinks the LSD is toast. I also tried lowering one rear to the ground and spinning the other by hand. In gear I could not turn it, in neutral I could turn it with same force as when all 4 were in the air. What does that say???

 

I put mine up in the air today to see what would happen.

In neutral:

1. If I spin a rear, the other turns opposite and the fronts turn the same direction as the rear I am spinning. There is a bit of slop between rear the and the front.

2. If I spin a front, the other turns opposite and the rears turn the same direction as the front I am spinning. There is a bit of slop between the front and the rear.

In gear:

1. If I spin a rear, the other turns opposite and the fronts don’t spin.

2. If I spin a front, the other turns opposite and the rears don’t spin.

At idle, all wheels spin about the same speed.

1. If I grab a front, the other turn faster but the rears stay the same.

2. If I grab a rear, I get a little kick and it starts spinning again.


Tom