differential button

  #1  
Old 08-01-2005, 06:47 PM
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Default differential button

just got a 1990 audi quattro 20v. there is a button on the console for the differential (change form fwd to awd) wondering if the car starts in awd or in fwd and when i press it that it changes and does it restart when the car goes off. dont have the owners manual. ne1 knows please tell.
thanks
 
  #2  
Old 08-02-2005, 10:11 PM
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Location: Swamplands of NJ
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Default RE: differential button

Audi’s don’t switch from 2WD to AWD like on some Sabaru’s and some other car makers. On Audi's all four wheels are pulling all the time. I think what you are referring to is the rear differential lock. It only works if you are going less than 15MPH, go any faster and it automatically disengages. There are very few situations that would require you to lock the rear differential. If you find yourself on ice this feature is very useful. Never ever use it on pavement! You run the risk of permanently damaging the AWD system. Engaging the differential lock also disables the ABS.
 
  #3  
Old 08-03-2005, 04:23 AM
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Default RE: differential button

Actually, most all wheel drives only power all 4 wheels in good traction situations.

Should. On most, if the front wheels have no traction, you will sit and spin, as that is the definition of differential. Or, vice-versa, the rears are on ice. Tractive power is split between rear and front. One axle spins, you sit still, or lock up and hope the other wheels are able to drive.

Most will lock up the center differential so that the front and rear axkes are locked, but if you have ice under 1 front wheel and 1 back wheel, you will still sit and spin.

You would need to have locking front, rear and center to have true AWD, or true 4 WD.

Ah, well.

Cheers,

George
 
  #4  
Old 08-03-2005, 02:49 PM
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Default RE: differential button

Most late model Audi’s use a Torsen or torque sensing center differential. The TT uses a form of viscous coupling as found on the VW Golf models and I don’t consider it as true Quattro system. According to Audi there is always some amount of torque going to all the wheels. The Torsen differential can split the driving torque anywhere from 25% to75% between front and rear axles. Under hard acceleration the rear wheels get 75%, under hard engine deceleration the front axle can end up with 75% of the torque. Normal coasting should give you close to 50/50 split. Audi went to the Torsen differential so as to eliminate the problem of ABS not working properly. It also eliminates the problem of having to lock the open center differential as was the case with 4000 and 5000 Quattro models. This especially a problem if you are constantly alternating between hard pavement and a slippery surface. The Torsen differential also compensates for the rotational differences when the car is traveling in a turn.

To full understand and appreciate the Torsen differential put the car on a lift and turn the drive shaft, then turn the wheels by itself, forward and backward! The Torsen system is fully mechanical and uses no computers to work. The older Audi V8 Quattro used two Torsen differentials one for the center and another for the rear eliminating the differential lock switch.
 
  #5  
Old 08-05-2005, 06:30 AM
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Default RE: differential button

I'll defer to you on the Torsen. Haven't the foggiest as to how well they work.

Seems, though, that back in the 80's they used the same ads for Chevy, etc, full time and all wheel drive. Quite possible the AUDI system is revolutionary.

Holding back on that.

Cheers,

George

Well, hell, the reason I am skeptical is that, while hunting one year, the guy ahead of us, with a "full-time" 4 wheel drive died on the hi-way home. I looked at it, found that all the components of the right front hub had sprung out somewhere back up the hi-way.

All wheel drive, it just sat and spun on the right front axle. Since it was late on a Sunday, all the places I called were either closed or did not have parts. I finally told the owner to try locking it into 4 wheel drive, and he made it home. The "torque sensing" all-wheel drive saw a free wheel, the busted front right, and applied all the torque to it. Result, dead vehicle. I am not sure that the AUDI will not do the same.
 
  #6  
Old 08-05-2005, 12:20 PM
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Default RE: differential button

The Torsen differential has been around for a long time and is used on many military vehicles like the Hummer. During the 80's many automakers tinkered with AWD systems. They used mostly viscous coupling system because they were cheap and they are still widely used today. There were also automatic 4x4 systems like the one used on the AMC Eagle. Others used buttons, switches and levers that required driver input. The 959 Porsche used a computer and electromechanical clutches to split the power and was probably one of the most advanced AWD systems. There are many other variations as well. Many of the 4X4 trucks that have full time systems have an open differential that can be locked for better traction for off road. On open differential system if you find that the rear wheel(s) are spinning apply the hand brake, this will transfer power to the front. If the front or one side is spinning, slight application of the brakes might help. I have a Mitsubishi Montero 4x4 and will say that the Audi system is well balanced and works remarkably well. During a few Nor’easters I literally used my Quattro as a plow and never got stuck. The truck will spin the wheels on occasion.

The Torsen differential* is a purely mechanical device; it has no electronics, clutches or viscous fluids.

The Torsen (from Torque Sensing) works as an open differential when the amount of torque going to each wheel is equal. As soon as one wheel starts to lose traction, the difference in torque causes the gears in the Torsen differential to bind together. The design of the gears in the differential determines the torque bias ratio. For instance, if a particular Torsen differential is designed with a 5:1 bias ratio, it is capable of applying up to five times more torque to the wheel that has good traction.

These devices are often used in high-performance all-wheel-drive vehicles. Like the viscous coupling, they are often used to transfer power between the front and rear wheels. In this application, the Torsen is superior to the viscous coupling because it transfers torque to the stable wheels before the actual slipping occurs.

However, if one set of wheels loses traction completely, the Torsen differential will be unable to supply any torque to the other set of wheels. The bias ratio determines how much torque can be transferred, and five times zero is zero.

*TORSEN is a registered trademark of Zexel Torsen, Inc.
 
  #7  
Old 08-05-2005, 04:43 PM
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Default RE: differential button

I had a 4x4 chevy PU with fulltime 4 wheel drive and blew a universal joint in the rear drive shaft. I just put it hi-lock (locking the transfer case) and drove home.

It took me about 5 weeks to find the time to work on it so I had a big front wheel drive vehicle for awhile.

Audi's are different. The diff lock just locks the rear up to about 15 MPH. It is to help you get out of snow banks and other gooey places.
 
  #8  
Old 08-06-2005, 01:33 AM
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Default RE: differential button

Offramp,

This is exactly the situation I described with the Chevy truck.

"However, if one set of wheels loses traction completely, the Torsen differential will be unable to supply any torque to the other set of wheels. The bias ratio determines how much torque can be transferred, and five times zero is zero."

The front right hub lost all its driving members, all the torque was sent to the free spinning axle. And, "5 times 0 is 0".

I wasn't sure if they repealed ALL the laws of mechanics, yet.

Cheers,

George
 
  #9  
Old 08-09-2005, 12:27 AM
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Default RE: differential button

I think we are really nitpicking here! I have gone through several Northeast winters and the Quattro worked flawlessly. I got through all the snow, sleet and slush without any problems, I never felt the wheels spin or had to engage the Diff-Lock. Blowing a hub or joint and being able to limp home is not something I am concerned about or would want to do. If you destroy a CV joint or a universal, there is a good chance that something is going to flop around violently and possibly causing even more damage. If I was concerned about traction on ice I would get a spare set of wheels with winter tires and metal studs. Fortunately I can get away from using studs, what ever frozen stuff does fall it usually melts the next day.

"5 times 0 is 0" In theory “yes” in practice “no”. Even if one of the axles is on ice there is enough internal friction and parasitic draw that you will never be at "0" but the resulting torque might not be enough to overcome what ever has you stuck. If one is stuck in a deep ditch or a ravine then even locking all the differentials is not going to get you out. I have seen 4x4's spinning all four wheels and the only direction they were heading was down!
 
  #10  
Old 08-09-2005, 12:41 AM
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Default RE: differential button

Yoshi, the way it was explained to me is this:

Leave the car in position 0 for normal driving. This position is front wheel drive, and will only spin one front tire in loose conditions.

Position 1 is all wheel drive and is what you would use in snow/mud conditions. People who race Audi's use this position for track use. Now it will spin one front and one rear in loose conditions.

Position 2 is like having a solid rear axle and should only be used when you are stuck. You would not want to use this on pavement, or even when turning. This will spin one front, and both rear in loose conditions.

Hope that is what you were after.

 

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