S8 Tranny Flush question.
08-21-2008, 09:36 AM
#11
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The real problem changing tranny fluid ...
Many dealers don't follow the proper fill procedure, which requires close monitoring of tranny temp, and inevitably fill the tranny when it's too hot. The result is that a perfectly good transmission is damaged running with a low fluid level.
08-21-2008, 11:20 AM
#12
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Well got her done this morning, only took 7 quarts.
I presume from all of the reading that 7 quarts is perfectly normal.
Vag-Com regsitered exactely 39C when this occured. (figured 1/2 way between 35c and 45c.
This is 03 s8/face lift with the shallower pan.
Vag-Com regsitered exactely 39C when this occured. (figured 1/2 way between 35c and 45c.
This is 03 s8/face lift with the shallower pan.
08-21-2008, 11:57 AM
#14
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Just curious
How much does the transmission fluid expand in absolute terms after it reaches 39-40C? I'm not nearly as versed as others on this forum, just wondering how much difference a few degrees one way or the other affects the total ammount of fluid in the tranny (is it several ounces or a pint or a quart)?
Phil C
Phil C
08-21-2008, 01:12 PM
#15
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Re: Just curious
When I did it, as the temperature climbed, more fluid poured out. I do not know how much comes out per degree Celcius but having the vag-com was important. I can fully believe that if you go too hot, you may be running low on tranny fluid and you will have no way of knowing until it might be too late!
08-21-2008, 01:25 PM
#16
And how so does this really affect transmission life?
The question is raised,"how much fluid is too much, and how much too little"?
To much, and the fluid is picked up by the whirring bits inside the transmission and then entrains air and foams. Air entrained fluid pumps poorly, if at all. An over fill situation is not conducive to crisp performance, but there is probably lubrication enough not to cause problems. Surely all the moving parts get bathed in fluid.
How low a fluid level can be tolerated? Well, certainly not lower than the pick up tube, else the circulation pump goes wanting. We know there are both fluid lubrication circuits and fluid shift actuation circuits within the transmission. Both utilize the same fluid. ATF
Most of us are familiar with the sounds associated with pump cavitation due to a blocked fluid filter. Would low fluid levels also give an audible warning?
An additional risk of low fluid would be heat removal. There must be fluid enough to make it's way to any external cooler coils, fill the pump, and perform the functions of piston activation etc. Over heating of too small a volume of fluid could be a real threat. Oil does not tolerate high temperatures well. But at what level do things get critical? I doubt that even a liter deficiency (one of eight) would seriously effect transmission life or performance unless the service conditions were made miserible by high ambient temperatures, trailer towing, or overly spirited driving. Transmission abuse can take place under any fluid fill condition ;-) Fluid breakdown at high temperatures occurs at the surfaces of slipping clutches, but this process is made easier if the fluid arrives to this location already over temperature.
Perhaps the risk of poor service would be in the significantly underfilled condition that would happen if the fill process took place without the engine running and having the transmission cycled through the gears. All fluid passages should be filled all of the time, and THEN the fluid level be established.
I have no valid statistics regarding failure modes for our ZF 5HP24 transmissions. From this form, I have seen failed "baskets" and failed actuator "pistons". I have heard many complain of blocked filters. (myself included).
The "hunt and thump" is not acknowledged as a "failure" but does affect drivability, comfort and sophistication.
I don't see anything that suggests that these failure modes would be made worse by fluid changes. Or made better for the matter.
Additionally, I don't feel that the subtle fluid level changes due to small divergencies in the prescribed fluid temperature will result in catastrophic transmission failure.
It would be fun to fill a cold transmission to the overflow level, and then remove the plug a second time after running it for an hour to see how much oil was expelled. Hot fluid pouring over my hand is not high on my "fun list" however.
One must remember that the Audi Gods, in their wisdom, removed the transmission level dipstick because too many transmission failures were due to poor understanding of this indicator by the operators. Isuzu did this as well with the GM transmissions. Is "dip stickless" now standard for all automatics?
Lots to think about. Sorry if this bores you. I'll try to get over it.
Regards
Cal
To much, and the fluid is picked up by the whirring bits inside the transmission and then entrains air and foams. Air entrained fluid pumps poorly, if at all. An over fill situation is not conducive to crisp performance, but there is probably lubrication enough not to cause problems. Surely all the moving parts get bathed in fluid.
How low a fluid level can be tolerated? Well, certainly not lower than the pick up tube, else the circulation pump goes wanting. We know there are both fluid lubrication circuits and fluid shift actuation circuits within the transmission. Both utilize the same fluid. ATF
Most of us are familiar with the sounds associated with pump cavitation due to a blocked fluid filter. Would low fluid levels also give an audible warning?
An additional risk of low fluid would be heat removal. There must be fluid enough to make it's way to any external cooler coils, fill the pump, and perform the functions of piston activation etc. Over heating of too small a volume of fluid could be a real threat. Oil does not tolerate high temperatures well. But at what level do things get critical? I doubt that even a liter deficiency (one of eight) would seriously effect transmission life or performance unless the service conditions were made miserible by high ambient temperatures, trailer towing, or overly spirited driving. Transmission abuse can take place under any fluid fill condition ;-) Fluid breakdown at high temperatures occurs at the surfaces of slipping clutches, but this process is made easier if the fluid arrives to this location already over temperature.
Perhaps the risk of poor service would be in the significantly underfilled condition that would happen if the fill process took place without the engine running and having the transmission cycled through the gears. All fluid passages should be filled all of the time, and THEN the fluid level be established.
I have no valid statistics regarding failure modes for our ZF 5HP24 transmissions. From this form, I have seen failed "baskets" and failed actuator "pistons". I have heard many complain of blocked filters. (myself included).
The "hunt and thump" is not acknowledged as a "failure" but does affect drivability, comfort and sophistication.
I don't see anything that suggests that these failure modes would be made worse by fluid changes. Or made better for the matter.
Additionally, I don't feel that the subtle fluid level changes due to small divergencies in the prescribed fluid temperature will result in catastrophic transmission failure.
It would be fun to fill a cold transmission to the overflow level, and then remove the plug a second time after running it for an hour to see how much oil was expelled. Hot fluid pouring over my hand is not high on my "fun list" however.
One must remember that the Audi Gods, in their wisdom, removed the transmission level dipstick because too many transmission failures were due to poor understanding of this indicator by the operators. Isuzu did this as well with the GM transmissions. Is "dip stickless" now standard for all automatics?
Lots to think about. Sorry if this bores you. I'll try to get over it.
Regards
Cal
08-21-2008, 03:53 PM
#18
Low fluid level will not give warning.
We had a car waiting for a rebuild (it had a broken tranny cooler) I drove it around a block. The rpm was getting higher and higher but the speed was the same until it just stopped moving. The filter pickup pipe is 1/2" from the bottom of the pan at the deepest point. You can run the tranny out of fluid completely, at least 6-7Qt low without any noise from the tranny.
08-21-2008, 04:26 PM
#19
Fluid pressure fluctuation
Tozo
Have you any thoughts on the possibility of the destructive pressure fluctuations mentioned in the referenced article. The warning seems a bit sensational if the pick up tube remains submerged in atf. I can see pressure fluctuations if the pump were allowed to suck air.
pressure spikes could readily blow seal, diaphrams and even VB castings. ;-)
Oh these auto boxes are so complicated!
Cheers
Cal
Have you any thoughts on the possibility of the destructive pressure fluctuations mentioned in the referenced article. The warning seems a bit sensational if the pick up tube remains submerged in atf. I can see pressure fluctuations if the pump were allowed to suck air.
pressure spikes could readily blow seal, diaphrams and even VB castings. ;-)
Oh these auto boxes are so complicated!
Cheers
Cal
08-21-2008, 04:48 PM
#20
Yes, it can. 3 years ago the second gear in my tranny went out because of pressure fluctuation.
Audi designers thought they have to change the pan and the filter size every 6 months. The older filter will touch the bottom of a facelift tranny's pan. I always cut at least 1/2" off of the pickup pipe. There is no reason to be so close to the bottom of the pan. Tipical German engineering. The smallest dent on the pan will ruin the tranny. Maybe that's what they wanted.