blown 87

I wonder how that can happen?

Maybe "flushing" is the wrong term.

What you are really doing is changing the fluid, all of it instead of some of it like when you just drop the pan.

All of the machines that I have seen simply put new fluid into the transmission via the cooler lines from a new fluid container and drop the used fluid onto a used container.

Greg N

BrianG

If you drop the pan and change the filter and fluid, sometimes you are left with a cooling system and torque converter full of old ATF. That could constitute as much as 50% of the total volume of ATF in the system.

In those situations, I drop the pan, replace the filter, refill the fluid, break the ATF cooler return line at the transmission and bleed it until fresh ATF runs out. (monitor the ATF level closely) Most Auto-boxes run the ATF out of the torque converter, and to the cooler before returning to the pan. This effectively "flushes" the transmission as well as getting the crud off the bottom of the pan and into the garbage can, as well as getting a new filter in the thing. Best of both worlds, with none of the risk!

Shark Attack

Sounds like he knows someting you don't, Like your trans is in a early stage of failure and he knows this and doesnt want to touch it for fear you will blame him for the failure because he was the last one to work on it? ( boy is that a run-on sentance if i ever saw one)

happens all the time to me in the computer business.

dr bob

Yep, flush the CPU and all hell will break loose. All your data gets scattered to bits, magnets that hold the letters onto the hard drive platters change polarity, there's just no end to what can be blamed on changing the ATF in a computer! ( boy is that a run-on sentance if i ever saw one)

docmirror

I am not a trans specialist, but I've heard and seen this scenario before too. The explaination I got was different, but the end effect was the same. If trans fluid is left in for a long time, it oxidizes, and gets that burnt smell. The plates in the auto trans build up some of this stuff on them, and work with the existing fluid okay. If the fluid is changed, the oxidation is gone, and the new fluid will start slipping on the plates in the trans. Aparently you can adjust the bands to make it work for a while(older auto boxes, prolly US types) but eventually the car will not pick up first or second gear, as the clutch plate within the trans just won't mate. It happened on a F350 truck I had. I don't know if the 928 trans could have this failure mode or not. The shop I took my truck to didn't mention anything about it, so after it failed, they didn't have anything to say. I got the fluid, pan cleaned, new filter, and blow out the coolers all at the same time. Ford C6 with 4x4 case. 928 may be way different.

Doc

Bill Ball

I believe Steve C does flushes, but I know there are some do's and don'ts. Hopefully he will see this thread and chime in.

Shark Attack

Ok smart ***.....(LOL) what i meant was. I go in to repair someones point of sale system.... i fix a printer, 2 days later the whole thing crashes from a power outage... the customer doesnt think that the power outage may have screwed them all up.. NOooooooo.. it must be the work the tech did 2 days ago......

Doug Hillary

Hi,
heinrich - are the aliens on the way here to OZ as well?

For what it's worth, I'm in the "don't flush" camp!

Over the years I have often seen problems occur soon after "flushing" has been done. Sadly many Shops forget about the filter or vehicle Owners don't want to pay the Labour to have it done. This has been with various vehicles/transmissions and in a variety of operating circumstances

Some believe that it disturbs areas where friction material and wear metals have safely accumulated and introduces this throughout the lubrication system again

IMHO the best way with a 928 is simply to drain the fluid, drop the pan and change the filter - draining the TC at the same time of course. If the fluid quickly changes colour again after refilling but the transmission operates well, a second change of fluid after a 1-2k miles will probably be all of the "flush" it needs

The Benz transmission we enjoy is robust and well filtered. It responds very well to a "Euro" specification Dexron 3 fluid

Certified ATFs are very complex indeed. They are the most complex of all fluids used in a motor vehicle as they perform a multitude of tasks such as cooling, cleaning, lubricating and acting as an hydraulic medium to just name a few
It is always important to use the specified ATF as many are especially formulated for specific applications

I prefer to use Castrol's Transmax Z (which is fully synthetic) in my 928. It is very expensive but it is one of the best ATFs ever made and it has held all of the relevant Euro Approvals (MB, Voith, ZF etc) for a decade or two. It will easily last for four years or 100k kms in normal use. I use it in some trans-axles and manual gearboxes too where an ATF is called for

Regards

heinrich

Remember though the original post said "flush" it really asked if it was OK to replace the fluid. I think it is never not OK to replace the fluid (and filter).

SharkSkin

That's what I call the "Coffee-pot Syndrome". I picked up the term from an old engineer that mentored me early on in my computer career. What it means is this: If you are performing field service in a customer's server room, whatever it may be -- adding a patch cable, dropping off a box of tapes, installing equipment -- and a coffee pot on the same floor breaks within a few weeks of your visit, it's your fault. You are now expected to fix the coffee pot.

Bill Ball

If you drain the TC, as you should, the amount that remains unchanged compared to a dry installation is about 2 quarts, IIRC.

0utlier

Just to clarify... When I said "flush" I meant dropping the pan, replacing the filter, filling with new ATF. Not just replacing the fluid. I apologize for not being more clear.

heinrich

We're getting into all sorts of things here. Doing as you say above is fine. It's "flushing" that I'm hearing is the problem, right? In other words shuffling all the crap around inside the valve bodies ... right?

Doug Hillary

Hi,
heinrich - yes, you got it!

Probably the flushing as we know it should be called "Pressure flushing"

Regards

blown 87

It is my understanding that the flushing machines use the pump pressure from the transmission and the only pump in the machines are the one that you use to empty the used container, at least that is the way that all 3 of mine have been.

They really just replace the used fluid with clean fluid, using the pump pressure from the transmission that sends oil to the cooler.

I can see how fresh fluid could clean the old varnish off of the plates and other parts of the transmission and cause a failure.

My thinking on it is that if it is junked up with varnish and other crap, it is on its way out any way.

I can tell you this much, never ever hook one up to a power steering pump to flush that out because that is how i ended up with machine #3. That was a $4,500 lesson.