FAILURE - Why it Happens.  
Nine out of every ten transmission failures are the result of heating and fluid contamination. Just like the oil in your car's engine, automatic transmission fluid suffers from heat, friction and degradation. This process causes the vital components in your transmission to clog with sludge and varnish deposits.  
 
FAILURE - How to Avoid it.  
All of the contaminated Automatic Transmission Fluid (ATF) must be drained from the system. The components of the transmission must be cleaned, and the old fluid replaced with fresh ATF.  
 
THE PROBLEM - A Complete Drain -> http://www.rlb.50megs.com/trktransfluidcycle.html
Unlike oil, most of the transmission fluid doesn't drain into the pan. It stays in the various components. A conventional service which simply drains the fluid, removes only 30% of the old, dirty fluid which is full of sludge and varnish. When fresh fluid is added, it is contaminated by the remaining dirty fluid.  
 
When you simply change what's in the pan, and replace with the few quarts, you have at least 2-3 quarts still in the Torque Converter, and other areas inside the Transmission, and radiator (and/or cooler if present).   You never get fresh start with fluid unless you completely drain the entire system.  Basically, when you drain the transmission pan, you're only replacing 30-40% of the fluid.  The remaining BAD fluid is still present within the system, and mixes with the new, contaminating it.
->this is one reason I'm flushing my system! But also because I'm switching to synthetic But I've also been told after flushing is when you encounter problems in older trannys. C3RVETTE38600.9519212963

Hhhhmmmmmm......should I or shouldn't I? Oh, what th' heck!
I have a few problems with this "tech-tip". In just a couple of seconds, I found as many tech. errors. I don't know who the author of this piece is, but I feel it needs to be corrected somewhat.
First off, I have no problem with the idea of completely changing the fluid.

The first thing I saw that was misleading, was concerning the "Torqie Convertor"(sic)
Contrary to what is stated in this article, the T.C. does NOT require any fluid in it to pump the fluid. The T.C. hub turns the pump gears, and that is what moves the fluid thru the unit. The only fluid needed in the TC(new/reman) is a small amount to lube the bearings/thrust washers on start-up. We normally put about a quart of fluid in a "dry" convertor before installation.

Then the author calculates the amount of fluid in the TC as 3-4 quarts. Maybe on a Chevette. A typical 350 trans TC will hold closer to 6-7 quarts, a 400, 7-8 quarts, and the 700, 6-7 quarts. Also, most trans coolers, whether inside the radiator, or external/add-on, will hold less than 1/2 quart, depending on the size of the cooler.

There were some 350 TCs that DID have drain plugs installed from the factory, assisting in draining the convertor during a fluid change. Almost all Brand "X"(Ford) convertors have drain plugs. They are there for a reason.
I know I'm just nit-pickin', but sometimes people get incorrect info on stuff like this, and then when they show up at my shop on Monday morning, armed with this "Take it to the bank" info, I have to bite my tongue, and 'splain to them that what they have is not really the truth.

The bottom line here is, yes, it is a good idea to change your trans fluid. Is it nessesary to do a "flush"? Not really. I like the shops that sell these "flush-jobs", it's job security for me!!
Filling a trans that has not been serviced regularly with new fluid can, and a lot of times does, create more problems. You need to realize that trans fluid is high-detergent, meaning it does a good job of cleaning. If you put a large amount of new fluid in an older trans that has not been taken care of, you will "wash" some of the clutch material off, as well as loosen, and circulate a lot of sludge thru the unit, that was perfectly happy sticking to whatever it was stuck to before.

My advise to anyone who asks is, if you've never changed the fluid, don't start now. Change the filter, put half of the old fluid back in, and top off with new fluid.
Then change the fluid on a regular basis after that. Preventative maintenence is the best solution.
As far as using synthetic fluid, I'm all for it. It does survive the heat better than the "organic" fluids. I do NOT recommend just draining the fluid out, and replacing it with a synthetic, it will be leaking before long. The synthetics have smaller molecules, and it also will clean the gunk off the seals/gaskets, and leak. If you are having your trans rebuilt, that is the time to put in the synthetic fluid.

There, I've said my peace! Adams' Apple38600.8134259259

Absolutey correct.

Joel and I have both heard many many "true" stories that we have had to do battle with.  Who starts this stuff??  The uninformed.

Preventive maintance is the best solution.

Heat is the biggest killer.  Over loaded and overworked transmissions create heat.  A few big burnouts are fine, but allow some cooling time in between every few times.  Let it run at idle, or better yet, cruising to keep the air flow moving, and let it cool.

kstyer38600.853125

I've changed the fluid in my TH350,but I've only had the 79 maybe 1.5 to 2 years so I dont know its history,but have no issues at all. I wanted to switch to synthetic for the obvious benefits,& figured why just drain out the 5 qts in the pan,& mix it with the rest of the dirty dino oil left in it...seemed kinda pointless,although I've never used synthetic tranny fluid before.I have a 3:55 rear end & do 3600 rpm at 70 mph.Also,I havent noticed any tranny problems at all,including leaks,up to this point. Now...I'm getting conflicting advice..."Do it,..synthetic is great stuff,& you'll reduce operating temps,better mileage,smoother operation,etc!"  ...or...."If you switch,you'll get all kinds of problems,including slipping,bad shifting,leaking all over the place,etc!"  -Now which is it,or will I get some of both?  --Before flushing,I will have the pan dropped & filter changed 1st!

 

C3RVETTE38600.9603240741

Problems, Problems, Problems!
Synthetic fluids are great, and will reduce operating temps, as well as friction, inside the trans. The real problem is mixing it in with the older type("dino") fluid. No mater how much you "flush" your unit, it will still have the older oil soaked up into the clutches.
Synthetic fluid will start leaking if you just put it in an older(un-"fresh") unit. Its just the nature of the beast, in this case.
I would stick with standard type fluids, and service at regular intervals, until the time to o/h the trans. Then, you could go with the synth.
On another note, if you drop the pan & change the filter before the flush, you'll be flushing all the "crap" back into the new filter you just put in ! Flush first, then change the filter!

Hmmm...so,if you flush 1st w/o changing out the already dirty filter & possible sediment in rhe pan,you DONT just push & mix anything thru thats already in there? And you will have leaks no matter what?  I wont be rebuilding this tranny,cuz when I get the cash & get around to it,it will be replaced with a 200 or 700 OD tranny.

[QUOTE=Ron 78]Nope do'nt run synthetic through an old tranny,or an old engine,its thinner viscosity and cleaning characteristics,will leak right through those old gaskets and seals and turn either unit into a sieve[/QUOTE] I was told on another forum,that that is an "old wives tale"! Are you an old wife?  I've been getting alot of conflicting advice,not just here,but combined with 2 other sites that I asked the question.So,now I've got synthetic fluid sitting here,and an appt. coming up that I set after my initial inquiry! Now it sounds like I'm just shooting craps & rolling the dice on this one,if I do go ahead! I was also considering the same for my engine,but it has a small oil leak,so I wasnt doing that until it was tracked down.So much for that,eh?C3RVETTE38601.5921990741

This is from a guy I talk alot to on 'another forum' who works for DTS-Drive train Specialists- whose products have been featured on "Trucks","HorsepowerTV",etc. "B.S.!
 
SYNTHETICS ARE TOTALY COMPATIBLE W/ CONVENTIONAL FLUIDS!  FACT NOT FICTION!  DETERGENTS AND DESPERSENTS ARE HIGHER QUALITY, AND BLENDING HAS NO EFFECT ON ADDING TO SLIPPAGE.
 
IF THEY STARTED LEAKING AFTER A FLUID CHANGE THEY INSTALLED THE GASKET WRONG, OR IT WAS GONNA LEAK ANYWAY!
 
TO MANY TIMES HAVE I SEEN SYNTHETIC STOP AN ALREADY LEAKY ENGINE, TRANS, AND REAR AXLES.
 
THE CRUD IN ANY TRANNY, BEFORE OR AFTER, IS ALREADY IN THE FILTER!  IT NOT LINGERING AROUND INSIDE SOMEWHERE.  THE FLUID IN ANY AUTOMATIC TRANS IS RAN THROUGH THE FILTER AT A VERY HIGH RATE, SO ITS BEING FILTERED ALL THE TIME.  IF THERE IS CRUD THOUGHOUT THE UNIT, IT HAS BEEN WAY TO LONG SINCE ITS LAST SERVICE, AND IS ON ITS WAY TOWARD FAILIER NO MATTER WHAT IS DONE!
 
12+ YEARS PUTTING AMSOIL IN OLDER AUTOMOBILES W/ MANY MILES ON THEM, AND I HAVE "NEVER" HAD A CUSTOMER LOOSE A TRANS DUE TO DIRT, SLIPPING OR OVERHEATING!  AND TRUST ME, I HAVE SOLD MORE THAN 30 CASES OF AMSOIL ATF OVER THAT 12 YEARS OR SO.  AND THE MAJORITY OF THE FLUID WAS INSTALLED BY ME W/ JUST A FILTER CHANGE.  ONLY ONE CUSTOMER IN 18 YEARS HAS USED THE FLUSH METHOD!  "MY TRANNY BUILDER"  I HAVE BEEN DOING TRANS SERVICES SINCE 1984.  TAUGHT HOW TO DO IT WHEN I WAS 12 YEARS OLD W/ MY DAD'S TRUCKS!
 
THE ONLY TRANNIES I HAVE LOST PERSONALLY, INCLUDING FAMILY, WERE DUE TO A LEAD FOOT AND MASSIVE ABUSE.
I BROKE THEM, THEY DIDN'T FAIL! "
 
 

Guess I'll have to turn in my business license, and go flip burgers or sumpthin!
We'll just have to agree to disagree!!!