This is concerning a car with an automatic transmission. My owner’s manual says that you can run the car for 20 or so minutes and drop the pan beneath the transmission and change the fluid yourself. However, two sources (a mechanic friend and person at a lube shop) both told me that in order to properly change the transmission fluid, you must take your car to a place that has the machinery to suck the fluid out of your transmission. Their reason was that during the life of a transmission, the fluid forms a varnish at certain points in the transmission. If you try changing the fluid yourself, you will not be able to remove those deposits and when you fill the transmission with new fluid, the new fluid will dislodge those deposits and potentially ruin your transmission. Thus the need for the machinery to remove the old fluid. Lastly, I do know of a person that changed his transmission fluid himself, and his transmission did go south shortly after he changed the fluid, but in his defense, the transmission was acting screwy to start with. So I need the straight dope on this subject: change the fluid yourself, or take it somewhere withe the proper machinery to remove the fluid.
Well, I can say this. I have changed the fluid in a few transmissions the way your manual specified. I have not had one crater thereafter (well not shortly thereafter…a few years maybe but I don’t relate them).
Your friends may very well have a valid point that vacuuming out the rest of the fluid may be better but I wouldn’t be concerned about it.
Mt $.02
Changing the transmission fluid is (usually) a hell of a mess, from what I observed at my mechanic’s shop, but it’s essential on many cars because a car’s transmission often has a screen and it can clog up over time, decreasing fluid flow. And, yes, a lot of fluid cannot be removed from simply dropping the pan and emptying it. Transmission shops usually offer the procedure you’re talking about, but I wouldn’t let a transmission shop touch any of my cars on a bet.
From what my mechanic says, if you change the fluid every 40,000 miles or so, you shouldn’t have a problem with much build-up.
BTW, lots of shadetree mechanics swear that changing transmission fluid invites all kinds of transmission problems. Sounds anecdotal to me.
Jiffy Lube will do this for about $45.
The major issue with changing transmission fluid, other than the mess, is the fluid trapped in the torque converter. That’s the coupling between the engine’s flywheel and the actual gears in the transmission.
Just dropping the pan and changing the fluid in the pan and filter leaves about half of the total fluid behind in the torque converter. The places that pump it out usually do so by intercepting the fluid as it’s traveling to the cooler built into the radiator and replacing that with new fluid as the trasmission pumps the system. They have a glass cylinder with a dividing piston. New fluid on one side and used fluid on the other.
But, with this method you can’t drop the pan and change the filter!
If there is a system that both changes ALL the fluid and still allows for a filter change, let me know 'cause that’s where I’m going next time!
There’s not. This is what I’d do, if I cared enough.
Go to an auto-parts store and buy a filter/gasket kit for your transmission, some STRONG tranny cleaner, and a few quarts of whatever is their cheapest fluid for your transmission. Drop the pan, catch the fluid, clean the pan out, this is important. You must clean the pan! It will have a magnet on it to collect metal shavings, but likely the magnet is overwhelmed. Take the magnet off, clean it completely off, clean all the shavings out of the pan, deposits, etc. Clean the bottom as well as you can without disturbing anything, also. Trash the old gasket and filter, put the new ones in, add the cleaner, and fill the transmission. Drive down to the Jiffy Lube or wherever you go to get caraped, and have them suck out all the fluid you just put in along with the cleaner and all the crap it’s removed. Then have them fill it with their best fluid.
Sure, you’ve probably ran a bunch of nastiness through your brand-new filter, but how else are you going to get three exclusionary tasks finished without f’n up one of them? Besides, it’s surely alot cleaner than the old one.
So enjoy your like-new transmission that doesn’t slip anymore.
–Tim
P.S. While your pan is off, I HIGHLY recommend having it drilled and tapped for a drain plug so it’s not so messy of a job next time. Might as well change your oil pan gasket while you’re down there, if it’s leaking or old.
Actually, the reason they didn’t put a drain plug in it in the first place is so that one must remove the pan to remove the fluid. That way a user is not tempted to simply swap fluid without swapping filters as well.
That having been said, the drain plug isn’t that bad an idea, IF- and only if- you do your own auto maintenence. If not, don’t bother, since most of the “add on” drain plugs WILL- not “if”, “when”- leak eventually.
Personally, I TIG welded a new bung to my pan for the plug.
Anyway, the idea of “varnish” is ludicrous. Tranny fluid is highly detergent- it’s designed that way to keep the packs of friction plates from “glazing” and thus burning and slipping.
In fact, it’s an old-car nut’s trick to dump a bunch of ATF in the engine- say, four quarts plus one of oil for a five-quart system- to free up sticky lifters and rings, to decarbon the reciprocating stuff and generally wash the junk out. You will be astounded at the junk that decent ATF will free up from an old motor after a very brief time- like less than 50 miles. Then dump the ATF, slip on a new filter and fill with fresh motor oil.
Anyway, if any “varnish” is forming inside the tranny, it’s due to excessive heat, not the fluid itself. One old mechanics’ rule of thumb is if the fluid exceeds about 250 degrees, the life of the tranny can be measured in minutes.
Now, if you want to flush the converter, find the tranny cooler line where it connects to the radiator. Drop the pan, change the fluid and filter, replace the pan, and fill the tranny to the proper level.
Now, remove the upper tranny cooler line from the radiator. (It’s the upper one for older GM products- I have not the least clue which is the correct one for any other model- check your manual if you have one.) Jam a long scrap of rubber fuel line, or better yet, a long chunk of clear aquarium tubing over the now-loosened hose.
Have your Trusted Sidekick start the engine and let it idle… watch the fluid as it comes out of the hose. (Hopefully it’s not gushing out of the fitting on the radiator… if it is, you chose the wrong line.)
The “old” fluid will be noticibly darker when it comes out of the hose… when the flow gets lighter and cleaner, the converter is flushed.
Now, you’ll have to keep a VERY careful eye on the fluid level in the tranny. You may have to let it run briefly, shut it off and top it up, then start and run it briefly again.
If you’ve gotten three or four quarts or so out and you haven’t seen a color change, go ahead and stop, you have most of it out anyway. Keep an eye on the level!
Voila`, you’ve flushed your own tranny. Double-check the fluid level- step on the brake and run the selector through the gears, then check it warm and in neutral- and check the pan for leaks. Dispose of the filter and used fluid appropriately, and you’re done.
For what it’s worth, my car has 141,000 miles on it. The transmission fluid has NEVER been changed. Never. I have yet to have one transmission problem. (A relative told me many years ago that transmission fluid changes are completely unnecessary. From what I’ve seen, he was right. This may indeed be merely anecdotal, but I’m happy with the way things are.)
Now that I’ve written this, I’m sure my transmission will quit on me on my way home from work. 
nineiron: “A relative told me many years ago that transmission fluid changes are completely unnecessary. From what I’ve seen, he was right.”
This has certainly been my experience with my Chrysler minivan. In 85,000 miles I’ve never needed to change the transmission fluid. I have had to replace the transmission twice though.