Transmission flush or drain & fill

[Richard Foster]

I am confussed about the difference betweem flushing your transmission or doing a drain and fill. And when you should do one or the other.

[SilverFox]

The rul eof thumb is....if you have regular on time maintenace of the trans, FLUSH IT.....if you are crappy service mechanic and not timely, and the trans fluid is very cruddy, DRAIN IT.

FLush - COLD or HOT ATF is pumped through your cooler lines to displace the COLD or HOT ATF in the trans...they do this until it comes out clean :) It will cycle ALL the ATF.

Drain, only 60% fluid is changed, there is A LOT of ATF hidding in the cooler lines, torque converter, and inside the the gear train of the trans and at least a quart or more inside the VB and worm channels.

The reason for the differance is that many FLUSHERS rip the crud out of the trans - this crud helps OLD high milage trans hold up and that crud might be blocking leaks also. Flush a trans like this, and if might leak or not last. There is really no explination for the reason, however it just so happens that an OLD trans flush (like AOD C4 C6) it tends to kill them off quicker :(

You probably have a 4R70W trans, they are designed to be flushed for the most part, as long as the fluid is in good condition still and you don't have any trans issues.

WHAT I RECCOMEND if you pan drop:
Pull the TOP cooler line going to the trans, slap a short section of hose on it to drain to pan, start car, let the trans pump all the ATF it can, kill car. Hook the mess back up, drop pan.....you won't get NEAR as messy, trust me. Also, if you want to keep the car for long long time, install a drain plug at this time :)

Note that you can also unhook the cooler line at the radiator as long as you know it is the cooler line that is running FROM the trans (4R70W bottom line)

[Richard Foster]

Thanks, Silver Fox - I have a 99 Towncar with 25,000 miles. Sounds like I should do the flush, right. Do you know which line should I disconnect at the radiator? top or bottom.

Thanks, Dick

[98TC-Cartier]

Top line at the rad is the inlet to the trans cooler. Add a piece of 1/2" garden hose, it will nicely slip over the factory rubber line and have someone HOLD it into an empty jug to catch the old fluid. Otherwise you will have trans fluid spraying out all over the place.

[interceptor_74]

yes use the top line the bottom one is for the power steering

[Richard Foster]

Thanks for the help, I will give it a try next week sometime.

Dick

[mercman1951]

Let us know at what mileage your trans starts slipping at after the preventative 'flush'... If you have never had the trans fluid replaced, and you have 50,000+ miles, I wouldn't start flushing it now. I've never heard this was a good idea to do...especially if you haven't done it "regularly", or depending on your owner's manual.

The trans starts to need that gunk built up in the fluid to shift right as the bands wear. Take away the gunk...slip city. Trans rebuild time. I've had many a trans last over 100,000 miles that never had their fluid replaced...and I have also seen 'paranoid' people that were talked into trans flushes...only to have their trans "prematurely" let go 5,000 miles later...scam, scam scam. Unless you religiously replaced the fluid 15,000 miles, 30,000 miles, etc...if it's been the original fluid since day one and you have more than 50,000 miles.... I'd leave well enough alone. Just my .02.

http://www.freep.com/apps/pbcs.dll/article?AID=2007705010379

http://autos.aol.com/article/general/v2/_a/fluid-flush-fallacy/20060505115609990001

[98TC-Cartier]

His 99 TC only has 25k on it, at that rate, not likely to have wear related problems for another 100k or about 28 years if he does nothing to it :)

BTW old wives tales about not flushing old dirty ATF date way back to whale oil days, and back then it was a problem. They rank right up there with synthetic oil causing oil seal leaks.

The biggest cause of old trannies breaking is change of use patterns. Youn'n buys grampa's 20 year old 70k miles car which hardly ever got above 35 mph and always got 25+ mpg in the city cause it was driven so lightly.

Now the throttle is 3/4 to the mat at every light, the whole drive train is going WTF as it has never been used this way in all its life. Unknown to everyone, it is totally worn out from corrosion because it never ever got warm enough to boil off the moisture inside. Engine oil changes were done every 3k miles just like book says, too bad that was maybe once a year. All other fluids except the anti-freeze are factory fill.

Newby owner changes the black ATF fluid and then wonders why the tranny died 3 months later. If it hadn't been changed it would have died in 2 months instead of 3.

My father-in-law has a 94 LHS, has about 30k miles on it. It will take about another 5 to 8 years for the mileage to get to 40k.
I expect it will be broken shortly after he sells it too.

[98TC-Cartier]

........ or depending on your owner's manual.
http://www.freep.com/apps/pbcs.dll/article?AID=2007705010379

http://autos.aol.com/article/general/v2/_a/fluid-flush-fallacy/20060505115609990001

Yes there is a lot of scams going on, but quoting chrysler, what a joke. they have fiddled with their 7xxx fluid formula a number of times and still haven't got it right. Tons of vehicles with premature tranny problems, not counting the trannie cases which get broken and are not repairable. How about their engine which only holds 3 quarts of oil, not the most reliable 4 banger on the market either. The bozo who used an additive to upgrade the fluid to Chrysler spec is out in left field as well.

Taxi duty in Vegas is anything but severe service, yes a lot of miles and heat, but trans fluid likes to run nice and warm. Now for really severe service, take one of those minivans and run a summer shuttle service in and out of Death Valley 12 hours a day and there is no way the transmission would last two summers.

The second article says brake fluid system are sealed, well that used to be true before everyone started using plastic in their master cylinders. Almost all plastics can absorb moisture, and it can travel right thru the plastic reservoir on the master cylinder and into the fluid.

Now the second article raised a lot of valid points, the industry has gone flush crazy, lots of $$$, but if you follow the severe service maintenance shedule in the owners manual you won't go wrong. And if you read it carefully, and examine your driving habits, you will notice your usage is usually outside of the regular maintenance schedule.