The Great Debate

Higher mileage automatic transmission that you don’t know the maintenance record of . Transmission works perfectly , fluid change / flush or not ? One school of thought says yes , another says no . I personally disassembled an auto trans. & there was gunk hiding in a lot of little nooks & cranny’s . I can see the logic behind the school of thought that says don’t do anything to cut this gunk loose .
There’s also the school of thought that says , bunk , change that fluid and or get a flush , the trans. fluid needs changed . I can also see the logic behind this school of thought . I also have read that lubricants don’t wear out , they just get dirty .
There is a man locally that has a shop at his home & makes a living rebuilding auto transmissions . He evidently is good as he has a very good reputation in this area . My father-in-law had a transmission rebuilt by him & he said the man told him it was a mistake to change the fluid in a transmission that had fairly high mileage & hadn’t been serviced regularly .
A family member had a ford ranger that needed a transmission . He purchased a salvage yard transmission , installed it & it worked properly . He changed the fluid & filter & the transmission immediately went south . The salvage yard had warranted the transmission so he removed it , took it back & they gave him another one . Did the exact same thing with the 2nd transmission . It worked properly , fluid change & it immediately went south , he parked the truck after that & didn’t fix it again .
Now I’m in the same boat with a jeep liberty I bought that needed an engine & I’m in the process of installing a low mileage engine in it . To mess with the automatic transmission fluid or not , that is my dilemma . The transmission is / was working properly when the jeep was parked due to engine problems . As always , input is what I’m looking for & would appreciate .

Change the transmission fluid. Lubricants may not wear out (personally…I think they do) but they do get very dirty which wears out transmission parts. No flush…just drop the pan and change the filter (if one exists) and the fluid.

I won’t tell you what to do, but if I were in this situation I’d do a drain and refill (not a power flush). If the transmission died after that, I’d say that it was already on borrowed time.

I agree with @missileman ad @lion9car. If normal maintenance won’t fix it, it’s already doomed.

A non-maintained transmission that died shortly after long-delayed maintenance would have also died at the same time even if it hadn’t received any maintenance.

This is similar to the situation where people come down with flu shortly after getting a flu shot. In reality, the flu virus was already in their system, and they would have gotten sick whether or not they had gotten the shot.

Just as a flu shot can’t give you the flu, changing transmission fluid via a drain & refill (as opposed to a power flush) will not kill one’s transmission.

Additionally, while the trans fluid itself may not “wear out”, over time it becomes dirty and some of its additives become depleted. Similar to the situation of having old antifreeze/coolant, even old antifreeze will still protect against freeze-up, but because its additives have been depleted, the entire cooling system is subject to rust and corrosion.

Does new trans fluid have detergents that deteriorate over time ?

Yes, that is one of the reasons why you should change the fluid (& filter) every 30k miles.

Just read an article & the link didn’t work , will try again .

Just drop and clean the pan, replace the filter if it has one, and refill.

I prefer to not use “one size fits all” fluid, as that is often a cause of problems.

“I prefer to not use “one size fits all” fluid, as that is often a cause of problems.”

This needs repeating, so pardon me, as Joe Mario says, I prefer to not use “one size fits all” fluid, as that is often a cause of problems."

Use the exact specified fluid, using the car manufacturer’s product when possible.


If simply changing the fluid killed an automatic transmission then the trans was toast anyway. Old wives tale, coincidence, whatever terms you wish to apply to it.

There’s only one reason to worry about gunk coming loose. Any gunk will be stopped for the most part by the filter. It’s possible if enough gunk was broken loose that it could partially clog the filter and restrict fluid flow which could then lead to altered pressures, shifting oddities, and potential transmission failure.

If a trans is in that bad of a shape then it would not be a idea to perform a subsequent fluid/filter change soon after the first one.

The transmission fluid will not wear out but the additives in the fluid will.

I do not flush but only a pan drop or fluid change but on cars that I’ve had from new or nearly new. If I were going to flush, I would still do a pan drop and filter change after that. I believe this is what Tester has recommended here in the past and also what my transmission shop has advised in the past. No flush but if you must then a pan drop too. That reminds me I need to schedule this next week.

“If simply changing the fluid killed an automatic transmission then the trans was toast anyway. Old wives tale, coincidence, whatever terms you wish to apply to it.”

This bears repeating over and over again. Maybe we can kill off this old wives tale. I wonder how many transmissions “give up the ghost” each year because their owners buy into this nonsense?

My 2002 Sienna has a drain plug. Often, I drop three quarts and add three quarts. The fluid looks at all times brand new, and the transmission shifts perfectly at 218,000 miles.

I did this because back in 2002 Tom and Ray told a man who bought a new Jaguar that was an acceptable thing to do.

I did change to Mobil-1 Synthetic ATF a long time ago.

The closer I get to having to get rid of that 2002 in the spring the better it seems to run.

I look at transmission fluid in the same manner as engine oil.

Either you replace all the fluid and filter, or why bother?

I have a tranny fluid exchange machine. And here’s how it works.

You disconnect one of easiest of the transmission cooling lines to the radiator.

Then take one hose from the exchange machine and connect to the radiator, and the other hose to the cooling line.

Fill the exchange machine with fresh fluid.

Start the engine, and let the pump within the transmission pump the old fluid into the exchange machine while it pumps new fluid back in.

On some vehicles, the transmission has to be put into gear to engage the pump.

So it’s an urban myth. A tranny fluid exchange or FLUSH will not damage a transmission. It’s how the tranny was treated prior to the fluid service. Be it a pan drop or a complete exchange.


Anybody read the link ? Doesn’t seem like Pennzoil thinks it’s an old wives tale .

“A tranny fluid exchange or FLUSH will not damage a transmission.”

Tester…I believe with all my heart that you know what you are doing when it comes to transmission fluid exchanges. With that said…all technicians and/or mechanics are not created equal. It takes a perfect storm of a competent individual, the proper fluid and the proper machine to do a transmission flush/exchange in the right manner. I think that “perfect storm” is rare in the transmission flush/exchange world and that’s why I recommend against them.

In my car there are 12 quarts of fluid, at 146k I was feeling a little negligent, fluid still looked ok on the dipstick, but decided to do a complete change. 6 qts and filter had been done previously as that is all that is possible with a regular change, unless you crack the housing, but I am like why do I want that old fluid mucking up new fluid. Trailvoy had a solution.

Get a couple of extra jesus clips, disconnect the tran line from the radiator, dump 12 quarts in as needed while the old stuff goes into a recycle bucket.

Glad I did, the old fluid in the bucket was toasted looking, blackish rather than reddish!

Here’s one of the earliest transmission exchange machines.

All you did was fill the upper tube with new tranny fluid. And when the pump kicked in inside the tranny, it would force the piston inside the tube to inject the new fluid into the tranny.

But that’s old technology!


Either you replace all the fluid and filter, or why bother?

Because when I drain and add, it produces perfect transmission operation at 218,000 miles. And, if circumstances did not dictate I get rid of the 2002 in the next few months, it would almost certainly produce perfect transmission at 300,000 miles as well.

If you guys with your brand of car, believe you need to replace that filter all the time, go for it. You may be correct for your brand. Even with a desire to increase billable work, Toyota dealers do not recommend dropping the pan and replacing the filter, which they call a screen.