Is it true that you shouldn't change the transmission fluid on high mileage vehicles?

Question

Everyone and their mothers has told me to never change transmission fluid on a vehicle with high mileage (150,000+). My father who is good friends with a guy at a Ford dealership even said the same thing. He said if it wasn't changed on a regular basis to just leave it be because the transmission is so used to the old fluid.

Again, he said to leave it be ONLY if it hasn't been changed on a regular basis. Is there any truth to this at all? Also, when are you supposed to change the transmission fluid? Every 30k or 40k miles?

Answer 1

Yes, if you haven't serviced it on a regular basis, doing so now could actually cause it to start slipping. Especially if you flush it.
If the transmission is already damaged, those metal shavings and other debris is providing the friction to keep it working. If you flush that fluid out and put in new fluid, there might not be enough friction to work anymore.

Draining it and refilling it might not be as bad though. It depends on the amount of damage. But if you drain it and check out the fluid, you will get a better idea. Since just draining it doesn't remove all of the fluid, if there is damage, the old fluid that still remaining in the converter might be enough to keep it working. Kind of like a fish tank, you leave a little of the old water in there for the algae and bacteria the fish need. The inside of the transmission is worn and damaged, it needs the friction at this point.

But if there is damage, at one point or another you will need to replace or rebuild the transmission. It wont matter how much friction you have if everything is broken and worn down to nothing. But replacing the fluid really isn't going to help at this point, as far as keeping the transmission in good shape. That's why a transmission service is called "preventive" maintenance.

And no, the transmission doesn't get "used" to the old fluid. New fluid is better for the transmission and will prolong the life, however if the transmission is already damaged, the friction provided by the old fluid (and all the debris in it) is what is keeping it from slipping, for now. It will eventually completely tear up the tranny and it will need to be fixed. But changing the fluid at this point could very well cause it to start slipping.

The best thing would have been proper maintenance in the first place... We can all agree to that. At this point, I think your best option is draining out some of the old fluid and adding new. Flushing would completely remove all of the old fluid, so just drain it instead. You will be able to inspect the old fluid for debris and see how bad it is.

Contact any reputable tranny shop and ask them if they will flush your transmission fluid. Then tell them you have 150k + miles and have never serviced it before. 99% of them will tell you no, and why. Then again, some will tell you sure, because they know you will be forced into getting a rebuild right after the service.

Answer 2

Changing Your Transmission Fluid

Answer 3

I would have to agree with your father I have in the past had used vehicles and thought I would do the right thing by changing the tranny fluid (the tranny shifted fine) after I changed it miraculously it started to slip it almost seemed like it was holding the tightness into the tranny and as he stated if it was changed on a regular basis then change it on a regular basis but otherwise don't cause if it is a used vehicle you know nothing about you don't know if it was changed on a regular basis the only time I would change the used is if it was already slipping and it was a last resort

Answer 4

My daughter has a F-250 that here tranny was slipping so gt a junk yard tranny to keep her truck on the road because she could not afford to have it rebuilt or down time. I have heard many different stories on bone yard trannys. If the fluid is clean and smells good changing the filter and fluid can cause the tranny to slip from it being use to the old fluid. Should I change the filter and fluid or just add the new fluid that was lost in the replacement?

Answer 5

That's hooey. Change the fluid. A transmission being used to old fluid is nonsense. Change fluid and clean or replace the filter screen every 25-30,000 miles. Automatic transmissions can be destroyed by running old, burnt and contaminated fluid in them.

Answer 6

I've never heard this before, and don't believe it. A long lasting transmission is best achieved by keeping the trans-fluid clean along with the filter. I suggest you change the fluid and filter asap and then maintain it per the owners manual.
By the way, transmissions don't get used to anything, they just deal with it till they can't deal with it any more, then you deal with it..

Answer 7

Change the fluid, the myth is not true, the longer you wait the more damage is being done to the internal working of the transmission! Have it flushed and change the filter in it ASAP!

Answer 8

Full transmission service (change fluid + filter) should be done every 30k miles, unless of course if you want the transmission to die out and spend $1000 + to replace it.

Answer 9

I have no proof that this is the case but for whatever reason I tell my customers the same thing. Just like the fuel filter. I hear people tell me that they never had the fuel filter changed until the car had 130k miles on it and then a month later the fuel pump went out. Dont know if there is any truth to these but I hear it enough that I say the same thing to my customers because if it is true I dont want them coming back to me and telling me I ruined their car.

Answer 10

being a long time auto tech. i can aswer your question but you won't like it. yes you should change your fluid at manufacture intervals. if the fluid hasn't been changed regularly it can cause problem when you do change it. what happens is the dirt and crud in the old oil breaks lose and can cause valves in the trans. to stick and let some worn sealing rings to not seal as good any more. sorry for the bad news. it is very important to keep all your fluids changed .