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AWD Mercury Mountaineer. 1997 Automatic transmission. 8 cylinder 5.0L 150k miles. fairly good shape. Pretty awesome truck.

From 1st to 2nd gear, the transmission shifts with a light clunk. Also, at times the transmission shifts late from 1st to 2nd.

I want to change the transmission fluid and the transmission filter, thinking this will fix an issue or improve the shifting.

I was advised to not change the transmission fluid or filter because it may disrupt "something" and make the transmission shifting situation worse.

Has anyone heard this before? Doesn't anyone disagree? Agree? Any advice?

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I've heard it many times, but never seen any proof. See the question and my answer to this post. I have never read about it in any of the Factory Service Manuals I have been through, so I shelve it up as false.

I think the idea is that new fluid, with all the fresh detergents, shock dirt and debris loose and cause problems. Again, never seen it happen.

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  • Thanks for the answer. Sounds reasonable and logical. So, it's not that the fluid change would "break" the transmission. The concept is: if the transmission is somehow partially dysfunctional, it will fail eventually. Changing the transmission fluid of a partially dysfunctional transmission may accelerate the failure, for some reason or another. I'll change the fluid and the filter. If it goes bad, at least it's on my terms and close to home. (Reminds me of the 'Married with Children episode' where Al hasn't gone to the dentist for 25 years. He has no cavities!) Thanks @Nick.
    – 1c1cle
    Commented Sep 16, 2013 at 17:52

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