Is there any solid evidence that a tranny flush at high mileage is bad? You always hear people say that if you don't flush it at 30k miles and at regular intervals, then its not good to flush later on. What evidence is there to support this or is it all hearsay?

 

I thought the jmatic fluid was good to 50k-60k miles min.

 

Being in the auto repair business for 30 years...i have heard that the more miles on the trans the wise choice is not to flush it.In the business if the trans has over 100,000 miles and has never been flushed or drained most shops wont flush the fluid out do to fears that sludge or varnashed material could loosen and cause problems.

If the trans has been maintained i dont see a problem with flushing the fluid every 30-40 thousand miles.

 

The reason why shops refuse to flush it if it was not kept on a regular flushed interval is because the metallic debris that the magnets and filter cannot catch severely contaminates the transmissions valve body, torque converter, etc. This metallic silt comes from the long term wear of the clutch bands against the plates. The friction clutch bands will become severely worn in such an unusual way that FRESH NEW FLUID will actually be TOOO SLIPPERY. When this happens the transmission shifts erratically, not at all, or just slips gears constantly. If the shop flushes it and it slips... the customer holds the transmission shop liable, even though it was the customers irresponsibility for not properly maintaining it.

For a better understanding, picture how your smooth your brake rotors are and how flat the brake pads are. Smooth on smooth makes for superior braking (grip). Now place sandpaper grit between the pad and the rotor and see how it would brake... it wouldn't. But if you put sand paper to sandpaper it grabs instantly. My example might be difficult to apply inside an automatic transmission but that's the simply way to put it.