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Transmission Service:
Flush or Drop Pan and Change Filter?

Alex Steele
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Your answer with regard to an F-250 transmission problem following a transmission service left a little heartburn. You are correct that many shops, independent as well as franchised dealers, tout the fluid exchange as a recommended service for automatic transmissions. The fact that the questioner had 90K on his odometer should have prompted the repair facility to drop the pan and change the filter as well as inspect for any debris.


If you service the transmission at 30K mile intervals, regardless of what the owner's manual says, you will not have transmission problems. In fact, BG products [which we highly recommend and use] will cover a substantial amount of the cost should there be an issue. If that first transmission interval is ignored, a reputable shop with qualified technicians will remove the pan and change the filter to prevent a question of new fluid contamination.


The heartburn comes from your suggestion that the old way of doing it by adding the 4-6 quarts of new fluid to the remaining 8-10 quarts of old fluid. You wouldn't suggest that for engine oil, nor should you suggest or imply that for transmission fluid. All of the additives, detergents, and dispersants of the new fluid are immediately going to work trying to treat the old fluid and are used up very quickly.


In addition, the exchange method generally moves 3-5 additional quarts of fluid through the system to "flush out" the junk. While it is profitable, it is not a cash cow by any means. If removing the pan is warranted due to the mileage and past service or lack thereof, the shop should inform the customer and charge accordingly.


I enjoy your column and almost never disagree with your assessment. If you still defer to your original stance of mixing old and new fluid, we'll just have to agree to disagree.




Sorry about the indigestion. That was not the idea, at all.


In response to the previous reader's question I gave my opinion on a so-called transmission flushing procedure probably costing more than it's worth. Again, it's an opinion, and we both may be somewhat correct.


Like a 3,000 mile oil change is considered a standard service interval on an engine, 30k is about the same thing on an automatic transmission. And for years transmissions have been holding up just great using the standard procedure -- simply dropping the pan, changing the fluid which spills out and replacing the filter. Note: Certain transmissions do have a drain plug on the torque converter which enables replacement of the majority of fluid.


Now a new device hit the market which can replace nearly all the transmission fluid at a higher cost, but the service is still being recommended at the same intervals as the previous method, which worked just fine. My question; why is the new procedure being recommended at the same intervals? Why not 50k or 60k miles if it's doing so much more?


The standard procedure does not change all the fluid, but the engineers have believed for a long time that servicing at the recommended schedule will maintain a ratio of new-to-old fluid which is more than adequate to maintain the fluid quality and life of the transmission.


Will the new method at the same intervals extend the life of the transmission? That would require extensive long-term testing by qualified engineers to determine for sure. In my opinion, even if it does extend the lifespan a little bit, doing the more costly procedure over the life of the vehicle may not pay off.


Personally, if it was my love-truck and cost was no object, I would probably use the fluid exchange machine, drop the pan and change the filter every 30k, myself. If it was a car I wasn't planning to keep forever, I wouldn't waste the money.


In another article I discussed the misrepresentation of the term "flush". When a transmission is replaced it's part of the job to flush the transmission cooler and cooler lines to remove debris, and make sure the cooling system is clean and free flowing for the brand new unit. This requires a special tool which uses high pressure air and transmission fluid to "back flush" against the normal flow of fluid, which is the proper way to remove foreign matter.


The "flushing" machines don't flush. They use pressure equal to that of the transmission's internal hydraulic pump, and replace fluid in the same direction as normal flow. So it's not really removing anything but the old fluid. Actual back flushing of the transmission itself could lead to a hard failure.


Again, it's just an opinion, but maybe this will help clarify where I'm coming from.

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