Q: Your answer with regard to an F-250 transmission problem following a transmission service gave me 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 90,000 miles 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 30,000-mile intervals, regardless of what the owner's manual says, you will not have transmission problems. In fact, BG Products, a highly respected company my shop uses, 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 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.
A: 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. As a 3000-mile oil change is considered a standard service interval on an engine for trucks running extreme duty cycles, 30,000 miles is about the standard interval on an automatic transmission. And for years, transmissions have been holding up great using the standard procedureâ€”drop the pan, change the fluid that spills out, and replace the filter. Note: Certain transmissions do have a drain plug on the torque converter, which enables replacement of the majority of fluid. There's a new device that 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. Why is the new procedure being recommended at the same intervals? If it's doing so much more, why not do it at 50,000 or 60,000 miles?
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 were my love-truck and cost were no object, I would probably use the fluid exchange machine myself, drop the pan, and change the filter every 30,000 miles. If it were a car I wasn't planning to keep forever, I wouldn't waste the money.
In another article, I discussed the misrepresentation of the word "flush." When a transmission is replaced, part of the job is 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 that 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.
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