The oil-change shop always suggests this, and I usually decline, figuring they’re pushing a service that really isn’t needed all that often. But I always wonder: what’s the straight dope on transmission flushes? How frequently should they be done?
Read your car owners manual. It will tell you how often to change the transmission oil and under what circumstances. If you are a belt and suspenders kind of person, and drive under normal conditions, you could follow the “severe conditions” schedule or whatever other term your car’s manufacturer uses. The transmission flush simply replaces the old fluid with new fluid, it is faster than simply draining it, but I doubt if it provides any further benefits.
For most cars, a transmission service is recommended every 30,000 miles, or every two or three years. If a lot of mountain driving and/or towing is done, twice as often is generally recommended. The maintenance schedule in the owner’s manual should list this.
We are talking about automatic transmissions, where the fluid actually propels the car (fluid drive in the torque converter) as well as lubricates the transmission and hydraulically operates some of its internal components. The fluid is subjected to great stress and does break down. To give you an idea how hot it gets, automatic transmission fluid is typically cooled by piping it through the radiator, which runs around 200 degrees F with a warmed-up engine. Manual transmission fluid is not subjected to anywhere near the same stresses (service for it is a simple drain and refill, maybe every 60,000 miles, and often is overlooked with little or no discernible effect).
There are two ways to service the transmission. The conventional service involves removing the pan, replacing the filter, and refilling. Typically only about 2/3 of the fluid can be replaced this way. The other method is a flush, which replaces virtually all of the fluid but does not address the filter. Neither way is perfect. I think the best approach is to alternate these services – conventional at 30,000 miles, flush at 60,000, etc.
The appearance and odor of the fluid changes as it breaks down. It goes from red/pinkish red to dark red, to orange, to yellow, to brown, to black, and from a sweetish smell to an increasingly burnt smell. Generally when it reaches dark red to orange and smells somewhat burnt it’s time to change it. Be aware, however, that after a conventional service the fluid may still appear somewhat deteriorated because of the significant amount of old fluid still there. Don’t change it if it’s black, as things are pretty far gone then, and fresh fluid may push the about-to-go transmission over the edge (in other words, it will need to be overhauled now instead of next week/month/year when it would go out otherwise).
Most quick-lube places (like most chain operations) train their personnel to sell rather than to develop good mechanical judgment. As a result there seems to be a fair degree of overselling. In the worst cases, transmission flushes, air filter replacement, and other items may be suggested at nearly every visit, regardless of the actual condition of the items. Keep records of the maintenance done and approve each item when it’s due rather then just because it’s suggested.
Another excellent post, GaryT