Monday, August 24, 2009

Should I flush my transmission fluid?

If it is a newer vehicle with less than 80,000 miles, I would recommend regular servicing of the transmission every 25,000 miles, including changing the fluid and internal filter. It is important that you maintain the transmission early on in the life of the vehicle, or you could set yourself up for real trouble if you change the transmission fluid later on (discussed below).

Regular servicing of the transmission can provide longer life expectancy, smoother shifting, and increased performance.

Some fast lube places service transmissions by "flushing" the transmission using a special machine that acts like a blood transfusion machine. It pumps out the transmission fluid, filters it, then recirculates it back into the transmission several times until the fluid is clean. I have mixed emotions about this procedure.

Yes, I will agree that with this machine it is possible to remove more of the old dirty fluid from inside the transmission than the conventional method of removing the transmission pan, draining the fluid and replacing the internal filter, but I feel very strongly that the internal filter should also be changed. Metal and plastic debris become lodged in the filter, and I have seen the filter itself break down and become compacted to the point that it restricts fluid flow.

If the fluid has become contaminated with water or some other foreign fluid, then I would recommend the flush method, but for regular maintenance I prefer the tried-and-true way of draining the old fluid and replacing the filter. If you have any doubt as to what method would be right for your vehicle, seek the advice of a qualified transmission shop.

I really feel the reason these fast lube places have adopted this flush method is because it lessons the chance of "mechanic error" while removing the transmission pan and filter, not necessarily because it is the best way to perform transmission maintenance.

Higher Mileage Cars

If your vehicle has high mileage (> 80,000 miles) and regular transmission servicing has been preformed as recommended, seek the advice of a qualified transmission shop if you experience a problem such as:

* Slipping
* Hard or erratic shifting
* Slow to shift when engine is cold (first thing in the morning)

If your vehicle has high mileage (> 80,000 miles) and the transmission has not been maintained, I would not recommend replacing the fluid and filter. The fluid that has been in the transmission all this time has become dirty and gritty. This gritty fluid is actually providing needed friction for the worn internal parts of the transmission.

Changing the fluid and replacing the filter would remove this friction that the internal transmission parts have become dependent on. If you have not been regularly maintaining the transmission throughout the life of the car, you might actually be doing more harm than good if you replace the transmission fluid at this point. For example, putting new clean slick transmission fluid in an older high mileage vehicle could cause the transmission to slip.

If you are not experiencing a problem, have over 80,000 miles on your car, and have not kept up regular maintenance on the transmission, my advice would be to leave the transmission fluid and filter alone. If you are experiencing a transmission problem like those listed above, seek the advice of a qualified transmission shop before allowing your regular mechanic to service the transmission.

I learned about this problem several years ago...the hard way. We had two cars in our shop with these "minor" transmission symptoms that I have listed above, and both of them had to be towed out to the transmission shop after servicing. The old fluid was gritty due to metal shavings caused by normal internal wear and tear on the transmission clutches and was acting like liquid sandpaper.

This "sandpaper" was producing the friction needed for the transmission to pull itself. When we changed the filter and replaced the old fluid with new fluid, the clutches inside the transmission had nothing to grab on to. Sometimes even the best of us learn lessons the hard (and expensive) way. Flushing your transmission is vital to the transmission performance, But you must flush it regularly, once the fluid is damaged, it is best not to flush your transmission.

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