Taking care of your car's transmission is one of the most important aspects of maintaining your vehicle, as the cost of repairing a malfunctioning transmission can range from $1,000 up to $3,500. If you buy a used car, however, you should forgo force flushes. It could actually do more harm than good. Instead, you should have a fluid change done.
Transmission Flush vs. Fluid Change
A true transmission flush is different from a transmission fluid change, although service centers might call either a flush.
As CostHelpers explains, a fluid change involves removing the oil pan from the transmission and draining the transmission's oil (which is different from motor oil) by running the vehicle. Once the fluid is drained, the oil pan is replaced, and more transmission oil is added to bring the fluid up to its proper level. Between 5 and 7 quarts won't drain from the transmission. It will instead mix with the new fluid.
A power flush uses specialized equipment to change out all of the fluid in a transmission. Hoses are connected to the transmission, and new fluid is pumped in as old fluid is pumped out. During this process, the fluids are forced backward through the transmission, flowing the opposite direction from their normal path.
Metal Filings and Flushes
Over time, metal filings can break free in a transmission. They usually settle in the bottom of the transmission, where they don't cause any harm. It's not unusual to have a few in a transmission's oil pan, and, the Automatic Transmission Rebuilders Association notes, even a moderate number aren't cause for concern.
During the normal operation of a car, these metal filings remain at the bottom of the oil pan. The fluids don't move fast enough to pick them up.
The pressurized flow of fluids in the opposite direction, however, is able to dislodge these particles. In order to complete a transmission flush in a timely manner, service centers must pump new fluid in at a quick rate. This new fluid flows fast enough to pick up these small metal particles, which eventually settle in other parts of the transmission.
As long as these filings sit in a drain pan, they won't cause harm. If they're moved to other parts of the transmission, though, they can damage it. Therefore, if your transmission has metal filings in it, you shouldn't have it flushed.
If a transmission is properly maintained, the risk posed by metal filings is low. Transmasters recommends having your transmission serviced every 15,000 miles or once a year. There won't be many particles dislodged in this short timeframe.
Thus, as long as your transmission is regularly flushed it's not likely that metal particles will collect in its drain pan. If your car has had the transmission flushed every 15,000 miles, you should continue having the service performed.
Used cars don't usually come with all of their service records. While some car owners may keep documents related to a major transmission repair or rebuild, few will have a receipt for every oil change, brake job and transmission flush. Therefore, if you're purchasing a used car, you likely won't know whether it's transmission was flushed every 15,000 miles or year.
Without knowing your car's previous service history, it's impossible to determine whether a transmission flush would be safe to perform. Changing all of the fluid would help the car's transmission, but it could also disturb metal particles that might cause damage.
Unless your car came with a complete set of service records, you should forgo a transmission flush. It isn't worth the risk.
Instead of flushing the transmission, you can have its fluids changed, which won't send metal particles throughout your transmission. Although this won't replace all of your transmissions fluids, it will replace some -- and it doesn't pose a threat to your car's transmission.