What Is an Automatic Transmission Flush?

Most light cars and trucks have automatic transmissions, with some having up to 10 forward gear ratios that shift on their own, faster than you can. Automatic transmission fluid (ATF) makes this possible, functioning as hydraulic fluid, cooling fluid and lubricant. Depending on age, mileage and use, ATF eventually wears out, which means a transmission flush is in order.

When to Do a Transmission Flush

Inside the transmission, ATF endures heat, friction and force, burning off additives and conditioners. This can result in shifting hesitation, slipping gears or surging. Towing, hauling and stop-and-go driving increase stress, wearing the transmission fluid faster. Under these conditions, a flush would be needed around 20,000 to 30,000 miles. With mostly highway miles, a transmission fluid change is needed less often, perhaps every 30,000 to 60,000 miles.

On many vehicles, ATF condition can be checked via the dipstick. On so-called “sealed” transmissions, you might have to pull a fill plug or drain plug to get a sample. New ATF is bright, clear, red and sweet-smelling. If your ATF is dark, opaque, brownish-red or smells burnt, it’s time for new transmission fluid.

Flush vs. Drain-and-Fill

Generally, there are three ways to change ATF: transmission drain-and-fill, ATF transfusion or transmission flush. Transfusion and flush procedures are similar and might be referred to interchangeably, but they each have their own advantages and disadvantages.

• Transmission Drain-and-Fill: Remove the transmission pan or drain plug and drain the ATF. If removing the pan, replace the transmission filter, too. After reinstalling the pan or plug, refill the transmission. This is only a partial ATF change, generally replacing 20% to 30% of the fluid, but it’s the easiest to do yourself and is the least stressful on the transmission. To get more of the old ATF out, repeat the procedure every few thousand miles.
• ATF Transfusion: Connect an ATF transfusion machine to the transmission cooler. The transmission will push old fluid into this machine and new fluid into the transmission. This fluid change is more complete, generally replacing 80% to 95% of the old fluid.
• Transmission Flush: A flush machine connects like an ATF transfusion machine, but uses an onboard pump to push fluid under pressure through the transmission system. This might include heated detergents and backflushing against the normal flow of fluid. Some say this can dislodge deposits that may have stayed in place during a no-pressure drain-and-fill or low-pressure transfusion, but it’s hard to say if this is the fault of the transfusion machine or due to a lack of automatic transmission maintenance.

When changing transmission fluid, it’s a good idea to use a transmission flush to clean deposits and condition seals:

1. Warm up the engine and transmission by driving for 10 to 15 minutes.
2. Add transmission flush to the transmission via the dipstick tube or fill port.
3. Engage the parking brake and idle the engine for 10 to 15 minutes.
4. Meanwhile, with your foot on the brake, move the shift lever through the gears, staying in each position for about 30 seconds. Do this two or three times.
5. Continue with your preferred transmission fluid change procedure.

Consider servicing your transmission sooner rather than later, because the most thorough cleaning and best ATF can’t make up for wear caused by burnt ATF. The best way to judge is by checking its condition regularly.