Transmission Flush Vs. Drain—What's The Difference?

Properly servicing your automatic transmission is one of the best ways to ensure that it lasts for as long as possible. With transmission rebuilds and replacements costing well into the four-figure range, it makes sense to maintain the one you have. Most modern transmissions are relatively hands-off, but the fluid is often a serviceable component.

When it comes to maintaining the fluid, you'll generally have two options: flush or drain. For most people, draining is a far better option, but you'll need to understand how your transmission uses its fluid to see why this is the case.

What Does Automatic Transmission Fluid Do?

The fluid in your automatic transmission is far more than a simple lubricant. Although it does help to lubricate internal transmission components, it also serves numerous other purposes, including acting as a hydraulic fluid. Your transmission relies on fluid pressure to transmit power from the engine and select gears, so fluid problems can drastically impact your transmission's performance and reliability.

If that wasn't enough, your transmission fluid also helps evenly distribute heat and keep the transmission cool. Its role as a coolant is so critical that some cars include transmission coolers to help circulate the transmission fluid and keep it from becoming too hot. An overheating transmission can fail just as quickly as an overheating engine, so keeping cool is essential.

Why SHOULD You Drain and Change Your Fluid?

Your transmission is a mechanical system, and its parts will naturally wear over time. This wear generates small particles that can gunk up your transmission fluid, reducing its ability to lubricate and cool. This particulate matter will appear even in otherwise healthy transmissions, even those that come with "lifetime" fluid from the factory.

Draining your transmission allows a good portion of this contaminated fluid to flow out, although a significant amount will remain behind. As a result, a drain isn't a complete fluid replacement. Instead, your technician will add as much as they drained, leaving most of the old fluid inside the transmission.

Why SHOULDN'T You Flush Your Transmission?  

If you aren't getting all the fluid out, you may think that flushing is a better option. Unfortunately, flushes are often a mistake. Flushing out your old transmission fluid can create leaks and reduce the friction of the fluid, leading to slippage and other issues. In some cases, it may even result in permanent damage to the transmission.

Instead, routinely draining your fluid will keep the transmission clean and operating correctly. Each drain procedure removes a little more fluid, and the clean fluid will help flush out contaminants for the next time you drain it. Over time, this process will safely keep particulate matter out of your transmission without the risk of severe damage.

Contact an auto transmission fluid drain service for more information.