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Transmission Fluid Change vs. Flush: Which Is Right for You?

By Stephen Fogel, June 1, 2018

Your car’s transmission is responsible for converting the engine’s power into motion for the wheels. Transmission fluid plays an important role in this conversion

In a manual transmission, the driver selects the appropriate gear ratio with a lever and a clutch pedal. In an automatic transmission, gear ratios are selected by the transmission itself. Both types use fluid to keep things moving — though the type of fluid differs.

Every so often, this fluid needs replacing — we recommend doing this every 30,000 to 40,000 miles for most cars with automatic transmissions. But should you have it changed or flushed? And what’s the difference between the two, anyway? 

Let’s look at why you need transmission fluid, or you can skip ahead to learn what a fluid change is, what a transmission flush is, and why you should only get a flush when your owner’s manual says to, or when the fluid shows serious problems.

Why you need transmission fluid

Your transmission contains many moving parts that generate heat and need to be lubricated and cooled. Transmission fluid dissipates this heat, usually by circulating through a radiator-type cooler. The fluid also has lubricating qualities, which keeps the transmission operating smoothly.

In an automatic transmission, the fluid plays an extra role, as it’s pressurized to select certain gear ratios based on your speed and other driving variables.

Over time, your transmission fluid wears out and gets dirty. It loses its lubricating qualities and picks up debris. Replacing the old fluid with fresh, new fluid at prescribed intervals is the best way to keep your transmission running smoothly and reliably.

This is why a transmission fluid service is part of every vehicle’s maintenance schedule. If you check your owner’s manual, you’ll see how often you should have this done.

The default transmission fluid service is called a transmission fluid change. But a new way of servicing transmissions, called a flush, came into widespread usage in the mid-1990s. Both of these services replace your transmission fluid with fresh fluid, but in very different ways. Let’s take a look at both and see which makes more sense for your car. 

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What is a transmission fluid change?

This is the tried-and-true process that your owner’s manual specifies when it calls for a transmission service. Your mechanic drains the transmission fluid from the transmission, replaces the filter, and then refills the transmission with fresh, new fluid. Gravity does all the work here.

In a manual transmission, this procedure lets you completely drain and replace nearly all of the oil. But in an automatic, this process will replenish only about half of the fluid. The other half stays in the torque converter and the lines running to the cooler. This is still a major improvement over not changing your transmission fluid at all. Some experts believe that it may actually be better to change the fluid gradually at the prescribed intervals, rather than all at once.

Another benefit of the transmission fluid change is that it lets your mechanic get a close look at the transmission pan and the filter. The pan is the access panel on the bottom of the transmission, and it’s removed when the fluid is drained. The presence of metal particles or other debris in the pan can indicate a problem that needs to be fixed before it damages or destroys the transmission. The condition of the transmission fluid filter can also reveal the presence of an issue that needs further attention.

The transmission fluid change is a low-impact process, with plenty of opportunities for your mechanic to determine the actual condition of your transmission. This makes it an ideal preventive maintenance procedure. 

What is a transmission flush?

The transmission flush process was created over 20 years ago. A machine was invented that could autonomously replace all of an automatic transmission’s fluid, without any supervision. At the time, this machine was greeted as a major advance, promising major cost savings, since there was no need for a mechanic to operate it. Many auto repair shops bought these expensive machines.

The transmission flushing process is straightforward. Simply fill it with the proper type and amount of transmission fluid, hook it up to the transmission cooler lines or pump inlet, and let it run. The old fluid, dirt and sludge is pushed out, and the new fluid replaces it. If necessary, a solvent can be run through the transmission after the old fluid is removed to clean it out.

But flushing has downsides. It usually costs more than a fluid change, because the cost of the machine must be included. Some shops see this as a high-profit procedure, so they mark up the price more than usual. These shops will suggest a transmission flush whenever possible, because it’s very good business for them.

Because the flushing process does not require the pan to be removed or the filter to be changed, the diagnostic and preventive benefits of seeing what’s in the pan and changing the filter are lost. Additionally, some mechanics believe that the high pressures produced by some of these machines can damage the sensitive seals and valves inside your transmission.

» LEARN MORE: Get an estimate for your transmission fluid service

Don’t get a flush unless you have to

Many manufacturers don’t include a transmission flush as part of their recommended maintenance schedules. Honda has actually recommended against using flushes in a service bulletin. The carmaker says it doesn’t want additives, solvents or non-Honda transmission fluid used in its transmissions. 

When in doubt, stick to the manufacturers’ service recommendations in your owner’s manual. If the manual doesn’t call for a flush, don’t do it.

The one exception to this would be if your transmission fluid becomes contaminated with debris. In this situation, you want all of that bad fluid out of your transmission, and a flush will do this. Your mechanic should follow the manufacturer’s instructions for the best way to perform this procedure. To be thorough, the pan should also be dropped and the filter checked and replaced.

Keep in mind that if your transmission fluid is in such bad condition, there may be other related problems that will need attention. These should be addressed at the same time.

 

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