Transmission Flush: The Definitive Guide

A question a lot of people with automatic transmissions ask during their car’s lifetime is whether or not they should get a transmission flush. You may have heard of a transmission fluid change before but you probably aren’t so familiar with an automatic transmission flush. It might sound quite different, and that’s because it is. Whereas a fluid change only removes some of the fluid, a transmission flush works to replace all of the old fluid with new fluid while eliminating contaminants and dirt as well.

People with automatic transmissions often prefer a transmission flush because it’s more effective than a fluid change, because removing all of the old fluid and adding new fluid helps to improve the fluid’s effectiveness, keeps transmission temperatures down, and helps to extend its lifespan.

What is a Transmission Flush?

transmission flush machineSo what exactly happens in a transmission flush? As we have explained a flush removes all of the old fluid and swaps it with new fluid, but how? With a professional transmission flush machine of course, which is either a cooler line flush machine or a pump inlet flush machine.

Cooler line flush machine:

With a cooler line flush machine there are two lines that will be used. The line in connects to the transmission and is responsible for draining the fluid while the line out connects to the cooler and is responsible for adding the new fluid. New fluid is added to the flush machine and when the car is turned on, the transmission’s pump naturally cycles out the old fluid and quarantines it while new fluid flows in, so no damage to the car can occur. The machine does not produce any pressure and all of the work is done by the vehicle.

Pump inlet flush machine:

A pump inlet flush machine works in a different way. After the pan and filter are removed, the machine attaches to the pump intake where it lets in new fluid only, and as the fluid passes through the transmission it gets disposed of into a collection tray. All of the old fluid is removed and replaced with fresh, new fluid. A new filter is then installed, the pan re-attached, and topped off with new fluid in order to reach the proper levels. This method uses more fluid and costs a little extra but it’s often more effective.

**Important Tips When Getting a Transmission Flush**

  • Always have the pan dropped, as this can help to diagnose any potential problems
  • Always have the filter changed, as particles collect over time and need to be removed
  • Make sure that nothing gets forcibly pumped back into the transmission as this can cause damage
  • Be sure that only the preferred transmission fluid specified in the owner’s manual is used
  • Unless you know what you’re doing, always have a qualified mechanic perform the transmission flush

Why Should I Get a Transmission Flush?

A transmission flush is more effective than a fluid change because it replaces all of the old fluid while removing residue and contaminants that have built up over time. In a regular fluid change, only some of the transmission fluid is removed, which is only the fluid that can be drained out by natural gravitational forces. A normal fluid change only removes around a third of the total amount of transmission fluid. Fluid can still remain in the cooler lines and the torque converter though; up to 40% can remain in the torque converter alone! That old fluid ends up mixing with and contaminating the new fluid, reducing the effectiveness of the fluid altogether. That is why some suggest getting multiple fluid changes in order to dilute as much of the old fluid as possible although this tends to be more expensive. This is what ATRA thinks about the service.

An automatic transmission flush on the other hand replaces all of the old fluid with fresh, new fluid which helps to improve its overall performance. As you probably know, automatic transmissions produce a lot of heat and requires transmission fluid for cooling. However fluid doesn’t last forever, and the more heat it is exposed to, the faster it will break down. So any old fluid that has been packing on a lot of miles can end up diminishing the properties of the new fluid if they end up mixing. So while a fluid change is an effective service for replacing some of the old fluid, a transmission flush is the most effective service for your transmission. But don’t take our word for it, this is what ATRA thinks about it.

But you might not understand how severe overheating is. Over 90% of all automatic transmission failures are caused by overheating and the leading cause is because of ineffective transmission fluid. When fluid temperature rises above 220°F the fluid ends up deteriorating quickly, lessening its ability to cool and lubricate the various parts of the transmission. Around 220-240°F is when varnish and sludge starts to form. 240-260°F and seals start to harden. 260-295°F and plates start to slip. Over 295°F and seals and clutches burn out and the transmission fluid forms carbon. Now you can really understand why so many people insist that you take care of the transmission fluid, because serious problems can occur otherwise. Even a 20°F drop in fluid temperature can double the life of the transmission!

transmission flush - transmission fluid life expectancy

When to Get a Transmission Flush

Now this really depends on your personal preferences. Transmission flushes can be performed as frequently as you’d like, but they are often pricier than a fluid change and you can’t do it yourself unless you have the proper equipment. So unless you want to burn a hole in your wallet, we suggest getting fluid changes every 30,000 miles/2 years and transmission flushes every subsequent period, or even perhaps after every two fluid changes. You could also replace fluid changes with transmission flushes altogether if you really want your transmission to last, but that’s if you have the money to spend.

A better method to gauge when to get your transmission serviced is to check the condition of the fluid. This can tell you a lot about your transmission’s health and whether there are any problems. Color, smell, and consistency are what you should be measuring. Generally, fluid should have a red translucent color, no odor, and a thin consistency with no particles in it. Worn fluid is often dark brown, might smell burnt, and can have particles floating around. If the fluid is black, smells like burnt toast, or has big particles or metal shavings in it, this could mean there are serious internal transmission problems. Transmission failure might be imminent if you allow it to run on the current fluid. Ideally you want to have your transmission serviced before it gets to this point.

If you are currently experiencing common transmission problems (such as grinding, shifting problems, shaking, strange noises, or slipping gears), this could also indicate that the transmission needs to be serviced.

transmission flush - transmission fluid colors

transmission flush - transmission fluid colors

-Make sure that the fluid doesn’t get too dark, you want to avoid burnt fluid as much as possible.

When Not to Get a Transmission Flush

A transmission flush isn’t some magical, cure-all service that can solve any problem. If your transmission has serious internal problems or is already failing, a transmission flush can’t save it now. A transmission flush is meant to be part of routine maintenance and not something you get when you start experiencing problems.

If it’s already failing, our best suggestion is to ride that baby to the grave, because getting a transmission flush now might cause it to fail even sooner. The reason most transmission flushes fail is because the owners bring in their cars too late, when problems have grown too big. And as soon as the transmission fails, the owner rushes to the conclusion that it’s the fault of the transmission flush and not the failing transmission itself.

If you want to try to salvage your car then by all means get a transmission flush, but don’t count on it to perform any miracles. A transmission flush is not supposed to be a solution, and the bad things you hear about them are often from owners who don’t take care of their cars until problems occur. Ask an owner that gets a flush on a routine basis compared to an owner who doesn’t get one until problems come up, you’ll see that you get answers on both ends of the spectrum.


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