We both know why you came here. Your transmission is one of the most expensive parts in your vehicle and you’re wanting more information on whether a flush or fluid change is the right choice. Regardless of the costs of both auto vs manual transmissions, both require service to properly perform. If you haven’t already noticed, although similar in concept, both a flush and a fluid change are two completely different services. 

Fluid Change

In a fluid change, also known as a transmission service. the pan is drained of transmission fluid and the filter is replaced but it doesn’t remove all the fluid. This service removes between 30%-40% of the fluid whereas the rest can be in the cooler lines and the torque converter. In this process, the gasket can be replaced and the fluid screen cleaned. Between 4-6 quarts of transmission fluid is needed for this service. 

Every 30,000 miles or 2 years is the recommended service time for fluid changes, but this can be reduced to 15,000-20,000 miles under heavy use such as towing or driving harder than the average person. Your owners manual will give a better service timeframe for your make and model, but always consult a certified mechanic if in doubt. 

As mentioned before a fluid change doesn’t remove all the fluid in the transmission. If the fluid was contaminated there could still be build up throughout the system and the old fluid will be mixed with the new. In a sense, this fluid might not be as “new” as you think and it still will work, but could possibly have diminishing performance compared to a flush. You can always self-check your transmission fluid to determine levels and the condition.

Flush

When a transmission flush is performed all of the fluid is drained and new fluid is given. To make sure the fluid has removed the use of either a pump inlet or a cooler line flush machine is used. The other name for a flush is a transfusion hence the machines used. Roughly 80%-95% of the fluid is removed and 10-12 quarts is replaced. It is impossible to remove all the fluid from the transmission since some of the fluid resides in the cooler lines and the torque converter. The filter is changed during the process and the gasket and cleaning of the screen are also performed if applicable. 

Every 45,000 miles or 3 years is the recommended service time for a transmission fluid flush. Of course, towing and harder driving can reduce the service interval. Flushes, since it is more extensive, can replace fluid changes altogether. 

The best performance comes from all new fluid. Your transmission will receive the most protection to the gears and bearings from wear and run cooler. One downside is that flushes may dislodge buildup in the valve body but unlike a fluid change, you can use additives to assist in cleaning up buildup.

Which to Choose?

First, consider the last time your transmission fluid was serviced. If unsure you should keep track of your service intervals and miles. Next, check your fluid levels and the contamination levels for this will give the best sign of the fluid condition. As stated before, a transmission fluid change and a flush are two different services and should be well evaluated.  In general, flushes are more expensive but this service reduces the most contamination in the fluid. One stipulation is transmission failure after a flush but it can’t be blamed on the flush itself. It is due to improper maintenance and not meant to fix existing problems with the transmission. Regardless, if you are unsure about what service you need it’s best to consult a certified mechanic for they will be able to directly tell you what service you require.