Services that you DON’T need: Transmission Flushes

airdrie car maintenance

A popular up-sell at lube shops and other auto service businesses is the transmission flush. A transmission flush (as described in this article) involves connecting a machine to your vehicle’s transmission cooler lines, and letting the machine pump new transmission fluid into the vehicle while it draws the old fluid out.

So, what’s wrong with that? Indeed, changing your transmission fluid is very important maintenance. The problem with a transmission flush is how the service is being performed. Most car manufacturers recommend only draining your transmission fluid, and then refilling it. Read on!

Fact #1: On many vehicles, transmission flushes are an incomplete service.

transmission flush
The transmission pan removed, showing the metal filter inside.

Hooking up a transmission flush machine is a quick, easy money-maker for shops that perform this service. However, it neglects the most important part of servicing the transmission: changing the filter. About 60% of vehicles have an internal transmission filter that should be replaced every time the fluid is changed. Changing the filter is time consuming and messy, but it’s part of the complete job. This is where we like to illustrate the difference between a transmission flush and a transmission service.

From our page on transmission fluid:
transmission service involves a lot more work than a flush. In addition to replacing the fluid, this service also includes replacement of your transmission filter, if equipped. Your transmission filter is usually accessed by removing a pan or cover on the bottom of your transmission. While we’re inside there, we also clean the inside of the pan, including special magnets that are installed to trap metal filings and debris. Some transmissions also require certain adjustments while the pan is removed. Checking your vehicle’s transmission control module for software updates, (and installing them if required) completes our full and correct service of your transmission.

Fact #2: Car manufacturers recommend against transmission flushes.

Most automakers recommend against using an external machine to flush transmission fluid. Among other reasons, this is because these machines, (especially when connected improperly) can block or pressurize the wrong passages inside the transmission and cause damage. Some machines will run the transmission pump dry as they remove more fluid than they add back in. Flush machines, unless meticulously cleaned out between different vehicles, can also pump a small amount of the wrong fluid into your transmission. Even if there’s only a small chance of your transmission being damaged by a flush machine, why risk it?

Fact #3: Transmission flushes can damage your transmission.

As mentioned above, a transmission flush (using an external machine) may not be good for your transmission. Here is an excerpt from a General Motors technical service bulletin that provides their opinion on transmission flush machines:

airdrie transmission repair

Fact #4: A conventional transmission service doesn’t replace all of your transmission fluid. (And that’s okay.)

Proponents of transmission flushes will often argue that a flush is a better service because it replaces more of your transmission fluid. It’s true that removing your transmission pan or draining your transmission via its drain plug (as your car manufacturer intends) only removes about 70% of the fluid inside. Some fluid is trapped inside the cooler or torque converter, and can’t be drained out. It’s also true that a flush machine will replace a higher percentage of the fluid; most machines advertise about 90%.

However, replacing 70% of the fluid is not a problem. Car manufacturers know how much fluid will be drained out during a normal service, and they have adjusted their maintenance schedules to account for this. As long as your transmission fluid is being replaced in accordance with the schedule in your owner’s manual, you’re giving your transmission all the maintenance that it needs.

Going from bad to worse: What are transmission flushing chemicals?

Transmission flushing chemicals are solvents or detergents that are added to your transmission before flushing the fluid. The vehicle is allowed to run for 10-15 minutes before the flush, in order to circulate the chemical through your transmission. The theory is that these chemicals will help to loosen up or clean out dirt and varnish from inside the transmission. However, there are a few reasons why transmission flushing chemicals should not be used:

  1. These chemicals are completely unnecessary. Because of transmission fluid’s high detergent content, almost every transmission on the road is absolutely spotless inside already. If your transmission has excessive clutch material or debris inside, this is due to a pending failure of the unit, and no flush will fix this. 
  2. Almost every car manufacturer recommends against using transmission flushing chemicals, and most will void your transmission warranty if they can prove these chemicals are being used.
  3. These chemicals can damage your transmission, which is why most automakers recommend against them. One of the reasons for this is because most transmission flush machines only achieve 80-90% fluid replacement, some of the chemical will always be left inside your transmission after the flush. 
airdrie transmission repair
While your transmission fluid will get dirty over time, that’s because it’s doing its job! The inside of your transmission is very clean.

Do you have any questions about servicing your transmission properly? Please give us a shout any time!