When they help and when they hurt your car
If you’ve ever read the phrase “lifetime smooth” when referring to vehicle maintenance, you might be tempted to breathe a sigh of relief. That sounds like one less bill to worry about. However, in this context, lifespan is more of a marketing term referring to the length of your vehicle’s factory warranty, not its actual lifespan beyond that.
Although a transmission flush is sometimes considered a dealer upsell, it is one of the best routine and preventative maintenance items you can perform, likely saving you from a more expensive repair later. However, a transmission flush is not always a one-size-fits-all solution, and there may be other factors to consider.
Depending on the vehicle’s current mileage, transmission design, and service history, a drain and fill service might be the best option.
What happens during a transmission flush
Cleanses the inner brain
A transmission flush (also called a fluid exchange) is a complete maintenance procedure that replaces the entire volume of fluid in your transmission. Transmission fluid is one of the most essential fluids in your vehicle, responsible for operating the torque converter, internal hydraulics as well as lubricating, cleaning and cooling the transmission itself.
Using a specialized machine connected to the cooling lines, a technician will essentially replace all the fluid while the engine is running (their machine works in tandem with the transmission’s own internal pump). As the pump pushes old oxidized fluid into the machine’s waste tank, it simultaneously reinjects an equal amount of fresh fluid into the system.
There are three main reasons why a transmission flush can be a good preventative maintenance item:
- Eliminates contaminants: Over time, your transmission generates microscopic metal shavings and carbon deposits from the clutch plates. A flush acts as a purge of the system, ensuring the removal of these abrasive materials. The new transmission fluid will also contain new detergents, which will help keep the system clean in the future.
- Cleanses the internal brain: The valve body is the hydraulic brain of your transmission, a maze of passages and solenoids that route fluid to the clutches so your car can shift gears. As this fluid ages, it breaks down into a sticky varnish that can cause difficult shifting. Flushing uses cleaning agents that safely dissolve buildup before new fluid is introduced, effectively unclogging the arteries in the valve body.
- Restores friction balance: Transmission fluid is a chemical cocktail containing friction modifiers, additives that allow internal clutch packs to seize and loosen. When these chemicals wear off, you might experience a slippery feeling. Because a flush replaces the entire volume of fluid, it restores these frictional properties to the entire transmission.
While I was a service manager at the Luxury Auto Mall in Sioux Falls, a customer with a Saturn Relay minivan (the location was a GM dealership before the 2011 rebrand) reported a multitude of acceleration and shifting problems. When the service technician performed a multi-point inspection, he found that the fluid was as black as coffee.
The Relay minivan had just under 73,000 miles, purchased by this person a few months earlier (in spring 2013). Normally at this time we recommended transmission fluid services at 50,000 miles for all GM vehicles. However, this particular Relay had never been in our workshop before, and the new owner wasn’t very clear about the minivan’s previous maintenance history.
In both cases, however, the previous owner and the new owner used the minivan as a family taxi and thus filled it to capacity with cargo and occupants. That constant payload in the minivan over 73,000 miles and missed transmission fluid maintenance could have been to blame (we ended up having to replace the transmission and recommended switching to synthetic oil).
In reality, the transmission fluid in a vehicle subjected to heavy use like this and harsh South Dakota winters should have been changed at 50,000 miles, if not sooner. The same goes if you own a vehicle that you frequently tow or if you drive in traffic jams. Although “lifetime” fluids are high quality, they don’t last forever, especially in situations like this.
Your Brake Fluid Is Failing: Why Flushing Isn’t Just an ‘Upsell’
Brake fluid flushes are essential for your wallet and your safety.
Drain and refill vs. full exchange
Two different services with distinct objectives
The difference between a drain and fill system and a flush is like a sink full of dirty dishes.
Draining and filling is like pulling the sink strainer for a moment, then turning on the faucet; you added clean water, and that helps, but some of the original food particles and foam are still swirling around. As we saw above, a flush is more like running the faucet while the sink drains, then letting the water run through the pipes until the sink is empty. At this point you fill it again with new water.
During a drain and fill, the technician removes the drain plug or drops the transmission pan to empty the reservoir, replacing approximately 30 to 40 percent of the total fluid. The rest remains inside the torque converter, valve body and external cooling lines. This is a great maintenance item, but not a total system refresh.
At Luxury Auto Mall of Sioux Falls, we sometimes recommend draining and filling instead of flushing. When vehicles with 100,000 to 150,000 miles and little or no maintenance history arrived, a flush wasn’t always the default option. With higher mileage like this and no solid service history, we were concerned about dislodging debris with a more forceful flushing of the system.
In these scenarios, a gentler drain and fill was the safest solution, as breaking up existing debris in the old fluid can lead to a host of transmission problems later. Transmission design also plays a role. For example, Hyundai vehicles equipped with a seven-speed dual-clutch transmission require draining and filling rather than a full flush.
Topping Up Coolant Isn’t Enough: 5 Reasons You Need a Flush
Although new vehicles may have “lifetime” coolant, there are some exceptions.
When to Service Transmission Fluid
Mile intervals are best, but there are other barometers
Unlike your engine, which probably has an oil life monitor that lights up in the instrument cluster, most cars don’t have a sensor that monitors the quality of the transmission fluid. Instead, rely on these rules of thumb:
- Mileage intervals: Many manufacturers recommend fluid servicing every 30,000 to 60,000 miles. In your owner’s manual, you can find all the recommended maintenance items listed for your make and model, along with a mileage interval indicating when this maintenance should be performed.
- Visual test: If your vehicle has a transmission dipstick (which is admittedly rare these days), check the color. Fresh liquid is usually bright red. If it appears dark or burned, it has lost its ability to handle heat and keep the transmission clean.
- Performance indices: If you notice shuddering, hesitation, or difficult shifting while accelerating, degraded transmission fluid could be the cause.
Protect your vehicle and your wallet
Choosing between flushing and draining and filling depends on the design and service history of your vehicle. Either way, both are better for your transmission than taking your chances with old fluid. A professional fluid service typically costs between $200 and $450, while a transmission replacement can easily cost $5,000 or more.
When in doubt, consult your owner’s manual and speak to a trusted mechanic, as they can help you choose the most appropriate service for your make and model. Either way, your gearbox will eventually thank you.
