From gasoline and oil to coolant and power steering fluid, motor vehicles demand all kinds of solutions to run smoothly. While some of these fluids don’t necessarily have to be topped off as soon as they run out, like windshield washer fluid for example, others are integral to the engine’s operations.
Of all the solutions to monitor closely, transmission fluid is one of the most important. The transmission is responsible for delivering power to the wheels, so they actually spin. As such, it plays a critical role in getting you from point A to point B.
Transmission fluid facilitates this process by reducing friction, building pressure, and preventing overheating. It also keeps gunk from accumulating on the mechanical components it flushes past. The transmission houses your vehicle’s gears (in gasoline powered cars) and transmission fluid keeps those gears from wearing down prematurely.
Of course, since you can’t drive in a sterile vacuum, impurities will inevitably make their way under the hood. Excessively high temperatures, leaks, inadequate maintenance, and cross-contamination can also hurt the quality of the transmission fluid—and, subsequently, its effectiveness—over time.
Fortunately, there’s an easy way to determine if the solution is still doing its job: check its color. As long as you know what each hue means, you’ll be able to tell right away whether the fluid needs attention.
How Do I Check the Transmission Fluid?
Most mechanics recommend checking the transmission fluid every six months or so. If you’re putting a lot of miles on your vehicle, though, it may be wise to check the fluid every three to four months so you can addess any issues right away to ensure optimal performance.
When that time rolls around again, park on a level surface and pop the hood. Unless the operating manual instructs otherwise, leave the engine running. Letting the car warm up for a few minutes will simulate normal operating conditions. If you were to check the level after turning off the engine and letting the vehicle cool down, you could get a low reading that’s false, because transmission fluid contracts as the temperature drops.
Locate the dipstick for the transmission fluid, making sure not to confuse it with the one for the engine oil. In most cars, the transmission fluid dipstick is farther back than the engine oil one. It may say “transmission” on it, and it will probably have a yellow, pink, or red handle.
Pull out the dipstick and wipe it to ensure an accurate reading. Then, dip it back in the reservoir before pulling it out again. Review the fluid line on the indicator at the end of the dipstick to determine whether the solution is low. Finally, wipe the dipstick on a paper towel or clean, white rag, so you can evaluate the color.
Note: If you cannot find a dipstick for the transmission fluid, your vehicle probably has a “filled-for-life” or “sealed” transmission. In order to check the solution, you’ll have to take your car to a mechanic that can lift it off the ground for you. Signs that the fluid in a sealed transmission needs attention include:
- A grinding sound
- A delay in accelerating or decelerating
- Red, brown, or black fluid leaking from under the front of the vehicle
- A smell like burnt toast
What It Means if the Transmission Fluid Is…
Bright Red
When transmission fluid is new, it’s a vibrant shade of red. As long as it’s translucent, a red solution means the fluid is in excellent condition, and there’s no need to flush it anytime soon.
Dark Red
As transmission fluid flushes cycles through the various mechanical components, it picks up contaminants, which darken its color. It also oxidizes over time, causing the solution to lose some of its original luster. If your transmission fluid is dark red, though, it’s still in good condition and is effectively doing its job.
Light Brown
Over time, dark red transmission fluid will take on a brownish hue. This happens as the solution ages and accumulates more impurities. While light brown fluid doesn’t facilitate optimal performance, it will still lubricate and cool critical components as long as it’s fairly transparent. At this point, though, it’s wise to flush the system and replace the fluid entirely, or you’ll have an opaque, sludgy mess before you know it.
Dark Brown
Dark brown transmission fluid is no longer able to do its job. If the solution gets to this point, it needs to be changed as soon as possible. Otherwise, the transmission will be subject to excessive wear and tear—not to mention incredibly high temperatures—every time you get behind the wheel. If left unchecked, this can eventually result in total system failure.
Black
Once transmission fluid has oxidized fully, it will turn black. At this stage, it’s no longer effective and needs to be flushed out and replaced right away. Depending on how long the solution was essentially useless, additional repairs—or total replacement—may be necessary to restore the transmission to its full functionality.
It’s worth noting that black transmission fluid is often accompanied by a burnt toast-like smell. If you notice such an odor when the engine is running, stop driving your vehicle, and drop it off at your mechanic’s as soon as you can.
Pink
If coolant or some other liquid finds its way into the transmission—through a damaged cooler line, for example—it will water down the fluid, resulting in a pinkish hue. Such an issue demands immediate attention because even mildly diluted transmission fluid won’t be able to do its job. If left unchecked for too long, you’ll end up having to rebuild or replace the transmission.
What Other Features Should I Evaluate When Checking the Transmission Fluid?
Color is just one aspect of transmission fluid that indicates its condition. It’s also wise to evaluate the solution’s consistency and smell. Don’t forget to check the level, either, as the fluid may need to be topped off periodically.
Consistency
Regardless of its color, transmission fluid that’s not entirely transparent is problematic. If it’s foamy, frothy, or milky, for example, it’s probably been contaminated with coolant or water. In such a scenario, it’s imperative to not only flush the system and replace the solution but also to repair the underlying cause of the leak.
Smell
When it’s in decent or better condition, transmission fluid is practically odorless. As it oxidizes, though, it can develop a kind of burning smell. In most cases, if you notice such an odor, the solution has been degrading for a while, and at least some mechanical components have probably already sustained damaged.
Level
Transmission fluid dipsticks have indicators to let motorists know how much solution is left in the system. When evaluating the color, make sure to check the level, too. If it’s getting low but the fluid is in good condition, you can top it off to ensure adequate lubrication for the miles ahead.
How Long Does Transmission Fluid Last?
On average, transmission fluid needs to be replaced every 30,000 to 100,000 miles. This is a fairly wide range because the speed at which the solution degrades depends on a variety of factors, which you can read about below.
Transmission Type
Generally speaking, the transmission fluid in automatic vehicles degrades more quickly than the fluid in manual vehicles. This is because automatic transmissions generate more heat, which shortens the lifespan of the solution.
Fluid Composition
Transmission fluid comprised of organic compounds breaks down faster than solutions comprised of synthetic materials. Because synthetic fluid can withstand much higher temperatures, it’s ideal for motorists who frequently push their vehicles to the edge.
If you often use your pick-up truck to tow heavy trailers, for example, you’ll want to stick to synthetic transmission fluid. The same goes for adventure seekers who frequently take their cars up and down mountain passes or off-road on treacherous terrain.
Driving Habits
Motorists who tend to drive aggressively—by accelerating and decelerating rapidly, for example—demand a lot from their transmission. The constant strain on the various mechanical components generates a lot of heat, which speeds up the degradation of the transmission fluid.
Driving off Into the Sunset
The transmission plays a critical role in your vehicle’s operation. To avoid having to rebuild or replace the system, check the fluid at least twice a year.
Once it’s degraded according to our color guide, have your mechanic flush the system and then refill it with new solution. Combined with other essential maintenance, this will ensure your car, truck, or SUV remains reliable for tens of thousands of miles.
