Why You Need To Change Your Transmission Fluid

Car transmission mounted to the frame of the car

Changing the transmission fluid routinely is one of the most important preventative services you can do for your car. Most people don’t bother and most mechanics won’t even remind you. Not changing the transmission fluid can cost you thousands of dollars later down the line if you’re planning on keeping your car for long.

It’s a good idea to change the transmission fluid because, over time, the fluid inside the transmission becomes darker from all the gears constantly grinding and engaging each other which results in metal particles floating around. Eventually, the transmission fluid gets darker and the transmission filter gets clogged. If it gets bad enough, the transmission will begin to slip and not shift gears properly.

What this means for you, is that if you wait too long, on an automatic transmission, you risk damaging your transmission by not replacing the ATF fluid. If the mileage is already too high and there’s no record of a previous transmission flush service, there is a risk the transmission can fail even quicker.

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Your car’s transmission fluid deteriorates due to the constant gear changing every time you stop-n-go and accelerate. Over the years, the fluid color turns from light red to black from all the metal particles circulating. Eventually, the viscosity thickens and your transmission can begin to slip.

By then, you’ll either need a rebuild or a new transmission. To prevent this, the transmission fluid should be serviced every 30,000-50,000 miles on automatic transmission and 50,000-80,000 miles for manual or standard transmission.

When a dealership says, ‘lifetime,’ what they really mean is 100,000 miles. Most of us easily drive more than that over the course of the years or purchase vehicles that already have that kind of mileage.

If the transmission fluid was never serviced before and it already has high mileage, there’s a chance the transmission can fail prematurely. Make sure any car you’re planning on buying has records of routine transmission fluid changes.

Transmission History

The first cars in the 19th and early 20th centuries were all manual. There were multiple levers, switches, and operating cranks to keep the car moving. As things improved over the years, the experience of driving a car became more mainstream.

With the new improvements, the automatic transmission was introduced in the 1930s. This enabled driving without having to be so involved in gear changing. Interestingly enough, most cars in Europe right up until the 21st century were manual whereas here in the US automatics gained traction earlier on.

Over time, automatic transmissions became more complex and various iterations appeared. From the 3-speed and 4-speed, automatic transmissions eventually evolved into 6-speed and beyond. It’s not uncommon to have 9-speeds in luxury line vehicles. There’s also CVT, dual-clutch, and more. All of them have one thing in common….they have transmission fluid to lubricate the gears.

Why Change The Transmission Fluid?

Transmissions have many moving parts all working in synchronicity to move you forward or backward. Automatic transmissions are much more complex than a standard transmission due to the hydraulic pressure plate, TCM, ECU, and more amount of gears.

Symptoms of a failing transmission are:

  • Hard Shifting
  • Transmission slipping when engaging in certain gears
  • Won’t go in ‘Reverse’
  • Check engine light and/or A/T lights come on
  • Vehicle won’t go into gear

I had a friend who went to a car service shop and insisted on having his transmission fluid flushed at 140,000 miles. They strongly advised against it, warning him that if it was never done before, there’s a chance it’ll start slipping and he’ll need to rebuild it or buy a new transmission.

He decided to do it anyway. After 300 miles, sure enough, the transmission began to slip. When I asked him if the service shop offered him a less evasive procedure called, transmission drain, he replied, ‘they did not.’ Normally the transmission will fail within 100-500 miles after a transmission fluid service.

Because there are fewer moving parts in a standard transmission, the fluid can be replaced less often but it still needs to be done. Regardless of the transmission type, waiting too long can cost you thousands of dollars down the line.

If the dealership says you never have to change the transmission fluid, what they really mean is you need to come in at that point and buy another car.

Transmission Flush vs Transmission Drain

I can’t stress this enough, but it’s really worth reading the owner’s manual. The reason being is because it tells you exactly when the manufacturer NOT your local dealer recommends you change the transmission fluid for your car. It’ll also tell you exactly what kind of transmission fluid works for your car.

A transmission flush is when a machine is hooked up to the transmission when the car is on a lift, and the mechanic completely removes all the transmission fluid not just what’s in the bottom transmission pan but in between all the gears as well.

A transmission drain is less evasive and oftentimes safer because it leaves the fluid in between the gears and only removes the sediments and fluid that’s in the transmission fluid pan. This means that the old fluid is topped off and mixed with new fluid. This procedure is especially important if the transmission fluid was not changed for a long time.

Draining the car's transmission fluid
Transmission fluid

Mixing the old and the new fluids minimizes the risk of the gears slipping. In fact, Honda strongly recommends against transmission flushes altogether because of this.

On one of my previous cars, no one ever changed the automatic transmission fluid and it already had 121,500. So I partially drained the transmission fluid initially and every 25,000 miles after that. I did a transmission drain specifically to ensure that it’s constantly mixing the old and new fluids.

If you’ve always had the transmission fluid changed since you bought the vehicle, then yes, you can do a transmission flush, otherwise, stick to transmission drains. After 4-5 services of draining, you can do a transmission flush with less risk.

How To Maximize Your Transmissions Life

Most vehicles nowadays are basically computers on wheels. Computerized brakes, sensors, driving monitors, accident logging, and shifting patterns. This means that driving a car has never been easier since the shifts are always perfect and perfectly controlled electronically.

The transmission is usually controlled by a TCM (Transmission Control Module) or ECU (Engine Control Unit). If you only have an ECU and TCM that means there’s only one computer that controls both the engine and the transmission. Not all vehicles have a separate dedicated TCM.

Sometimes the TCM & ECU computers need rebuilding or firmware re-flashing to improve shifts. Check with your local dealership for any factory recalls or firmware updates if you’re experiencing hard shifts.

If you’re TCM or ECU has transmission fault codes you can use a diagnostic scan tool to find out what the exact trouble code is. You can check out my favorite scan tools here.

As a driver, there are a few habits and maintenance tips you can be aware of that will help ensure your transmission is in good order.

On an automatic transmission, when you shift into gear (Reverse of Drive), give it an extra second or two until you hear or feel the gear actually engage. Sometimes it’s not audible especially if it’s a CVT. There are multiple gears that have to engage successfully before you can accelerate.

Inside of a transmission
Inside of a transmission

Trying to accelerate whilst the gear is trying to engage, can cause friction and metal debris. It’s quite audible on a manual transmission because when you don’t fully depress the pedals or engage the gear all the way, you’ll hear that grinding sound and it’ll vibrate through your hand informing you instantly.

The other tip is not as easy to do on newer vehicles because many newer cars don’t even offer it anymore. Check the transmission fluid dipstick. It should be a more reddish color. Use a white napkin to see the color. If it’s black, you need to service it soon.

Oftentimes, metal debris gives off a distinct odor. Experienced mechanics will know right away how good or bad the transmission is. If your car doesn’t have a dipstick then you’ll need to lift your car and drain the transmission slightly to check the fluid.

Lastly, when you do replace the transmission fluid, make sure to purchase the transmission filter kit. The kit generally includes the transmission gasket, filter, and sometimes a new drain plug. Once you replace the transmission filter, you can see all the metal debris that accumulated on the filter itself.

Conclusion

Transmissions tend to be an out of sight out of mind component of your vehicle. It’s easy to forget that oftentimes it’s doing a good amount of work to move your car and that constant wear-n-tear deteriorates the fluid.

Oftentimes, a transmission drain is much safer than a transmission flush due to it being less invasive. Check your owner’s manual to see when the manufacturer recommends a transmission fluid change and what fluid type to use. If you’re not sure, consult with a professional mechanic.

Dealerships tell you not to worry about it, and most people don’t even know until it’s too late. Hopefully, if this guide is of any help, you’ll be at the very least aware of the maintenance upkeep required to ensure your transmission lasts as long as possible and know what service type is best for you.

Did you ever have to change the transmission fluid in your car? If so, what is a flush or drain?

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Ernest Martynyuk

An automotive enthusiast who's been tinkering with vehicles since I was 15-years old. Repairing automotive electronics has been my main job for over a decade now and have a passion for everything technical regarding cars.

2 thoughts on “Why You Need To Change Your Transmission Fluid”

  1. If my transmission pan & filter were changed cause of a leak. Could that cause my TCM to break down. When I got my car back after repair I had to take it back cause of it still leaking. They gave me a warranty. So I decide to take it back. My car has been at Machanic over 4 wks the say the tcm went out. Now all the fuses also went out . I have no car.

    Reply
    • What year, make, and model is your vehicle? Sometimes the TCM unit is indeed at fault whether it’s because of software or hardware errors. Have your mechanic provide the codes that his scan tool is displaying which should tell you where is the fault is or if that’s a common issue for that specific model. Fuses going out could mean anything. If the wiring harness has a short or poor grounding that can affect the fuses. Which fuses are going out to be exact?

      Reply

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