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Transmission Fluid - To Flush or Not to Flush

Updated: Jan 25

Little secret...your transmission came with NEW fluid from the factory. Fresh fluid is a good thing. Healthy fluid will protect your transmission, not destroy it. Your transmission's greatest chance for survival depends on healthy fluid. So why do we hear comments like these?

"You shouldn't flush your transmission."

"My transmission started slipping after I replaced the fluid."

"I have a sealed transmission with lifetime fluid, I'm not supposed to service it."

Flushing your transmission means to completely exchange the old spent fluid with new fresh fluid. It's not a partial drain and fill, it's a complete replacement. Why would that be a bad thing? Before getting into that, let's look at the importance of good fluid.

Why Transmission Fluid Is Vital?

An automatic transmission is a large hydraulic pump that utilizes gears, clutches, valves, and fluid to multiply torque. The fluid is under constant stress from heat, pressure, and shearing forces. To do its job well, ATF (automatic transmission fluid) has been engineered across decades of abuse for optimal performance and durability. Check out just a few of it's modern properties:

  • Friction modifiers (maintain ideal clutch force and slipperiness)

  • Anti-wear & extreme-pressure additives (friction & wear control)

  • Corrosion inhibitors (protective barrier on metal surfaces)

  • Detergents (clean & protect surfaces)

  • Dispersants (prevent sludge, varnish and deposit formation)

  • Surfactants (reduce surface tension between fluid & parts)

  • Kinematic viscosity improvers (flowability of the fluid)

  • Seal swell additives & gasket conditioners (prolong o-ring, seal, & gasket life)

  • Boil-off inhibitors (heat protection)

  • Anti-oxidation & foam compounds (evaporation, cavitation)

  • Pour point depressants & high temp thickeners (keep fluid liquid, maintain viscosity)

Why not replace it?

If an automatic transmission is built around this vital fluid, why not keep that fluid in as-close-to-optimal condition as possible? Well the answer is, you should. So why the horror stories, debate, and differing opinions? The answer is not if, but when.

What happens to transmission fluid with time and mileage?

Transmission fluid does not exist in a vacuum. Additives will deteriorate or degrade as they perform their function. Transmission fluid is petroleum based, and can burn and chemically change with extreme heat. The fluid picks up moisture as it vents to atmosphere and experiences temperature changes. As friction modifiers break down, the fluid loses its slipperiness, accelerating clutch wear. Clutch wear is the lurkring symptom of spent fluid.

The more broken down the fluid becomes, the more the transmission's wet clutches wear, overheat, and ultimately slip. Fresh fluid allows the clutches to slip on the protective barrier of the fluid itself until full hydraulic clamping force completely engages the desired gear. With spent fluid, that fluid barrier is diminished, allowing for aberrant slip and wear of the clutches.

Without fluid protection, clutch wear accelerates. The material sloughed-off becomes a property of the fluid. Some is filtered out, but much of it remains suspended. This acts as an abrasive on clutches and bands. However, because it adds a friction coefficient between plates and clutches, it may mask slipping.

Sooo.....Should I Replace the Fluid?

The answer is yes...unless, you're too late. A transmission is very expensive and complex. Staying ahead of the fluid is your best insurance. I've been heard to say, "If people would just change their transmission fluid one time during ownership, they might never have a transmission problem."

I have replaced transmission fluid on vehicles with well over 200,000 miles with no history of transmission maintenance. If there's life left in the clutches, you're going to get the most out of them with full functioning fluid.

Unfortunately, many people only consider replacing their transmission fluid when they're having problems. If your transmission is already slipping, new fluid will make it seem worse. In reality, the damage was already done. The new fluid is not to blame, rather the old fluid failed and was hiding the problem.

Things to Consider before Replacing Fluid

How do I know if I have a slipping condition if the old fluid is masking it?

  • RPM spikes between shifts

  • Delay on acceleration

  • Poor shifting

  • Burnt smells

  • Check Engine Light - Codes indicating gear slip

I would also consider the vehicle history. Is this a pickup with a life-history of heavy towing, high mileage, high temps, and no maintenance? Hmmm...these clutches might be badly burnt or worn. Replacing the fluid might reveal a slip. Is this a light, compact car with highway miles and no shift symptoms? Let's change the fluid.

Fluid Says A Lot

New transmission fluid is red/pink and very opaque. Spent fluid begins to brown and eventually blacken. New fluid has a distinct smell, but burnt fluid smells potently burnt.

If the fluid smells badly burnt and is very dark, it's exhausted. Clutch damage is occurring. New fluid may prolong the remaining life of the clutches, but if they're too far gone, you may notice slipping with new fluid. Usually, transmissions at this point are symptomatic with noticeable irregularities before any kind of fluid service.

Other Considerations

Some fear the cleaning agents in new fluid will dislodge debris into the valve body. The valve body is the fluid control circuit or hydraulic brain for the whole system. It's the first place the fluid goes after the pan and filter. With slide valves, actuators, accumulators, and springs, it has the tightest tolerances in the system. I would venture a good cleaning would do more for system performance than the continuation of sludge buildup. Fresh fluid is far more likely to clean and enhance than grime-up and degrade.

"Sealed" Transmissions and "Lifetime" Fluid

Manufacturers have begun "sealing" their transmissions. This just means they've removed the dipstick to prevent dipsticks from working on them. They still have a fill port, and they can still be serviced. The term acts as a deterrent to the lay mechanic, referring the assessment and service of the transmission to a trained professional.

Lifetime fluid means your transmission will last the life of the fluid. Manufacturers may imply something beyond this, but that's really all you're getting. Because of the dependability of modern fluid, the OEM's can rely upon it to get most customers beyond powertrain warranty. So if your transmission's life is based upon "lifetime" fluid, why not give your transmission two lives, or three?

The Verdict

In case you haven't noticed, I'm a proponent of transmission fluid maintenance. Do it. Do it early. Do it on condition. Do it before the condition is bad. If you do, you'll join a community of owners whose transmissions outlast their vehicles. If you're handy enough to do it yourself, great! You'll save yourself a ton of money. If you pay someone $500 to do it for you, great! You'll save yourself a ton of money!

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