Rick's Free Auto Repair Advice

Should I flush my transmission fluid

transmission fluid

This article is about transmission fluid. If you want to read about transmission flushes, click here

Just about every time you go to a quick oil change place they recommend that you change your transmission fluid. Should you? Well, that depends. Because if it’s done properly with the correct fluid, it can be beneficial. But many shops don’t follow the manufacturer’s recommendations. Here’s what you need to know before you tell the shop to change transmission fluid.

What’s in it, what goes bad

Automatic Transmission Fluid (ATF) has three jobs; lubricate internal parts, keep the parts clean, and keep them cool. In addition, transmission fluid, like motor oil, contains a healthy package of additives to protect your transmission. Every type of ATF contains anti-wear additives, as well as rust and corrosion inhibitors, detergents, dispersants, surfactants (clean and protect metal surfaces), viscosity modifiers, anti-shudder additives, seal swell additives, agents that extends temperature range, and antifoam and antioxidant additives to inhibit oxidation and improve fluid stability. These additives break down over time and that’s why ATF must be changed according to the manufacturer’s schedule. As the additives wear, the transmission runs hotter and that starts a chain reaction that eventually kills your transmission. Consider hot temperature affects your transmission.

ATF temp and life expectancy

Every 20 degrees Fahrenheit decrease in fluid temperature can double the life of the fluid and transmission components. Many of the newer ATF formulas will last for 100,000 miles if they’re not abused. What’s considered abuse? Well, transmission temperatures usually run about 175°F. But as temperature rises, life expectancy drops.

175°F = life expectancy of 100,000 miles
195°F = life expectancy of 50,000 miles
212°F = life expectancy of 25,000 miles
235°F = life expectancy of 12,000 miles
255°F = life expectancy of 6,250 miles
275°F = life expectancy of 3,000 miles
295°F = life expectancy of 1,500 miles
315°F = life expectancy of 750 miles
335°F = life expectancy of 325 miles
355°F = life expectancy of 160 miles
375°F = life expectancy of 80 miles
390°F = life expectancy of 40 miles
415°F = life expectancy of Less than 30 minutes

What causes the rise in temp? Well, a drop in internal pump pressure causes clutch disks and band to slip and that friction creates lots of heat. And, if that heat can’t be removed by the transmission cooler, the fluid returning from the cooler will just add to the problem. Pulling a heavy load for long period is too high a gear can also overheat your transmission. And, driving with an overheated engine can toast your transmission too. Finally, “rocking” your car out of ditch can overheat your transmission.

Why you shouldn’t rock and roll

Most drivers think the correct way to rock yourself out of a ditch is to alternate between reverse and drive while gunning the engine. BAD! What you’re really doing is burning up the clutch plates and bands in your transmission. Think about this: You’re in reverse and the band is applied. You’re revving and spinning your wheels. The internal parts are spinning. Then you jam it into drive. One band releases and another engages, the transmission pump applies maximum pressure to the 1-gear clutch pack and you start turning parts. Because your wheels are spinning the transmission will shift from first to second and possibly third gear. With each gear change, there’s a small amount of clutch slip and that creates heat. Then you slam it back into reverse and repeat the process.
All that heat and friction is chewing up your clutches and bands. By the time Spring arrives and your transmission fails, you won’t put two and two together to realize that getting out of that ditch has cost you almost $2,000 in transmission repairs.

The correct way to rock

You can avoid transmission damage and still attempt to extricate yourself from a ditch. Here’s how. Place the transmission in LOW gear. Accelerate until the vehicle starts to move forward. Then let off the gas and allow the car to roll backwards. As soon as it reaches maximum backwards roll, apply just enough gas to force it forward again. I’m not talking “gunning” it here. Just enough to use the vehicle’s momentum to get you out. This method keeps the transmission in one gear and eliminates all the shifting and extra heat buildup. It works as long as you use just the minimum amount of gas to move the vehicle.

Flush versus pan drop

I’ve written a complete article on the advantages and disadvantages of ATF fluid flushes. Read it here. The biggest warning is this: If you decide to do it, INSIST that the shop flushes with the manufacturer’s recommended fluid. Not following that advice is what will get you into trouble. Shops that run ATF flush “specials” use low cost Dexron III fluid and then add a bottle of friction modifier to match the manufacturer’s required friction characteristics. This is known in the industry as a TOP COAT. And every vehicle manufacturer has published technical service bulletins WARNING against this practice.
Dealers are required to use the manufacturer’s ATF. GM, for example, would require its dealers to use Dexron IV at almost $7 quart. That’s over $100 in fluid costs alone. So a dealer flush usually runs almost $200. If the shop is charging less than that, chances are they’re using Dexron III and a top coat.
Finally, several companies make a “universal” ATF. Well, let’s just say that the oil company that makes it calls it Universal. Shops like it because they don’t have to stock as many fluids. But NO car maker recognizes it as a suitable replacement for their specified oil. If you flush with a universal fluid, you can void the warranty.

© 2012 Rick Muscoplat

Posted on by Rick Muscoplat


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