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Fixing Transmission Fluid Leaks Will Prevent Costly Repairs

Transmission fluid leaks can be annoying as well as expensive. Most people don't want unsightly, sticky oil stains on their garage floor or driveway. But that's one of the telltale signs of a leaker. So is a sudden appetite for automatic transmission fluid (ATF). When a transmission leaks, the fluid level should be checked every few days to make sure it doesn't get too low. If the fluid level goes get below the add mark on the dipstick, the transmission may hesitate when it is first put into gear, and it may slip when accelerating and shifting. If fluid is not added to bring the level back up between the add and full marks, the transmission may eventually run out of fluid altogether - and when that happens, the consequences can be very expensive.

An automatic transmission or transaxle can cost upwards of $1,500 to replace on a late-model car or truck. Yet for a fraction of that cost, most leaks can be fixed by simply replacing a gasket or seal.

A low fluid level is one of the worst things that can happen to an automatic transmission or transaxle because it overworks the fluid and makes it run hotter than normal. Less fluid means less cooling capacity and less lubrication. As the fluid's temperature goes up, it begins to oxidize and break down. And once that happens, the transmission is doomed unless the overworked fluid is replaced. Adding a couple of quarts of fluid to a transmission that has already suffered heat damage is like putting salve on a third-degree burn. The damage has already been done. Better to prevent the damage in the first place by fixing the leak, maintaining the proper fluid level and changing the fluid and filter periodically for preventive maintenance.

Some motorists will, of course, try the "cheap fix" first - adding a can of transmission stop-leak to slow the leakage. Most of these products contain ingredients that cause seals and gaskets to swell slightly. Though an additive may reduce fluid leakage temporarily and buy your customer some time, it is not a long-term fix for worn seals or aging pan gaskets. Replacing the worn parts is the only permanent cure.

The first thing to check is the fluid level itself because a low fluid level almost always indicates a leak. The level should between the full and add marks (the closer to the full mark, the better).

The check should be made after driving the vehicle so the fluid will be hot. Heat causes the fluid to expand. Checking it when it is cold may give the false impression that it is low. On most vehicles, the fluid level is checked with the engine idling and the transmission in park.

Under normal driving conditions, transmissions should not use any fluid. If the level is low, therefore, it usually means the fluid is leaking out through the pan gasket or past the input shaft or driveshaft seals.

The appearance and condition of the fluid should also be noted. Oxidization will darken the color of the fluid, changing it from a bright red to a dark brown. Oxidized ATF will also smell like burnt toast. If the ATF in the transmission is discolored and/or smells bad, it needs to be changed without further delay.

Another way to check the fluid's condition is to use a "blotter" test. Place one or two drops of ATF from the transmission on a piece of blotting paper (or a paper towel) and wait about half a minute. If the spot is widely dispersed and red or light brown in color, the fluid is in satisfactory condition. But if the spot doesn't spread out and is dark in color, the ATF is oxidized and should be changed.

With rear-wheel drive automatic transmissions, most fluid leaks occur at one of three places: the tailshaft seal, the transmission pan gasket or the input shaft seal. The tailshaft seal is often the first to go because of the in and out plunging motion of the driveshaft every time the suspension bounces up and down. Not only must this seal cope with the constant rotational friction of the driveshaft yoke, but also any contaminants that are pushed under the lip of the seal when the driveshaft plunges in and out. A buildup of rust on the driveshaft yoke can also be a source of wear on the seal.

Another cause of leakage can be a plugged transmission vent. All transmissions have to breathe, so a small vent tube is usually provided somewhere on the top of the case. If this tube becomes clogged, a buildup of internal pressure inside the transmission may force fluid to leak past the seals (even if the seals are in perfect condition). The cure here is to unclog the vent.

With front-wheel drive transaxles, leaks typically occur in one of four places: the two halfshaft seals, the input shaft seal or the pan gasket. The halfshaft seals don't have to deal with the plunging motions of the halfshafts because the inner CV joints handle that. But their location does subject them to road splash and contaminants from the engine compartment.

Transmission and transaxle pan gaskets are longer lived, but over time composite-cut gaskets lose elasticity. As the gasket ages, it loses flexibility, which results in a loss of clamping torque that provides a leak-free seal. Fluid can start to seep between the gasket and cover creating a path for leakage. Retightening the pan cover bolts may temporarily stop the leak or reduce its severity, but an old gasket that has become hard and brittle with age will soon be leaking again. Replacement is the only cure that will stop the leak.

Some pan gaskets are sealed at the factory with RTV silicone. This eliminates the need for a cut gasket and provides a seal that will generally hold up seven to eight years. But over time, the RTV can break down and lose its grip allowing leaks to form.

Molded rubber pan gaskets and gaskets that have a metal or plastic carrier with a molded rubber bead are the most durable of all, and will generally stay leak free for many years. More of these "high tech" gaskets are becoming available in the aftermarket for OEM replacement applications, and are an excellent upgrade over standard cut cork/rubber gaskets. But the high-tech gaskets also cost more due to their more complex construction. Even so, if your customer wants maximum assurance the leak has been fixed once and for all, recommend upgrading to a molded- or carrier-style pan gasket.

Once you've identified where the fluid is going, the leak can be fixed by replacing the faulty gasket or seal. Tailshaft seals on rear-wheel-drive transmissions are fairly simple to replace, and only require removing the driveshaft. On FWD cars and minivans, though, one or both halfshafts will have to come out if a transaxle halfshaft seal is leaking.

Be sure to check the wear surface on the driveshaft yoke or inner constant velocity joint for rust or nicks that might damage the new shaft seal. Installing a new seal on a rough shaft will soon ruin the new seal. Fine emery cloth or sandpaper can be used to smooth a rough surface. But if the surface of the shaft is grooved or worn out-of-round, it may leak even with a new seal. You may have to replace the yoke or inner CV joint to eliminate the leak. Some aftermarket replacement seals are offset slightly from the OEM seal so the lip of the seal rubs on a different area of the shaft. Another alternative is to install a repair sleeve if one is available for the application.

A leaky input shaft seal or a bad seal between the torque converter and transmission requires the most labor to replace. The transmission or transaxle usually has to come out of the vehicle to access the seal. The labor expense can be quite high so many people choose not to have such leaks fixed - which means they have to constantly monitor the fluid level and make sure it doesn't get too low.

Leaky pan gaskets are relatively easy to fix. Drop and pan, carefully scrape off all the old gasket, clean out the inside of the pan, install a new ATF filter in the transmission while the pan is off, then install a new gasket and button up the pan.

Though many quick lube franchises use RTV rather than cut gaskets to seal pans after performing transmission service, most transmission experts prefer cut or molded gaskets. The reason? Misapplication of RTV increases the risk of a comeback as well as transmission problems. RTV must be allowed to sit and cure for 30 minutes or more before fluid can be added to the transmission.

Drying time will vary with temperature and humidity (cold and dry takes a lot longer than warm and humid). Both sealing surfaces must also be free from oil and grease, otherwise the RTV may not adhere well to the surface. And if too much RTV is applied to the sealing surface, it may be squeezed out of place when the pan bolts are tightened up and end up inside the transmission. You don't want chunks of rubber floating around in the fluid because it may end up causing a blockage. But if used properly, RTV is a good sealing material for pans and other engine covers.

When installing a new pan gasket, carefully clean and inspect both the pan flange and mating surface on the transmission or transaxle case. Both must be clean, dry, flat, and free from deep scratches or pits. A bent pan flange can usually be straightened, but if the pan is badly corroded or deformed it should be replaced.

Always follow the gasket supplier's recommendations for installation. Molded rubber pan gaskets and carrier-style gaskets with rubber beads or special coatings are almost always installed dry. Using a sealer may cause the gasket to slip or have an adverse chemical reaction with the factory-applied coating. Cork/rubber and other composite gaskets, on the other hand, are often installed with a coating of sealer or adhesive - though the usage of such products is often more for assembly ease than to provide an initial seal.

If the replacement gasket does not have grommets that limit the amount of crush when the bolts are tightened, use a torque wrench and follow the vehicle manufacturer's recommended torque values. Overtighening cork/rubber gaskets is a common cause of leakage and failure, so don't make this mistake.

When tightening the pan bolts, bring the bolts up gradually to the recommended torque, and tighten alternating bolts on opposite sides to even out the clamping load. This will provide a better overall seal and reduce the chance of future leaks.

Finally, refill the transmission to the proper level with the recommended fluid. Do not overfill the transmission because this can cause fluid aeration and a buildup of pressure that can force fluid past the gaskets or seals you've just replaced.

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