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Transmission Health Check: Situation Is Fluid


An automobile's automatic transmission--the technology-packed gearbox that is built like a Swiss watch, controlled by a sophisticated computer and cooled by a red liquid that looks like cherry Kool-Aid--is all too often a repair disaster waiting to happen.

Indeed, a transmission failure will typically cost more than $2,000, which in many cases is nearly as much as replacing the engine itself. Once a transmission starts to disintegrate, there isn't much you can do except to replace the entire unit.

Dirty automatic transmission fluid is one of the big culprits in failures, which have grown more common in recent decades as engineers have made systems more complex, jammed them into smaller spaces and reduced the amount of outside airflow that cools them.

These trends have made transmission fluid all the more important because of the crucial role it fulfills. It must lubricate gears, bearings and other moving parts. It acts as a hydraulic fluid that operates delicate valves and transfers power in the torque converter, which is a high-powered fluid clutch that connects the engine to the gearbox. And it is the only coolant inside the transmission to transfer out heat.

Unlike motor oil, transmission fluid must provide lubrication but not be so slippery that bands and clutches inside the transmission would be unable to grab and transfer power when they are supposed to, according to Mark Ferner, an engineer at Pennzoil's lube research center in Texas.

As transmission fluid ages, it can oxidize or burn up. It starts out clear with a reddish tint but can end up opaque or brown with an acrid, burnt odor. Such signs are typical of transmission failure because they indicate that the transmission is overheating. (Fluid that loses its color but remains clear is not necessarily a sign of impending trouble.)

As the fluid oxidizes, it becomes less slippery and offers less protection to moving parts. It also makes the clutches and bands inside the transmission more grabby, so shifting is more labored.

All the while, higher temperatures accelerate wear. It is also common for transmissions to shed metal flakes, but the amount of flaking grows as a failure approaches, and that can jam valves and abrade moving parts.

A mistake some motorists make is to change the fluid for the first time only after they think trouble is coming on a high-mileage car. The new fluid--with its greater lubrication and fresh detergents--often will cause clutches to slip and will loosen deposits that can jam valves. So the new fluid may actually precipitate the failure of a transmission that is on its last legs.

Auto makers and transmission fluid makers have introduced newer fluids that are better suited to handling modern operating conditions. At the same time, they have extended the recommended change intervals. For example, Ford Motor Co.'s Mercon 5 and General Motors Corp.'s Dextron 3 are described as lifetime fluids.

But a lot of the country's top transmission experts believe that motorists who follow such advice are begging for trouble. It makes a lot of sense to change transmission fluid every 20,000 to 25,000 miles--about four times as often as the auto makers say.

Ferner, for example, changes his own transmission fluid every 12 months or about 12,000 miles, saying new fluid replenishes the detergents, contaminant dispersants and friction modifiers that get used up over time.

Sam Memmolo, a master mechanic with a repair shop in Georgia, adds that spending $100 or so on a fluid change to protect a $2,000 transmission is "a no-brainer."


Knowing what to ask for in a transmission fluid change is a little more complicated than getting a motor oil change. Auto makers do not provide drain plugs for transmissions, so garages have developed two ways to do the job.

Traditionally, a mechanic unbolts and removes the transmission oil pan, a messy job that often results in burnt knuckles. With the pan off, the mechanic typically changes the transmission filter, which is either a screen or a cartridge with a felt filter inside. A good tranny man can tell a lot about the health of a transmission from looking at the amount of metal flakes inside a filter. The service costs about $65.

But this method leaves several quarts of dirty fluid inside the torque converter. Some garages now offer an alternative fluid change, in which the old fluid is pumped out of the transmission through a coolant line. The cost is typically about $60 to $80.

Although this procedure results in a complete fluid change, the old filter remains behind. But a transmission filter may not need to be changed every 25,000 miles, because it will continue to allow unrestricted flow.

A third option--which many transmission mechanics recommend at least every 50,000 miles--is to spend $110 to $130 and have both services done at once.


Ralph Vartabedian cannot answer mail personally but responds in this column to automotive questions of general interest. Please do not telephone. Write to Your Wheels, Business Section, Los Angeles Times, 202 W. 1st St., Los Angeles, CA 90012. E-mail:

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