Motormouth. Take 2.

Automatic transmission fluid comes through in the clutch

January 19, 2004|By Bob Weber.

Like most stuff automotive, automatic transmission fluid keeps getting better. Many carmakers no longer specify transmission-fluid change intervals in a vehicle's owner's manual. Some transmissions do not even have a dipstick with which to check the fluid.

But a professional transmission technician will tell you that it is foolhardy to ignore occasional automatic transmission fluid (ATF) changes. To do so is to invite an untimely demise and expensive repair.

Though there are some synthetics, most ATF is a highly refined oil to which chemicals are added. ATF lubricates the gears, bearings and other mechanical parts in the transmission.

But it also performs tasks as a hydraulic fluid. The fluid under pressure applies the clutches in the transmission. When the fluid is routed to a one place, certain clutches are applied. When rerouted to another, the first clutches may be released and another set of clutches are applied. This is what shifts the gears.

The ATF also creates a hydraulic link between the engine and the transmission. Instead of having a mechanical clutch between the engine and transmission as in a manual gearbox, there is a torque converter between the engine and automatic transmission.

Half of the torque converter is bolted to the engine, and half is bolted to the transmission. Transmission fluid is flung from one section to the other, creating a fluid coupling. Flinging fluid heats it up.

Hot fluid is channeled out of the torque converter and to the transmission fluid cooler. Most ATF coolers are built into the radiator. Here, the ATF sheds some of its heat to the coolant, which sheds its heat to the outside air passing across the radiator.

Some vehicles, especially those ordered from the factory with a towing package, have an auxiliary ATF cooler. This may look like a miniature radiator with cooling fins or may be a loop of pipe. Auxiliary coolers dissipate heat to the outside air.

The normal operating temperature for transmission fluid is 150 to 200 degrees Fahrenheit. Ideally it should be kept at 175 degrees.

Anything that causes the fluid to get hotter, reduces its service life. The amount of shifting, hence the number of times clutches create friction as they engage and release, turns up the heat.

Rocking your car to free it from mud or snow puts severe demands on the transmission fluid, heating it very high and very fast.

Heat shortens the life of transmission fluid by causing it to oxidize.

In an ideal world, the fluid may last up to 100,000 miles. Beyond the normal temperature range, oxidation doubles with every 20-degree increase in temperature. Put another way, fluid life is cut in half with every 20-degrees it is above normal.

Oxidized fluid performs poorly and protects poorly, leading to transmission failure.

