WHY ATF WEARS OUT
An automatic transmission creates a lot of internal heat through
friction: the friction of the fluid churning inside the torque
converter, friction created when the clutch plates engage, and the
normal friction created by gears and bearings carrying their loads.
It doesn't take long for the automatic transmission fluid (ATF) to
heat up once the vehicle is in motion. Normal driving will raise
fluid temperatures to 175 degrees F. , which is the usual temperature
range at which most fluids are designed to operate. If fluid
temperatures can be held to 175 degrees F. , ATF will last almost
indefinitely -- say up to 100,000 miles. But if the fluid
temperature goes much higher, the life of the fluid begins to
plummet. The problem is even normal driving can push fluid
temperatures well beyond safe limits. And once that happens, the
trouble begins.
At elevated operating temperatures, ATF oxidizes, turns brown and
takes on a smell like burnt toast. As heat destroys the fluid's
lubricating qualities and friction characteristics, varnish begins
to form on internal parts (such as the valve body) which interferes
with the operation of the transmission. If the temperature gets
above 250 degrees F. , rubber seals begin to harden, which leads to
leaks and pressure losses. At higher temperatures the transmission
begins to slip, which only aggravates overheating even more.
Eventually the clutches burn out and the transmission calls it
quits. The only way to repair the damage now is with an overhaul --
a job which can easily run upwards of on a late model front-wheel
drive car or minivan.
As a rule of thumb, every 20 degree increase in operating
temperature above 175 degrees F. cuts the life of the fluid in half!
At 195 degrees F. , for instance, fluid life is reduced to 50,000
miles. At 220 degrees, which is commonly encountered in many
transmissions, the fluid is only good for about 25,000 miles. At 240
degrees F. , the fluid won't go much over 10,000 miles. Add another
20 degrees, and life expectancy drops to 5,000 miles. Go to 295 or
300 degrees F. , and 1,000 to 1,500 miles is about all you'll get
before the transmission burns up.
If you think this is propaganda put forth by the suppliers of ATF to
sell more fluid, think again. According to the Automatic
Transmission Rebuilders Association, 90% of ALL transmission
failures are caused by overheating. And most of these can be blamed
on worn out fluid that should have been replaced.
On most vehicles, the automatic transmission fluid is cooled by a
small heat exchanger inside the bottom or end tank of the radiator.
Hot ATF from the transmission circulates through a short loop of
pipe and is thus "cooled. " Cooling is a relative term here, however,
because the radiator itself may be running at anywhere from 180 to
220 degrees F. !
Tests have shown that the typical original equipment oil cooler is
marginal at best. ATF that enters the radiator cooler at 300 degrees
F. leaves at 240 to 270 degrees F. , which is only a 10 to 20% drop
in temperature, and is nowhere good enough for extended fluid life.
Any number of things can push ATF temperatures beyond the system's
ability to maintain safe limits: towing a trailer, mountain driving,
driving at sustained high speeds during hot weather, stop-and-go
driving in city traffic, "rocking" an automatic transmission from
drive to reverse to free a tire from mud or snow, etc. Problems in
the cooling system itself such as a low coolant level, a defective
cooling fan, fan clutch, thermostat or water pump, an obstructed
radiator, etc. , will also diminish ATF cooling efficiency. In some
cases, transmission overheating can even lead to engine coolant
overheating! That's why there's a good demand for auxiliary add-on
transmission coolers.
AUXILIARY COOLING
An auxiliary transmission fluid cooler is easy to install and can
substantially lower fluid operating temperatures. The plate/fin type
cooler is somewhat more efficient than the tube and fin design, but
either can lower fluid temperatures anywhere from 80 to 140 degrees
when installed in series with the stock unit. Typical cooling
efficiencies run in the 35 to 50% range.
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