Question:How often should the automatic transmission
fluid be changed?
Answer:Most owner's manuals say it isn't necessary. Yeah,
right. That's why transmission shops are making a fortune replacing
burned out automatic transmissions.
For optimum protection, change the fluid and filter every
30,000 miles (unless you have a new vehicle that is filled with
Dexron III ATF which is supposed to be good for 100,000 miles).
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.
ATF FLUID TYPES
What kind of automatic transmission fluid should you use in
your transmission? The type specified in your owner's manual
or printed on the transmission dipstick.
For older Ford automatics and certain imports, Type "F"
is usually required. Most Fords since the 1980s require "Mercon"
fluid, which is Ford's equivalent of Dexron II.
For General Motors, Chrysler and other imports, Dexron II
is usually specified.
NOTE: Some newer vehicles with electronically-controlled
transmissions require Dexron IIe or Dexron III fluid. GM says
its new long-life Dexron III fluid can be substituted for Dexron
II in older vehicle applications.
CAUTION: Using the wrong type of fluid can affect the way
the transmission shifts and feels. Using Type F fluid in an application
that calls for Dexron II may make the transmission shift too harshly.
Using Dexron II in a transmission that requires Type F may allow
the transmission to slip under heavy load, which can accelerate
clutch wear.
CHANGING THE FLUID
It's a messy job because there's no drain plug to change the
fluid, but you can do it yourself if you're so inclined. To change
the fluid, you have to get under your vehicle and remove the pan
from the bottom of the transmission.
When you loosen the pan, fluid will start to dribble out in
all directions so you need a fairly large catch pan. You should
also know that removing the pan doesn't drain all of the old fluid
out of the transmission. Approximately a third of the old fluid
will still be in the torque converter. There's no drain plug
on the converter so you're really only doing a partial fluid change.
Even so, a partial fluid change is better than no fluid change
at all.
A typical fluid change will require anywhere from 3 to 6 quarts
of ATF depending on the application, a new filter and a pan gasket
(or RTV sealer) for the transmission pan. The pan must be thoroughly
cleaned prior to reinstallation. This includes wiping all fluid
residue from the inside of the pan and scraping all traces of
the old gasket from the pan's sealing surface. Don't forget to
clean the mounting flange on the transmission, too.
When the new filter is installed, be sure it is mounted in
the exact same position as the original and that any O-rings or
other gaskets have been properly positioned prior to tightening
the bolts. Then tighten the bolts to the manufacturer's recommended
specs.
When refilling the transmission with fresh fluid, be careful
not to allow any dirt or debris to enter the dipstick tube. Using
a long-neck funnel with a built-in screen is recommended.
CAUTION: Do not overfill the transmission. Too much fluid
can cause the fluid to foam, which in turn can lead to erratic
shifting, oil starvation and transmission damage. Too much fluid
may also force ATF to leak past the transmission seals.
Add half a quart at a time until the dipstick shows full.
The transmission really isn't full yet because the dipstick should
be checked when the fluid is hot, and the engine is idling with
the gear selector in Park. So start the engine, drive the vehicle
around the block, then recheck the fluid level while the engine
is idling and add fluid as needed until the dipstick reads full.
