Automatic Transmission
Engine/Transmission Relations
An important part of transmission diagnosis is to
make certain the engine operates properly.
If the engine performance is
incorrect, the transmission will receive the wrong information.
Many times
what is perceived as a transmission problem is in actuality an engine problem.
So
before you make any attempt to diagnose a transmission problem, be
sure engine systems are
operating properly.
Keep
in mind that this article deals with automatic transmissions in general. You should
always check the proper service and repair manual for your vehicle for specific information.
What
Is An Automatic Transmission?
There are two basic types of automatic transmission; the front wheel drive
version called a
Transaxle because it incorporates a differential and
a rear wheel drive version called a
Transmission that does not.
Just like that of a manual transmission, the automatic
transmission's primary job is to allow
the engine to operate in its
narrow range of speeds while providing a wide range of output
speeds. An
automatic transmission locks and unlocks different sets of gears to the output
shaft
to achieve the various gear ratios. In an automatic transmission;
the same set of gears produces
all of the different gear ratios. The planetary
gearset is the device that makes this possible
in an automatic transmission.
Transaxle:
Basically a transaxle
is a fully automatic electronically controlled front wheel drive
transaxle
with four forward gear ranges including overdrive and one reverse gear range.
A
variable displacement vane type oil pump supplies oil pressure. Two
shift solenoids, operated by
the Powertrain Control Module (PCM), control
shift points and TCC apply rates, which are
influenced by various sensor
inputs. The transaxle unit includes a three element hydraulic
torque
converter and lockup clutching element, four multiple disc clutch packs, roller
clutch,
sprag clutch, two bands and a compound reaction planetary gearset.
Power is transmitted to the
drive wheels from the planetary gear through
a final drive gearset and differential assembly.
Transmission:
The normal automatic transmission is fully automatic
with four forward and one reverse speed.
It consists of a lockup torque
converter assembly and a three unit planetary gear train.
Electronic controls
for the transmission are integrated into the on-board Powertrain Management
systems. Sensors provide inputs to the Powertrain Control Module (PCM).
The PCM electronically
controls the transmission hydraulic system for gear
selection and automatic shift control.
Torque converter operation is
controlled by the PCM. These controls, along with hydraulic
controls in the
main control body, operate a piston plate clutch in the torque converter that
eliminates torque converter slip when applied.
Testing:
There are several ways to test an automatic transmission.
There are some that can only be
done by a shop that has the proper equipment,
and others you can do at home.
? Air Pressure Tests
To test the bands and servos inside the transmission, air
pressure can be substituted for
fluid pressure. When compressed air is
applied to certain ports, it will apply the particular
band or servo. When
a band or clutch engages a dull thud will be heard. There should be no
hissing
heard while band or servo is engaged. This can only be done by a repair
shop.
? Line Pressure Test
Transmissions
need to operate with the proper fluid pressure. If the fluid pressure is too
low or too high, shifting can be too soft or too hard. Different parts
of the transmission
operate with different fluid pressures and a transmission
pressure gauge can be attached to a
transmission to check these pressures.
These readings can be compared to a chart that will
pinpoint the
most likely cause of a problem.
? Shift Point Road Tests
Each shift in a transmission is supposed to occur at a specific time
under certain throttle
conditions. For example under light throttle a
1-2 shift will take place sooner then a 1-2 shift
under heavy throttle. By
knowing when the shifts take place and when they are supposed to take
place
you can figure out what is wrong with it.