TCC Apply Piston &
Cover Deflection
Cover Deflection
By Ed Lee
©2006 Sonnax Industries Inc.
A 604 transmission with a 740 code led to
many different approaches and attempts to troubleshoot the
underlying problem. As the possible causes were narrowed down,
the focus began to turn toward the converter. Replacing the
converter with a factory unit eliminated the 740 code, and
since the code would show up on any road test, this seemed like
the perfect vehicle to identify the root cause.
Time after time, the faulty converter was
removed from the vehicle and one change was made or a part
replaced before the converter was reinstalled. After many days
and many more R&Rs, the problem was found to be poor
converter flow, caused by a solid fiber thrust washer between
the cover and turbine hub.
Finding the root cause was time consuming,
but the information discovered will prevent the same mistake
from being made repeatedly in the future. Some other
information also was discovered during the process of
inspecting the insides of many low-mileage converters in a
short time. One was the uneven wear pattern on the new friction
material. The 740 codes were proof that a TCC slip had
occurred, but the technicians expected to see an even wear
pattern down the center of the friction material. However, none
of the converters had this wear pattern. Most converters had a
dark wear pattern at either the inside or outside edge of the
friction material (see figures 1 and 2). The mystery of why the
740 code happened may have been solved, but several new
questions had come up.
The pressure that exerted force on the
piston started at 20 psi and increased in increments of 10 psi
until the pressure regulator read 90 psi. A second gauge was
placed in line at the fixture to record any difference caused
by a leak.
With all six pistons, both the gauge at the
regulator and the gauge at the fixture read the same pressure,
from 20 to 80 psi. When the regulator was bumped up to 90 psi,
the gauge at the fixture would go up to only 84 psi. This was
also true when any attempt was made to regulate the pressure
even higher. It is interesting to note that the OE apply
pressure is about 80 psi.
Starting at the pressure of 20 psi, the
readings of the dial indicators were recorded, and each time
the pressure that was exerted on the piston was increased by 10
psi, the new readings also were recorded. This then was graphed
to show how the deflection of the pistons changed in
relationship to the changes in pressure (see figures 4 and 5).
Some observations
1) The pressures being equal, the uncut OE
piston will deflect almost twice as much as an uncut billet
piston.
2) The OE piston is weakened by machining
more than the billet piston. When the OE and billet pistons are
machined a like amount, the difference in deflection is almost
three times as much on the OE piston.
3) When 0.015 inch was machined from the
billet piston, it actually had less deflection, because
machining removed the 0.75° taper on the uncut piston.
4) The better the mating surfaces match,
the better the seal for TCC apply. The difference in the amount
of deflection between the piston and cover will shift the apply
force to the outer or inner edges of the friction material.
5) The higher the pressures, the greater
the deflection differences will be.
6) Piston deflection is inversely
proportional to how rigid the piston is (the more rigid the
piston, the less deflection). This may be why the newest
front-wheel-drive Chrysler pistons are about 65 Rockwell. The
downside is that they cannot be machined by conventional
methods, not even carbide: They must be ground to resurface.
Some suggestions
1) To keep the angles of the mating
surfaces as close to parallel as possible, use a billet cover
and billet piston.
2) If you use a billet piston and an OE
cover with an original lining, you may want to remove the taper
from the billet piston. When the cover deflects more than the
piston, the apply force is localized more toward the outside
edge of the friction material (see Figure 1).
3) If you use an OE piston and billet
cover, you will want to limit the amount of material that you
machine from the piston to about 0.015 inch maximum. When the
piston deflects more than the cover, the apply force is
localized more toward the inside edge of the friction material
(see Figure 2).
4) Use caution anytime you are machining an
OE cover: The friction-material surface is only 0.040 inch
thick at the center of the pads.
5) Knowing that the cover is always
deflected forward at the time of piston apply is a good reason
not to use drop-in linings. The turbine hub is held on the
centerline of the cover by the bushing in the cover, and the
TCC piston is centered by the turbine hub. Any amount that the
lining is off center moves the lining up the angle of the apply
surface of the cover, created by the deflection of the cover
(uneven contact of the piston).
6) Measure the thickness of the piston
inside of the spring retainer to see whether they have been
machined. The uncut OE piston measures about 0.100 inch, and
the uncut billet measures between 0.100 and 0.105 inch in the
same area. 
©2006 Transmission Digest