The 4T65E transmission features a "forward" clutch that is
engaged in 1st gear forward only. Once it shifts into 2nd, this
clutch is released. It is only applied in 1st. The inner piston
seal gets worn or the piston area gets worn where the seal
rides. What is happening is once you slow down and stop,
the line pressure which is at minimum because the throttle
isn't touched, fails to pressurize the clutch piston completely
to engage the forward clutch plates. When you "tip in" or hit
the throttle to take off, the line pressure jumps and builds
enough pressure to apply the piston, thereby engaging the
forward clutch. This is why the transmission hesitates or
bumps into 1st gear on takeoff when hot. The complete
remedy is to pull the transmission and do a total overhaul,
replacing the parts that are worn in the forward clutch
housing and a new improved inner piston seal. The other
remedy is to manually downshift into 1st gear by using the
transmission selector lever when coming to a stop. When
you select low range with the shifter lever, the line pressure
is raised from about 100psi to about 175psi which helps
engage the forward clutch piston and overcomes the
pressure leak on the inner piston seal.
Note that it also is possible that the EPC or (PCS) solenoid
before mentioned can also cause this problem as the EPC
solenoid sticks and doesn't regulate the line pressure to
about 100psi when idling in drive. This can be verified by
using a transmission pressure gauge and driving down the
road while watching line pressure. If the line pressure is
accurate, then the inner piston seal is the problem and not
the EPC (electronic pressure control) solenoid.
Note that this forward clutch in the 4T65E is the same exact
clutch that is used in the 440T4 transmission that was used
for years without this problem. However with the 440T4
transmission, the forward clutch remained engaged in all
"forward" ranges so this never developed into a problem.
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