A Different Way of Doing Things
The 4F27E transmission uses (2) ON/OFF solenoids and (3) PWM solenoids to produce each of the shifts
and converter clutch apply as the chart in figure 1reveals. The EPC solenoid is used to control line pressure
relative to engine torque. So far this sounds easy but from a strategy standpoint, it progressively moves
towards a “different way of doing things” when compared to what we are accustomed to. Lets start off on
familiar ground. Shift Solenoids A & B are the ON/OFF solenoids which when they are OFF, block fluid
from stroking valves while the
C, D & E are the opposite. When they are
OFF they allow fluid to pass through the solenoid and stroke valves. OK, so far not so bad. Sounds a little
like the 41TE/42LE transmission with their Normally Applied and Normally Vented
solenoids.
The first departure from the “norm” if you will, is that shift solenoid B could be better understood as a Lock
Up/TCC solenoid. To call this shift solenoid B made it difficult for me when I first looked at a solenoid
application chart like the one in figure 1. I kept thinking it was a gear shift solenoid not a TCC shift solenoid.
But now, having clarified the operation of shift solenoid B, it would be good to do a “simple” overview of
the function of each of the other “shift” solenoids and by doing so, the solenoid application chart will make
considerably more sense.
Pulse Width Modulated Solenoids,
PWM - ON/OFF
When this solenoid is in the ON position, it strokes the 3-4 shift valve which exhausts Forward Clutch and
servo release pressure.
When this solenoid is in the ON position, it strokes the TCC Control Valve which exhaust the converter
release pressure allowing for TCC apply.
When this solenoid is in the OFF position, it provides Forward Clutch Apply Pressure
When this solenoid is in the OFF position, it provides Servo Apply Pressure
When this solenoid is in the OFF position, it provides Servo Release Pressure
With this simple overview of the function of each of the shift solenoids, along with the hydraulics provided
for each of the shifts while the manual valve is in the D4 position (figures 2-8), a closer look will provide a
more detailed understanding of “a different way of doing things.”
Using the second gear TCC off hydraulic provided in figure 3, carefully look at the TCC Control Valve.
Notice that forward clutch oil runs through the valve from the PWM SSC to the forward clutch. Also notice
that line pressure is routed to and blocked by the TCC Control valve’s land next to the spring. When this
TCC Control Valve is stroked by SSB for lock up, TCC release oil becomes connected to the PWM SSC
solenoid for a controlled TCC apply. At the same time, the forward clutch apply oil switched from being fed
by the PWM SSC solenoid to the line pressure that was being blocked by the land next to the spring (See
figure 4). Now that is a different way of doing things!
SSA
SSB
PWM SSC
PWM SSD
PWM SSE
