How to Convert an Automatic Transmission into a Manual Transmission
To convert an automatic transmission into a manual transmission, without actually removing it from the car. Requires switching the factory installed automatic valve-body, for either a manual/automatic valve-body, or a fully manual one. The valve-body is the "brain" of the automatic transmission. It controls the clutches and bands inside, which is how it shifts gears.
The gear shifter knob on the steering column, or center console, is connected to the valve-body by a cable or linkage rod. The position of the shifter moves a small piston in the valve-body, that directs hydraulic pressure to the proper bands or clutches. As the car comes from the factory the combination of vehicle speed, engine rpm, and vehicle load, causes the automatic shifting of the gears.
Inputs from various sensors installed on the vehicle are fed to the vehicles computer, and the computer selects the proper gear for conditions. As the engine rpm rises to a pre-programmed shift point, the valve-body releases pressure from the first gear clutch pack. A split second later it applies fluid pressure to the second gear clutch, then second to third and so on, until you're in high gear. Downshifting works exactly the same only in the reverse order.
A manual/automatic or semi-automatic valve-body, works the same as the factory installed part, when the gearshift is placed in drive. However it allows for manually controlling the gearshift when the shifter is manually shifted. So when the shifter is placed in first gear, the transmission will remain in first gear, until you move the shifter into second gear. It stays in second until you shift it to third, etc. This feature comes in handy if you live in hilly country, as you can use the compression of the engine to help slow the car when going downhill. It also allows you to start out in second gear, if road conditions are icy (minimizes wheelspin).
A full manual valve-body relinquishes all automatic shifting features. The gear shifter must be physically moved, from one gear position to the next to acquire the next gear. Fully manual valve-bodies are most frequently seen on racing cars, as they require manual control to achieve maximum speed.
The epitome of the manual valve-body is one with a transbrake solenoid installed in it. The transbrake is a device that applies hydraulic pressure to the first gear clutch pack and the reverse gear band, AT THE SAME TIME. By placing the transmission in forward and reverse simultaneously, it allows the drag racer to floor the gas pedal, without the car moving. Kind of like power braking on steroids! Releasing the transbrake, removes the pressure from the reverse band, and the car is off like a shot. So you see you really can convert an automatic transmission to a manual one.
