By Wayne Maxwell
This circuit controls the lockup torque converter used with the GM Turbo 200-4R transmission. It will cause the converter to lock up 1 second after the transmission shifts into 4th gear (overdrive), except at ‘wide open throttle’ or if the brake is applied. Once locked, the converter will instantaneously unlock if : 1 – the throttle is ‘floored’, 2 – the brake is applied or 3 – if the transmission downshifts out of 4th gear. The reasons for these conditions are if the converter was allowed to lock up under wide open throttle (WOT) conditions, this would put undue stress on the lockup clutch and possibly cause
torque converter failure. At WOT, the converter is hydraulically locked up anyway, so little is to be gained. The converter
will unlock if the brake is applied, because this is a simple circuit that cannot distinguish between light braking or a panic
stop that may lock the wheels. Obviously, if the wheels were locked and the converter was locked, the engine would stall
and the transmission would stop pumping fluid and bad things could happen. Finally, the converter won’t lock up until the
transmission auto-shifts into 4TH gear. Since the lockup feature of the torque converter is designed to improve gas mileage only, there is just no good reason to lock it up before cruising speed is attained. The three input signals, WOT, Brake and 4th Gear are all active-low signals, using a switch-closure to ground. The TH200-4R has an internal pressure switch that is available at the connector on the outside of the transmission to provide the 4th Gear signal. Many brake switches that operate from the pedal also provide a ground when the brake is depressed. This signal can be used for the Brake input. The WOT signal probably will need to be added, either by a switch activated by the throttle linkage, or a vacuum switch that senses when manifold vacuum drops to zero. These inputs are applied to optical isolators to reduce noise and provide isolation for U1, the decoder IC. U1 is configured to generate an output when the proper conditions as stated above are met. The output of U1 starts a one second time delay before the output FET enables the transmission TCC solenoid, thus locking the torque converter. The time delay is achieved by allowing C3 to charge through R9 until the FET threshold is reached and conduction occurs, allowing the TCC solenoid to apply fluid pressure to the torque converter clutch.
The circuit uses commonly available parts and is easily duplicated. Most parts may be obtained from Radio Shack, including the case and 9 pin Molex connectors.