An electronic control system for operating a torque converter clutch during a transmission gearshift includes a vehicle speed sensor and engine speed sensor, a hydrodynamic torque converter located between the engine and the transmission, a solenoid-operated hydraulic valve, which engages and disengages the torque converter clutch in accordance with the energized and deenergized state of the solenoid in accordance with the logic of the control system. The converter clutch is locked while operating in the original gear ratio, then unlocks during the gearshift, and relocks after the upshift is completed. The speed ratio of the torque converter is rapidly determined in terms of the number of clock ticks occurring between pulses whose frequency is proportional to engine speed. The current speed ratio is determined during interrupts occurring at short intervals during a background pass.