Downshift Control Operation

Vehicles equipped with THM 400 transmissions are equipped with an ECM-controlled downshift system. When vehicle is at speeds below 70 MPH, a forced (detent) downshift is possible by fully depressing the accelerator. When the ECM recognizes the drop in manifold vacuum (high MAP sensor voltage reading), the downshift control relay is energized by the ECM. The relay directs battery voltage to the detent solenoid in the transmission to open an orifice to force the transmission to downshift.