R/V & G Series

Chart C-8A - THM 400 Transmission Downshift Control:

Wiring Diagram For Chart C-8A - THM 400 Transmission Downshift Control:

CHART C-8A - THM 400 TRANSMISSION DOWNSHIFT CONTROL

Circuit Description:

When the accelerator pedal is fully depressed, manifold vacuum in the engine drops causing the MAP sensor signal voltage to increase to approximately 4 volts. The ECM responds by grounding CKT 422 to turn "ON" the (downshift control relay. The relay then sends battery voltage to the detent solenoid, which causes a forced transmission downshift.


Diagnostic Aids:

^ Relay coil resistance must measure more than 20 ohms. Less resistance will cause early failure of the ECM "Driver". Refer to ECM QDR Check Using an ohm meter, check the coil resistance of all ECM controlled solenoids and relays before installing a replacement ECM. Replace any solenoid or relay that measures less than 20 ohms resistance.