Diagnostic Aids

If problem is diagnosed as being an internal transmission problem, perform transmission service.

Relay coil resistance must measure more than 20 ohms. Less resistance will cause early failure of the ECM "DRIVER". Using an ohmmeter, check solenoid coil resistance of all ECM-controlled solenoids and relays before replacing ECM. Replace any solenoid or relay that measures less than 20 ohms resistance.

Fig 1: Downshift Control Diagnosis Flow Chart - (4L60 CCC TBI)

Fig 2: THM 400 Downshift Control Electrical Diagnosis Schematic