Classic Thunderbird Club International
Gil's Garage
Gil Baumgartner
CTCI Authenticity Chairman
Testing Overdrive Electrical Circuit and Components on the Vehicle.

The overdrive electrical components can be checked without removing them from the vehicle. The black wire connected to the relay in the engine compartment supplies voltage to the relay when the ignition switch is turned on. The white wire with red tracer provides a ground that is initiated by the centrifugal governor switch mounted on the overdrive unit. The blue wire supplies voltage to the overdrive solenoid which activates the mechanical components of the overdrive unit.

Photo #1- Test light connected to the black wire as shown on the overdrive relay. Turn on ignition switch (do not start engine) the light should illuminate if the voltage circuit is working properly. TempGuage2
Photo #2- With the ignition switch on (do not start engine) ground the white wire with red tracer, the points in the relay should close sending power to the overdrive solenoid through the blue wire. If the relay is working properly and the circuit (blue wire) to the solenoid is good you will hear an (audible click) in the transmission which is the solenoid engaging. If you hear the relay click and the solenoid does not engage, connect a test light to the blue wire and repeat the initial check. If the test light illuminates and the solenoid does not engage the solenoid may be defective. If the test light does not illuminate the relay may be defective. TempGuage3
Photo # 3- Connect a jumper wire from the positive post of the battery to the blue wire on the relay, if the solenoid is working properly you will hear an audible click in the transmission. This step bypasses the relay. TempGuage3
Photo # 4- Shows a complete wiring diagram of the overdrive circuit. The kick down switch provides continuity between the governor and the relay. If the kick down switch is functioning properly the circuit is disrupted only when the kick down switch is totally engaged by depressing the accelerator pedal to the floor. See article in Gil's Garage under electrical titled Overdrive Kick down Switch. TempGuage1

Other articles available under electrical are Overdrive Governor Test and Overdrive and Power Window Relay Bench Test.


Last modified: February 10 2012