Diagnosis And Testing-Overdrive Electrical Controls
WARNING: This page is about a different car, the 2003 Dodge Dakota. However, it is still accessible from the selected car via links, so may be relevant.
The overdrive off switch, valve body solenoid, case connectors and related wiring can all be tested with a 12 volt test lamp or a volt/ohmmeter. Check continuity of each component when diagnosis indicates this is necessary.
Switch and solenoid continuity should be checked whenever the transmission fails to shift into fourth gear range.