I have a 1991 Volvo 240 and recently, it would not go into overdrive. My overdrive indicator dash light was not on. I got an overdrive relay from pull-a-part salvage and replaced it. The car still would not go into overdrive but the overdrive indicator light was not on and still came on and went back off if the button was pressed. Also, when I was driving, the dash overdrive indicator light came on and has remained on for the first time. Replaced relay with a new Hella relay. Dash light is still on and transmission won't go into overdrive. I checked the wires and could not see where it was touching and being grounded out on the transmission case itself. Any ideas?

 

Rarely, the solenoid itself fails. Most often is: 1. the relay 2. The wiring under the car becoming oil soaked and fabulously nasty enough to disintegrate the insulation 3. the switch in the shifter 4. the od wiring under the drive tunnel carpet leading from selector switch to relay 5. the solenoid 6. failure of the entire od unit on the transmission.
Incidentally, the od off light has a single wire that plugs to the back of the instrument cluster. Occasionally, that connection will affect the proper operation of the bulb. It has no bearing on the proper operation of the od however.

 

Thanks for the information. I will begin tracing down the problem in order as you specified.