Apoplogies in advance since I'm sure this has been discussed but I can't find it anywhere. I have a 1978 242 with the M46 transmission. I just got the overdrive working due to a wiring problem around the shifter.

I drove the car for about 20 minutes on the interstate going in and out of overdrive a few times to make sure everything was working correctly. After the 20 minutes, it seems like it is not engaging correctly. It appears to be going into overdrive, the light comes on and it shudders a little bit but never makes the shift to drop the RPM. If I let off the gas the RPM will drop and occasionally hook up if I roll into the throttle very slowly but most times it just seems to go back up as if I did not have the overdrive switched on.

I have double checked the fluid level and if anything, it is very slightly overfilled. I have Valvoline type F fluid in it that I just changed at the same time that I rewired the shifter. I also removed the OD "pan" and cleaned the mesh filter. Could this be a solenoid problem or is the OD unit on it's way out?
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Help, I bought an old Volvo, now what do I do?