the transmission locks up several times a week. My mechanic says this was a recall on the s90

I've got 3 S90's and after having broken 2 shifters due to this happening I've finally figured out the problem. If you have an S90 it's already getting close to 15yrs old. The spring in the ignition switch is not as strong as it used to be and when you start the car it doesn't get pulled back to the correct position. If the ignition switch isn't in the right position then it won't release the cable lock that keeps the shifter from moving out of park. So the first thing you do is wiggle the ignition until it's back to where if you go any further you'll turn the car off. If that doesn't do it then the emergency deal is to get a flat head screw driver and pop up the rosewood shifter cover and wiggle it back so you can see in front of the shifter pole. There will be a round cable end in a plastic receiver or holder which is the shifter end of the ignition cable lock. If you move that towards the rear about a 1/4 - 1/2 inch it releases the transmission shifter and you can get the car moving. I hope this is helpful and understandable.

I also found the brake lights switch on the pedal have two internal switches . one is used for the stop lights and the other for the shifter interlock solenoid. in my case the switch for the solenoid (brown wire) was not closing but the one for the light was working, replacing the switch fixed my problem.

if the cable is the problem, there Is a way to adjust it. look at one of the ends and locate the nut/screw