https://www.brickboard.com/FAQ/700-900/TransmissionAuto.htm#OverdriveRelay
If you find the fault is traced to the solenoid and you think the fix is adding a bypass plate I would grind a groove in the underside of the solenoid as described in the FAQ section above. The procedure is easy and it works.
I did this in the last couple of years and it restored the overdrive. Of course you lose the ability to shift it down to 3rd at higher speeds.
EDIT: I found the post and it includes the Oring information that I found at Lowe’s.
https://www.brickboard.com/RWD/volvo/1685245/bypassing_solenoid_restored_proper_engagement_od.html
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Any twenty minute job is just a broken bolt away from a three day ordeal
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