When you've found the OD relay, per Art's directions, you can do a temporary* diagnostic bypass with a jumper wire at the relay harness connector plug.
Just connect these two plug terminals together with a "jumper" wire:
• Input terminal #15 (2 wires, a Blue-Black and a Gray)
• Output terminal #87 (1 White wire)

A good-fitting jumper can be made with flat male crimp-on terminals that are the similar to the relay terminals. This jumper wire will simulate a relay that is always energized — which is the normal condition, until the shifter button is pressed.

* If the OD works OK with the bypass jumper (and you don't need the shifter button downshift option), you can just leave the jumper wire in place.

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Bruce Young, '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.