In my initial post I said I swapped solenoids, tested the relay, tested to get 12V at the unit, tested the ground, got it to click by grounding against the trans case.

In addition, I started the car up with the unit off and let fluid push out the two holes in the mounting face.

Then I removed the solenoid again and blew air down into the trans through both holes on the face. I also tested the soleniod with air: off = no air goes through the holes; on = air goes through. I am sure of this: I tested several times. So the solenoid works.

Fluid is clean and level is correct. Kickdown cable is free.

There would be no point in cutting a passage in an old unit (my initial plan) since I know 12V opens the passage on the installed unit.

There is a plug (12mm bolt head) on the side of the trans just below the solenoid. I removed that and probed around but nothing. Thought of blowing air through this but I worried that there might be check ball or spring. Neither Bentley nor Haynes have a schematic.

It's possible of course that the OD disengages when I drive the car but I've replaced the wiring and so I think this is unlikely.

Am I missing anything?

I have heard that there is something mechanical that breaks in the trans to disable the OD unit but I find nothing here about it. Does anyone know what the internal problem in the trans might be?
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240s: 2 drivers and some parts cars