Troubleshooting the common 2-3 flare in my wagon.
Vacuum from VCV is good: 13 at idle to 0 at full throttle.

Disconnected vacuum line to tranny, and car shifted much harder, so I know the modulator is doing something.
But when I apply vacuum to the modulator (green version), it will not hold. Drops almost as fast as I can pump with the Mityvac.
Adjustments on it (clockwise to firm shifts) seem to have no effect.

I'm planning a K1 on the car when I change the tranny fluid. Also have to do the burnt out bowden cable. I'd be happy, however, to not have to squeeze an allen in that tight area to replace the modulator.

So my question is:
Can a modulator be failing (no adjustment range, not holding vacuum) and still soften shifting (vacuum disconnected produced harder shifting, so it must be working to some extent...)?

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To the extent that the vacuum pump can "overcome" the leak, yes. But you need a new modulator. Replacing it isn't as difficult as you seem to believe.