As pointed out the lack of vacuum to the modualtor will make all shifts at full load strength. A stuck modulator will act similarly.

If one were to actually measure modulator pressure the difference is striking between pressure at vacuum and at zero vaccum. The proper adjustment by a gauge is done with the vacuum disconnected at 2000 rpms (it's the same at idle if you got good trans pressures, the 2000 standard is to be sure the pump is fully working).

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician