I think I'm familiar with that unit. It will need to see 100PSI in the tank before it will charge. When it is "pressurizing tank" it is actually warming the tank which raises the pressure in the tank assuming it has sufficient refrigerant. It can take several minutes before pressure comes up. In addition there are valves inline and at the tank that need to be opened before use and ideally closed after every use. If the valves are closed it won't see the pressure and won't charge.

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90 300TE 4-M
Turbo 103, T3/T04E 50 trim
T04B cover .60 AR
Stage 3 turbine .63 AR
A2W I/C, 40 LB/HR
MS2E, 60-2 Direct Coil Control
3" Exh, AEM W/B O2
Underdrive Alt. and P/S Pulleys,
Vented Rear Discs, .034 Booster.
3.07 diffs 1st Gear Start

90 300CE 104.980
Milled & ported head, 10.3:1 compression
197° intake cam w/20° advancer
Tuned CIS ECU
4° ignition advance
PCS TCM2000, built 722.6
600W networked suction fan
Sportline sway bars
V8 rear subframe, Quaife ATB 3.06 diff