According to the manual for my 87 the system automatically selects the lower vents when it senses warm air, and as the air cools it adjusts to the upper vents. Also, if the fan is set to auto, and the temp set to cool, the fan will not start till the evaporator cools. With a proper charge the AC should be blowing cold from the upper vents within the first minute. It could be delayed if you are not moving since airflow across the condenser is important for cooling. One way to check the charge is by looking at the sight glass after cleaning it with a rag and a small screwdriver. A few bubbles are ok but streaks or foam indicate air in the system.
You can also feel the line as it leaves the evaporator, it should be cool. I charge till the evap inlet and outlet are the same temp.

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Brian
87 300Dturbo 180K #14 head still running R-12 SOLD 12/2017
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