68, R134 conversion

[Print Bear <prntbear@xxxxxxxxxxxxx>]

I would like to recharge my '68 Auto Temp. I will have to use the R134. 
The controls seem to be working properly and the heat and cool happen 
through the correct ducts however, the compressor doesn't ever engage. 
Is there a pressure switch that would be preventing the clutch to 
operate due to low freon? I have checked and all the fuses are good and 
the fan is operating automatically.