1/ Tach voltage supply is most likely in same draw circuit as fan circuit, changing the fan speed temporarily drops the tach voltage and thats why it fluctuates...run totally separate ignition on hot wire (with FUSE !!!) to supply side of tach...that usually foxes the issue...make sure you include inline fuse and also ensure the power supply si only on when Ignition is on.
2/ AC voltage to clutch circuit...I am assuming the system has all the necessary components including the vacuum components...
The biggest issue here is usually the system is not charged with gas sufficiently to enable the low gas cutoff switch...when charging a new system, it is sometimes necessary to bypass the low gas pressure switch to get the compressor to cut in...
