Assuming you have the C68 Auto Climate Control.
The AC Coolant Fan Switch which screws into the high
pressure line Opens around 233-240 psi and Closes around 185-
The Switch has two wires going to it. A Dark Green wire
goes to the ECM. The Black wire is ground.
If you unplug the connector, (simulates over 233 psi), start
the engine, the Radiator fan should run.
If the Fan does not run, there is a problem with the ECM.
If the Fan runs the problem must be the AC Coolant Fan Switch
is closed all the time, the system pressure is never reaching
240 psi which would open the switch or the Dark Green wire is
shorted to ground.
If you remove the plastic wheel well housing you can get
a better look at the Fan Switch.
The switch next to it towards the firewall with the larger
connector plug is the High Pressure cut out switch.
For safety reasons, there would be no reason to short that switch
out unless for testing purposes to see why the AC Clutch won't engage.
The schematic also shows the auxiliary fan which you said you don't