Firstly, jdere; a test regime can only do so much…
The system does not ‘know’ that the compressor etc is not working - it just assumes it is, using available ambient air to cool.
Basically, the amplifier compares ‘in-car’ sensor’s resistance with that of the temp control - then uses the ‘error’ to correct the servo’s position (whirring). The aim of the amplifier’s program is to equalise the two, using flap positions, fan speeds, water valve etc to rapidly acquire, then maintain, set cabin temps. All such manipulations are done by the servo - se sole function of the amplifier is to position the servo…
So a fundamental factor, apart from, as Doug stresses, verifying servo buzzing is to know cabin temp. If cabin temp is above the ‘65’ set, the system goes into cooling mode. Which indeed opens the centre vents, and closes others including open air inlet - recirculation mode.
So indeed, if ‘cold’ air enters; it implies the system is not in recirculation mode - flap open to outside air. This may be a PO modification; since there is no AC cooling - he (or she) may have disabled the high speed recirculation switching. This is easily done by disconnecting the electric connection to the solenoid operating that flaps vacuum switch. Another possibility is that vacuum is not reaching the flaps’ vacuum operator, disconnected or defective, or flaps simply stuck, as you say - take your pick…
You do not mention the system’s behaviour in ‘heating’ mode - which is important for general diagnosis before detailed fault finding. Set the temp control to ‘75’, heat up the engine and set function switch to ‘auto’. The fans should run at high speed, the centre vents should be closed and all other vents, except defrost, open - the air flow should be ‘hot’. As the cabin temps reaches set cabin temp; the fans should drop to low speed - and the centre vents may open ‘ajar’.
This requires monitoring cabin temps; a thermometer is ‘useful’ - ‘feelings’ are deceptive…
Frank
xj6 85 Sov Europe (UK/NZ)