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Hi Alex,
From the symptoms you've described I guess you might have a stuck/jammed variable orifice tube. Yes this is just a guess because you haven't provided any AC pressure to confirm with.
Is the aluminium tube/pipe after the orifice tube frozen? Anyway this icing condition could be difficult to see if the orifice tube was installed at the evaporator's inlet instead of the usual place along the inner fender (for earlier years 740/940 the OT was commonly installed at evaporator's inlet pipe).
And I don't think your violet pressostat (VP) was activated. If it was it would take a good 10mins to 1/2 hour for it (VP) to stay ON (this means to OFF the compressor). Your rapid compressor cycling (turning on/off) is probably due to quick drop in refrigerant pressure at the low pressostat (at accumulator) due to stuck variable OT. This could be seen when an AC manifold gauge is connected.
Connect AC gauges to the low pressure port near the accumulator and take reading while AC is OFF. If you have full charge the initial reading would be high. Then switch ON the AC and see if there is rapid drop in refrigerant pressure until below the threshold pressure of the low pressostat (which is below 29 PSI) when it would turn the compressor OFF. And this happens repeatedly.
Variable OT has been known to cause problem despite its benefits. I had installed a few last time in my car. When it worked the cooling performance was still the best. But every time it jammed I replaced it with a new one thinking it was manufacturing defect. In the end I put back the standard OT for peace of mind.
Regards,
Amarin.
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