oil pressure warning

[kingofhearts1961]

i have used the retro fit air con on my 52 plate tuscan today, when driving and coming to a full stop the oil pressure warning beeper comes on saying the oil pressure is down to 1psi also the standing idle speed has dropped to 6k, when switching the air con off all is fine. anyone else had this happen and any pointers on this? i checked the oil level and that is spot on.

[NCE 61]

Idle speed normally drop's by about 200 RPM with aircon on as there is no proper idle control, best to increase the idle speed to 900 to 1000 RPM with the aircon off. As for the oil pressure 1 PSI at 600 RPM sounds too low, maybe the oil pressure sender is U/S, what is the oil pressure above 2500 RPM?