Nope there are no blow off valves as it would mean that refrigerant is vented to atmosphere which no one wants as it's a greenhouse gas. A leak test is almost pointless if the system is not running as that's when the pressures rise to levels that would show a leak.
Is there a safety pressure blow-off valve in the system anywhere? When I had it regassed yesterday, it passed the leak test successfully... which suggests the venting was controlled and it's no longer leaking.
Hopefully that'll find something!
I've got hold of a diagnostics process for the Radiator Fan Control Unit (which takes input from the HVAC controller and is responsible for operating the fans and the compressor) so I'm hoping to go through that (involves multimetering different contacts) and see where that takes me.
I know that if you run a system without a condenser fan running it doesn't take long until the high pressure cut-out trips, it's been a few years but two minutes sounds about right. How long it takes before a leak I really couldn't guess. I would hope that a HPCO comes in a long time before any damage could be done. Also if the HPCO was faulty the system would still cut off due to the low pressure cut-off, this normally takes a bit longer to kick in though IIRC, this suggests to me that two minutes shouldn't be long enough for damage to be caused.
Is it feasible that within the two minutes I forcibly ran the a/c for, the lack of a fan on the condensor would cause the system pressure to get high enough to blow a leak/blow a safety valve? Or does it indicate a deeper problem?
That could be a fair conclusion as your Haynes says the signal comes from the radiator control unit. I would have hoped there'd be a couple of fault codes generated though! What car is this on BTW?
If it's just the fan, the contol unit might not switch the compressor on unless the fan is running (i.e. faulty fans/wiring)? Or the control unit has failed and therefore no compressor or fans, perhaps?