I was working on a tyler enviregard rack today (repairing a leak) and noticed that there was no hold back valve on the dropleg! I looked for the ord valve and it had what looked to be a solenoid valve with a coil comming from the discharge to the reciever! It looks as though there was never a holdback installed from the manufacture! It got down to around 15* the other day and this rack was running fine 200psig head. Is it possible they are just measuring the reciever temp. and when it drops engergizing this valve and pressurizing the reciever? Its a r404a lt rack.