i've been sent to work on a mcquay water source heat pump 20 ton unit. its had several compressers and a few exv valves replaced over the ten years its been installed. what i've found is that as soon as the scroll comp. starts the head pressure goes to about 200 for a second then climbs to 400 and then it shuts down due to high head pressure. the unit has excellent water flow through the coil. any help please.