Cvhe chiller giving a prestart alarm of "Differential Oil Pressure Overdue". This has happened almost consistently since the warm weather has started and the chiller has been on line. To this point I have attempted to cycle on the oil pump via the service test mode. The oil pump does start as confirmed by the current draw but does not pump. The discharge line from the oil filter is cool confirming no flow. The oil temp is at design of approx 143F, and the oil filter was recently replaced during winter maintenance. I can cycle the pump numerous times until it finally catches as though it is priming itself and then oil pressure is properly established. Repeated attemts of starting the pump will usually result in a positive oil pressure until the chiller is off over night and attemts to start the next day when the cycle repeats itself. I am thinking that a key may be sheared between the motor and the oil pump gears, any other ideas would be appreciated before we have to open it up.