Abstract: A pressure sensor probe has a first port connectable to a relatively low pressure source in a compressor crankcase and a second port connectable to a relatively high pressure source at the output of an oil pump. A shuttle is slidably disposed in a bore and has a first end which closes the first port when the difference in pressure between the two ports indicative of normal operation exceeds a selected value determined by the relative areas of the shuttle exposed to the respective pressures and by a coil spring which biases the shuttle away from the first port. A passage is formed between the shuttle and the bore leading from the second port to a force receiving surface on the high pressure side of the shuttle. The length and cross sectional area of the passage is selected to provide a desired dampening or time delay between the high pressure source and the force receiving surface. A control circuit includes a reed switch whose state of actuation is dependent upon the position of the shuttle.