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If voltage is getting to the compressor but it does not run, the problem could be the thermal cutout switch falsely telling the ECU that the pump is overheated. (The compressor is only rated for about 15% duty cycle at maximum pressure, hence the need for a thermal cutout). In my case, although the pumping was becoming slow and the bearings were getting noisy, the failure of the thermal cutout was the immediate cause of failure. This switch (the third and thinnest lead, colored orange, emerging from the pump motor body) is normally connected to the ground lead of the compressor; it open circuits when the operating temperature gets too high. On mine, the cutout switch open circuited for no particular reason, telling the ECU that the pump should be switched off. I was able to get the system going again by grounding this lead with a jumper.