This should help
It's the instruction sheet for the macerator (beginning to see why you should keep this stuff instead of tossing it???). http://www.jabsco.com/prodInfo/overview/18590_MACERATOR.PDFIf you can hear the motor humming, it's not burnt out. If you can't hear it, the macerator is toast...time for a new one. If the macerator hasn't been used in a while, the impeller is prob'ly stuck to the inside of the housing...it can't spin, which is making your motor work overtime. You'll see instructions for freeing it without removing it from the boat in the instructions. Since all that straining is likely to crack an impeller vane, I strongly recommend that, IF you can get it working, you replace the impeller.