2006 sullair portable compressor

Engine shuts down after 30 seconds,flashes four code,in trouble shooting it says fuel related,plugged fuel filters or possible not receiving input frequency from alternator,at battery I am getting over 14 volts alternator is charging,the remedy in trouble shooting says on The w terminal should be (7.5 + 2.5 vac greater than 300hz) By the wiring diagram this goes to the ssam controller I am not sure how to check this I have good test equipment,fluke,should I check this with engine running and wire off terminal on alternator or hooked up,I am installing new fuel filters to rule out this but not sure about the alternator?any help would be appreciated . Jerryjungwirth, Red bluff Calif.