i may be wrong but the reason i mentioned the pcm is because i thought the tach gets its signal from the pcm on pin 10. and on the newer pcms, you need a 680 ohm resistor as LS2Goat pointed out. i used this diagram http://lt1swap.com/lsx_tach.htm as most guys on here also do to hook mine up. durring one of my conversations with brandon IIRCC, he said the pcm can be set for however many cylinders you are running.... maybe i misunderstood and thats NOT how it works???? if not then what is the pcm doing?
i agree with LS2Goat, check the resistor thing as that may be your problem. what year pcm do you have?
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