Tach works on start up then nothing.

[Letzgoracin301]

Ok I'm at a loss. 98.5 24 valve o-rings, studs, htt62/475 compounds and SDX 240's. no box right now just a boost fooler. When I start the truck the tach works and responds like normal. Then after start up sequence when all the idiot lights go out (about 15-30 seconds) the tach quits. I've changed the crank sensor 3 times. It all started when crank sensor went and was replaced the first time. The other 2 were junk from advance that didn't work. When my adrenaline was hooked up the iquad tach worked and seemed to respond normally. Tried a different ECM made no difference. Throws a code for tach sensor voltage. Anyone have this problem before? Thanks