The aftermarket tach should be wired to the grey wire with the black strip comming from the PCM. That is weird though that the aftermarket will work by itself and the stock is not accurate by itself. I really don't know exactlly what to tell you other than for some reason you are not getting a good voltage signal to the stock tach. You did say that you have tried a few clusters, other than that I would say its the cluster. Honestly my best advice is to just use the aftermarket tach and get a cluster without a tach to neaten up the dash and be done with it.

 

well i guess that will have to worki just thought i would see if any one had any ideas. but thanks for the help