what v8 r u runnin
usually if the original engine is a single coil and the new engine has multiple coils is when u get problems
theres things u can buy to make the tacho signal compatible or i have some tiny electrical devices that can modify the signal that might work
is it a lexus v8? if it is then u might need an adapter
7m gte with 3 coils and 1jzgte with 6 coils i havnt been able to get a good signal for the tacho
u can use an msd tacho adapter on the 7mgte to get the tacho to work ive done this and it works
soon ill be tryin the msd tacho adapter on a 1jzgte and hopefully itll work too