Re: What is the difference between a 4 pin icm and a 7 pin icm??
the 4-pin module is for older vacume advance HEI,
the 7-pin for '81~'86 HEI with ECM controlled timing.
I too have found slightly different ECM plugs on CCC
distributors;i think carb uses one style and EFI the
other style-AFAIK, the only difference is the coil
polarity-one uses red+white coil primary leads,the
other red+yellow.For the tach problem,they can get
stuck at max if power is cut to the tach when it is
above a certain RPM-if high revs don't get the tach
back to normal,you could try physicly moving the
needle back to zero and see if normal operation