ECU causing tach problems.

[68coronetGLwannabe]

I recently had my 4 pin Orange "MOPAR" ECU go bad. No big deal just went to Autozone and grab a new one. Well all they sell is the 5 pin one, I plug it in and my tach is all over the place. (After market Tach).  I take it back and get another one, plug it in and it does the same thing. I dig around in the parts pile and find another 4 pin one made by Wells. I plug it in and the tach is still going crazy. We end up taking a 4 pin Chrome "MOPAR"  one out of my buddies car and the tach works just fine. My new Orange Mopar one came in today and everything is back to normal. So my question is what is different in these ECU's that caused my tach issue?