MOPAR Performance Ignition "MSD" tach Qustion...

I have a 6AL in my car and after I installed it my vintage Sun tach wouldn't work, the higher it would rev, the lower the rpms on the tach read. Maybe need a MSD splitter? I pulled the tach and replaced it with a early vintage Auto Meter one and it works great! Both tachs had the green wire hooked up to the MSD box.