your merc has a grey wire at the voltage regulator with engine running look for an A/C voltage reading if present check at tach grey wire and hopefully you will get same result,if so check for power and ground at tach as well should be battery voltage if all readings are ok than replace tach. As far as ACARR83 ,I think your tach signal is green so check for A/C power there at tach and power on yellow and ground