Re: How do I hook up a tach?
Ken, On the controls of that '72 merc, there should be a molded male plug on the forward part. It will have three slots with metal connectors inside. These connectors are for +12V, -12V and Tach sender. Your Merc dealer should have a molded female rubber plug with spade-type connectors to fit the male connector. Simply connect the proper wires to your tach (+12V, -12V and sender), set the tach to mercury alternator, and plug it in. The verticle slot is for the tach sender. The two horizontal slots are for +12V and -12V, you will need to check with a voltmeter, since I cannot remember which is which.
