In the past your alternator would always work at the same level (i.e. voltage) regardless of battery capacity. Now, with your external regulator, you can vary the level of work your alternator produces. In order for your tachometer to work, you need to have your alternator work a little. If your external regulator is off, that means your alternator is off, which in turn means your tachometer if off. Going forward, you should always have the external at the lowest level to have your alternator engaged a little. That way your batteries will get some charge when you are motoring, and simultaneously your tachometer will work as well.