I've been wondering about my tachometers being out of calibration all season. One is a Stewart Warner on the fly bridge and the other is a Motorola on the lower helm.
The tachs ran off the stator wire from the alternator. The boat long ago was originally equipped with a Delco 60 amp alternator internally regulated. This was replaced with a Powerline 120 amp alternator and smart regulator. I replaced this system with a newer version of the same. The problem ever since I bought the boat is that the tachs read WAY off from actual rpm. They read high by approximately 400 rpm (checked with a photo tach).
There is a small hole in the back of both tachs. Is this for calibrating the rpms with a trim potentiometer? These are older tachs and don't use the DIP switches the newer versions use and I can't find any info on them ( 40 year old technology).
I don't want to stick anything in there without knowing what's gonna happen...lol....
Anyone have any ideas on this? Thanks, Ted