Re: 40 mariner outboard
Slowly blowing the fuse, regardless of what size, says that you have an overload of current just above the rating of the fuse, as compared to a short circuit which would blow it instantaneously. With that said, and if it is a 20 amp fuse then you are pulling a little over 20 amps.
Well sir at 1 ohm 20 amps would take a 20 volt battery which you don't have. Thinking through this as I type: so for 12v and say 21 amps, the resistance would only be 12/21 = 0.57 ohms. The lead resistance of your ohm meter would be on the order of 0.3-0.4 ohms and the wiring to that tach is probably 20 AWG for another 0.3 or so ohms, so you could have a short circuit in your tach and not be able to recognize it for the reasons given.
Wiring to the tach should be: purple (12v) input, black (ground...battery - term), and grey (tach) input. Pins on tach interface plug: Purple pin A, Grey pin B, and Black pin D
Go to the back of the tach and measure the resistance across the purple and black terminals and ensure that it is something greater than 5 ohms with the ignition off.
Then, change to DCV on the meter, turn the switch on and measure the voltage at those same terminals.
Come back with the results. Oh, and the size of the fuse that you have been blowing.
Mark
