Tach problem 93 Mercury 75 HP 3cyl

[Harvprov]

The new tach I have replaced seems to work up to 4000 rpm then it goes to zero. I have replaced the old tach with a new one as the old one never worked when I bought the boat. It also seems to go to zero if I try to trim the motor, I have check the connections all the way to the engine and everything seems tight, I have also checked continuity between all wires from engine to tach. Thought I would get out on this forum and ask if anyone has run into similar.

 

[schematic]

some tachs require a regulator or limiting resistor. Check the tach manufacturers website....

 

[Faztbullet]

Check rectifier and stator.

 

[wn6ngp]

activating your power trim puts a new electric load on the system. It sounds like your voltage may be dropping enough to cause the tach to fail. Hook a voltmeter to the 12v and gnd line going to your tach. Watch it when you activate the trim, I'd expect that it would drop substantially. If so then you need to figure out where the voltage drop is occuring in the +12 leg or the ground return wire.