78 Johnson tach not working

[28sstony]

I put a new tach and rectifier on my 78/150hp roughly 50 hours ago when I first rigged the boat. The tach originally worked after the boat ran for about 20 minutes. Now the needle will jump a little when the engine first fires but then drops back to zero. So basically its not working at all besides the ocassional needle jump when the engine very first fires.


What do I need to be looking at?

 

[ezeke]

Re: 78 Johnson tach not working

Run a temporary wire to the gray wire at the terminal block on the engine and back to the S post on the tach. If it works the wiring is likely damaged or your ground is weak. If there is no change, test the rectifier.

It takes very little to damage one or more diodes in the rectifier; just a loose battery cable can do it. Anything that allows voltage to run the wrong way to a diode can burn a rectifier.

 

[28sstony]

Re: 78 Johnson tach not working

How exactly do I test the rectifier with a meter?

The wiring is all fresh 50ish hrs ago.

 

[ezeke]

Re: 78 Johnson tach not working

Edit: if you have a rectifier with a separate regulator, you can test the rectifier with an ohmmeter: http://forums.iboats.com/showthread.php?t=190638

For water cooled see: http://forums.iboats.com/showthread.php?t=190636

Also read: http://continuouswave.com/ubb/Forum6/HTML/000888.html