Testing tach

[Joker2]

If I just disconnected wires from back of existing tach and rewired them to a different tach and had someone just hold it while running, do you think this will be a good test to se if my tach is reading corrrectly ? Evinrude 1994 150 HP Ocean pro. Have to find a tach first.

[phatdaddy]

don't know why not, i've got a tach on my bench i hook up to check motors.

[spareparts]

thats how i do it, I have a spare tach with leads wired to it, it has aligator clips on the other end, I check for voltage(purple wire) first, then check ground(black wire) if they are good to go, I wire up my spare tach, hook up the grey wire(signal) and see if my tach works. Its best to unhook the old tach first, in case its shorted to ground

[Destroyer]

Just make sure that on the back the tach is set for the right number of poles.

[Joker2]

I have heard that for my motor it should be set to 6 as I have a 12 pole alternator, correct ?

[Destroyer]

Originally Posted by Joker2

I have heard that for my motor it should be set to 6 as I have a 12 pole alternator, correct ?

Correct

Here's why: A signal that drives the tachometer is often taken from the permanent magnet alternator. Typically the signal is taken from the stator coil output as it is applied to the rectifier. The attachment of the battery and the rectifier to the circuit affects the ouput of the stator coil somewhat. The positive voltage peak is limited to about a volt above the positive battery voltage, and the negative voltage peak is limited to about volt below the negative battery voltage (or ground). This signal is a half-wave rectification, so it has half the frequency of the full-wave output. If the alternator has twelve magnets, the tachometer output will have six pulses per rotation. Hope this helps.

[Joker2]

Thanks !