Broken Tachometer (how to test)

[gazelle]

I have a 1997 90 hp Force. Currently there is a tach installed but it is not working. Was told at one time it did work with this motor but it was not installed for this motor.

I have seen in the archives that this motor most likely needs a 20 pole tach. I have no idea if the current one is a 20 pole. Since it worked at one time does this mean that it is a 20 pole or could an improper tach give a reading but an improper reading?


Assuming the tach is appropriate for the boat, any suggestion on how to tell if it is not currently working because of the wiring or the instrument? Will all of the wires connected to it carry a current? I can connect to the wires using a car electrical tester if this is the case. If so any idea what the readings should be for the wires?

 

[Frank Acampora]

Re: Broken Tachometer (how to test)

left outleftFirst check the back of the tach. There should be NO adjusting switch. It is a small white switch that you carefully turn with a small screwdriver to 2,3,4, or 6 pulse position. If the tach has one, it is NOT a 20 pole tach.
Next check wiring. At the bottom of the rear there are three brass terminal screws. One should be marked I (for ignition) or P (for power). this gets a 12 volt switched source from the I terminal on the ignition switch. This is so the tach goes off when the ignition is shut off.
Another terminal on the tach will be marked G (for ground) connect a wire from it to any good ground under the dash .
The third terminal will be marked s (for source or signal) Connect the purple wire from the engine to this terminal.

If the tach is connected this way and does not function, then it is probably bad. They are not repairable under most circumstances since most of the innards are on a circuit board.
Oh, yeah! I forgot! If you have a bad rectifier on the engine the tach will not work. So before you replace it, check the rectifier.

 

[gazelle]

Re: Broken Tachometer (how to test)

thanks Frank
Great specific info!

you said to
check the rectifier

I am not sure what it does but I believe I read somwhere that if it is bad then the batter would not charge, correct? I have not had any batter issues so if that is the case then I think it is the guage.



Thanks
Gazelle

 

[WillyBWright]

Re: Broken Tachometer (how to test)

The tach gets its signal from the charging system. If the tach needle zeros-out with the key on and that's that, you have a problem with the signal. Most such failures are caused by a blown rectifier. Most rectifier failures are caused by crossed, corroded, or loose battery connections. You can see if the battery is charging with a voltmeter. Voltage should rise with RPMs. At high idle, it should get up into the 13v range. At full throttle it should get into the 14v range.