1997 Mercury Tach

[stuckinthehole]

I have a 1997 Mercury 90 hp 2 stroke 3 cyl. outboard on my 19 center console. The tach is not working. When cranking the motor the needle does move a bit but once started it doesnt read anything. checked the voltage on the battery with the motor off and read 12.2 V and then started it and still reading 12.2 V. The gauge is new so it shouldnt be that and I am pretty sure the wiring is correct. I just finished a rebuild of the boat but the tach did not work prior to the rebuild. Heard that it may be the recitifier since the battery is not getting a higher voltage when the motor is running. Havent been able to troubleshoot much yet but should be able to try some stuff out on it tomorrow after work and give more details then. Any suggestions on what I should try first?

 

[MH Hawker]

Re: 1997 Mercury Tach

If the charging system isnt working the tech wont work, the tach gets its pulse signal from the charging stator.

 

[Texasmark]

Re: 1997 Mercury Tach

If the charging system isnt working the tech wont work, the tach gets its pulse signal from the charging stator.

Yes but it comes from within the rectifier/regulator module. So either could be the problem. Service manual says to disconnect the yellow wires that go to the stator under the flywheel. Neither should show continuity to ground...engine block, batt. - lead. Between the leads, there should be somewhere around 0.15 ohms give or take depending on temperature. If using a digital ohm meter touch your leads together and get the ohm reading. Subtract that number from what you read on your meter when you read the stator resistance to get the net stator resistance. If it looks like it is shorted, manual says that's ok. If you read an open ckt stator is bad.

The rectifier/regulator module is identified by the following wires going to it:

2 yellow, stator inputs.
Small red, power to the unit if it has a separate small red which mine of like kind does.
Large red, regulated voltage output from the unit to recharge the battery.
Grey, pulsed signal to operate the tach.....which also needs 12v and gnd to work besides the grey trigger input.
Black, ground.

Depending on rpms and battery condition an operating rectifier will put out 13.5 to 14.5v when operating properly. I would recharge your battery before you put in new parts; 12.2v is pretty well discharged. The engine recharge system is designed to keep a battery charged, not charge a discharged battery. It can overheat and die.

Mark