Tach non working on Mercury classic 50

[Open john]

How would I check to see if I'm getting a signal to the tach? All of mine came hooked up when I bought the boat, but the tach doesn't work. Could it just be an old worn out tach? Everything on this boat is a 1986. Thanks all.

 

[jimmbo]

Welcome to iboats

Could be a lot of things:
Broken Connection
Fried Rectifier, happens when someone gets the Battery polarity wrong for even less than an Instant
Improperly connected
Defective Tachometer

 

[Open john]

Welcome to iboats

Could be a lot of things:
Broken Connection
Fried Rectifier, happens when someone gets the Battery polarity wrong for even less than an Instant
Improperly connected
Defective Tachometer..

It does still charge my battery when running, but how would I go about testing the rectifier? I have cleaned all the connections, verified none of them are broke. Applied dielectric grease and reinstalled. All the connections are on properly per almost every wiring diagram I have read. The two things I'm down to is rectifier, or just old worn out tachometer. Thanks for the help!

 

[jimmbo]

Even if the connectors are good and are connected to the wire well, wires have been known to break inside the Insulation. You should check for continuity.
To see if the Charging system is working, just connect a Volt Meter across the Battery, as the RPMs increase so will the Voltage, usually topping out in the 16 Volt range. It is an Unregulated Charging System, but Maximum Output is about 6 Amps. However since you say the battery hasn't died, it is probably doing its Job.
One thing to be aware of, with an Unregulated Charging System, you should only use Wet Cell Batteries that are NOT of the Maintenance Free variety, as the Electrolyte Level needs to be check regularly and Distilled Water added when low. So AGM Batteries like the Optima are a No No

 

[Chris1956]

Most Mercury rectifiers are bridge rectifiers. That is one diode can be bad and the system will still charge the battery at half current. A bad diode can also not provide the tach signal. You might connect the tach wire to the other AC terminal of the rectifier and see if it then works.

 

[jimmbo]

Mercury is not using Full Bridge Rectification, just Half Bridge. Might be a way to keep the Current low enough to avoid a Regulator

 

[Chris1956]

Jimmbo, My Mercury's from the 70s all had full wave bridge rectifiers, when did they change to half wave rectifiers?

 

[jimmbo]

I saw a Wiring Diagram from Mercury and it showed the Rectifier as having 2 Diodes, not 4.

 

[Chris1956]

John, What is the model number of the motor? How many wires on the rectifier? A full wave rectifier has 3 wires (red, yellow, yellow), plus the rectifier body is grounded.

A half wave rectifier will have 2 wires, (red, yellow)

 

[Open john]

 

[Open john]

John, What is the model number of the motor? How many wires on the rectifier? A full wave rectifier has 3 wires (red, yellow, yellow), plus the rectifier body is grounded.

A half wave rectifier will have 2 wires, (red, yellow)

I have red, yellow, yellow, and my tach wire.

 

[Chris1956]

That is a full wave rectifier.

Move the tach wire to the other yellow wire terminal. If the tach works, replace the rectifier, as at least one diode is open.

 

[Open john]

Will try this in a little bit. Thank you sir!

 

[Open john]

Rectifier tested good. Continuity from tach all the way back to rectifier. I have continuity with power ground and signal. Checked voltage at signale with motor running. Have voltage. Seems like I just have a bad tach. New one already ordered. Thanks all for all the help.

 

[Chris1956]

Glad it worked out. Mercury charges a arm+leg for rectifiers. Sierra is better, but they were a $10 part at radio shack, for a much better one.

Tach setting is likely 6c or 12P whichever you have.