1985 Mercury 90HP inline 6 s/n 0a149573 Charging Problem

[Wills Mercury]

Ok. I just got a new (to me) 1985 Arima Sea Ranger with a 1985 Mercury 90HP inline 6 s/n 0a149573. It starts and runs easily. Boat has Perko switch for batteries 1 and 2 at stern. Boat has perko switch and isolator for battery in bow. Motor is really clean under the cowl and looks to have (1) Switch box replaced (black potting) and another looking original (amber potting). Starter solenoid looks a bit corroded and rectifier looks newer (black potting). Problem is charging. I put my digital volt meter on each of 3 batteries and all are 12.4 volts running. I rev it up a bit and same voltage. I start motor with perko switch on 1 &2. 12.4 volts. I move the Perko switch to 1, then I get 12.4 volts, I move the switch to 2, then I get 12.4 volts. The volt meter on the dash reads approximately 11 volts. My shop manual will not arrive until this Saturday. Any advise on using the multimeter to test the stator and rectfier would be much appreciated. Also what other things that I should look at to make sure is connected... Thanks All...

 

[Chris1956]

Put the multimeter on the battery, engine off. Note voltage. Start the motor, and rev it to 1500 RPM on the flusher. Note voltage. If the voltage with the motor running at 1500RPM is higher then with the motor off, the motor is charging.

Your charging system is maybe 9A Max, at higher RPM. If you have a good battery, you will only see a fraction of a volt higher, when the motor is running. Hopefully your Perko switch is make-before-break, or you will destroy your rectifier.

Your voltmeter on the dash can easily read the wrong voltage. If it shows a rise in voltage with the engine revving, it may be used to conform the motor is charging, but is likely inaccurate.

 

[Wills Mercury]

Thanks for the advise Chris. I will try that as soon as I get off of work on Friday and report back. In the mean time, can you tell me what is "make-before-break"...? This boat is really clean for it's age, so I should be able to find a part number on the Perko Switch and look up the specs. This is my first "older" boat, so "knowing what not to do" is as important as knowing "what to do"....

 

[Chris1956]

The newer Perko switches are make before break. When switching from battery one to battery two, the switch connects battery two (make), before disconnecting (break) battery one. This protects the rectifier from not being connected to a battery, which usually causes it to blow a diode.

 

[CharlieB]

The length of a wire run and any corrosion on connections can make a voltage lose. You can measure voltage at the regulator both not running then again at RPM. You should see the voltage rise with RPM, if not then it is time to start Ohms testing the regulator/rectifier and the stator. Be SURE to disconnect the wires before testing to ensure NO voltage from the battery.

 

[Wills Mercury]

Ok. So the results were (1) battery 12.0 volts before starting motor. Running at idle 12.1 volts at around 700rpm. Running up to 1500 rpm, I got 12.8 volts and at 2000 rpm, I got 13.0 volts. I'm not sure if this is good or bad. I'm used to an alternator running 14 volts from idle to top rpm. Any thoughts...? I'm looking up the Perko switch info tonight. Thanks for your time...!

 

[Chris1956]

Your results are exactly what is expected. Your motor has a simple stator-based charging coil, flywheel permanent magnet and a rectifier.

Alternators have a voltage regulator, a field coil, armature and a set of diodes. They are often three phase. Alternators are designed to charge much more at idle, then a generator (or stator).

In your charging system, as RPM increases, more AC electricity is generated by the charging coil, and converted to DC to charge the battery. At high RPM she will charge about 9 Amps. To do that, the voltage needs to rise, as RPM rise.