Volvo Penta MD2030 Tachometer / Charging Info

[372RTM]

I am on a boat with the above engine. We have an internally regulated alternator, about 60amps. As far as I can determine, everything is pretty standard Beneteau 2000 vintage, 36 feet.

We have an intermittent problem with the tachometer. It goes to zero, and when it does we stop charging our batteries. Occasionally the tach “waves” at us, occasionally up to the speed of the motor, for short periods of time. I do not believe charging resumes during this activity.

And then it all starts to run perfectly again, correct rpm reporting and charging resumed.

We have checked all connections we can see. The motor and alternator is in good condition, well maintained. Clean and no evidence of any issues with salt water corrosion on the various connectors.

Does anyone know what the problem is ? It’s hard to diagnose as it’s so damn intermittent, but can be for quite long times.

[tcwippy]

Wondering if you have figured anything out? I have pretty much exact same issue. When tach doesn’t work, alternator doesn’t work. Actually I think it’s the other way around, ie when the alternator doesn’t work, the tach doesn’t work, which makes sense since the wiring diagram shows the feed to the tach comes directly from the alternator. I think my issue is a bad On/off switch on the panel, as the blue/red excitor wire for the alternator originates at the On/Off switch. And when I press the switch on and off repeatedly, the alternator and tach come to life. I’m also thinking of bypassing the panel and running 12v directly to the alternator excitor post. Have read that on other alternators this works, but that it’s prudent to put a 12v light on the hard wired excitor wire because the terminal switches from hot to negative???

[SailingHarmonie]

Do you have any other charging sources active? Maybe solar?

Depending on the sophistication and programming of the solar regulator, a solar system can lift the voltage of the batteries above the regulated voltage of the alternator. The regulator then shuts off the field current completely, and the sine wave the tack uses then stops. The alternator stops charging and the tach stops working.

There can be other more mundane causes as well, but the solar one is frequently missed in troubleshooting.

[tcwippy]

No alternative charging sources. As I think further it’s not the On/off switch, as the instruments lights and starter button always work when the On/off switch is on. So now I’m focused on the feed to the exciter. There is an “Excitation Resistor” shown in the wiring diagram, but for the life of me I can’t find any info from Volvo about what it is our what it is. Have reason to think it’s a 33ohm /5w resistor somewhere between the alternator and the instrument panel. Essentially it takes place of the “alternator lamp” I described in my first posting. It provides an initial current to excite the alternator at start up.