Trojan Boat Forum

My starboard voltage gauge is dancing around 14 volts. It show 12.5 when I start the motor and when I go up in rpm's it jumps to 14 volts but jumps around. The port gauge starts at 14 volts and is steady. I checked with my digital volt meter at the gauge and get the same result. I Have 2 new batteries and checked for corrosion from my batteries to the block. Where do I start looking and has anyone had the same problem. I think it's a bad connection but open to any other suggestions.

Check the engine harness plug pins and the harness ground wire and power connections. Ensure connections are clean and tight.

Rather than checking the voltage at the meter, try measuring it as well at the battery, if you have steady voltage (12-14.5 volts) then try checking by the gage again but use a different ground wire (perhaps off the other voltage meter). Sometimes with poor ground connections, it can cause what you are explaining.

If voltage is jumping around at the battery, then you may have a bad voltage regulator in the alternator, and on the newer models, it is PART of the alternator, rare to have them jump around, they usually work, or dont.

Electrical is always a great time consuming excersize. Corrosion is the #1 cause of most failures such as this. Good luck.

I had much the same as you are describing, erratic operation, after checking connections , grounds etc , I replaced the volt regulator, all ok......now mine was an external regulator, the old design and not in the alternator......not sure what design yours is though.

mikeandanne wrote:I had much the same as you are describing, erratic operation, after checking connections , grounds etc , I replaced the volt regulator, all ok......now mine was an external regulator, the old design and not in the alternator......not sure what design yours is though.

I have a small can attached to my alternator. I will check the voltage around the battery.

I don't know what the small cylinder does. It's 2 inches long.

That could be the brush cover if that is a true marine alt.......look at your wiring diagram to see where the reg is if it is all still factory....keep telling yourself 12volt electrical is fun.

Can you send a picture of the "can"?

One other thing you should at least look at..... Belt tension on the alternator, any bit of slippage can also cause this.

And, There is also the possibility you have something loading the system only when the ignition is on, It would have to be a pretty good load (windlass, DC fridge, bilge pump) that could cause this..... These loads you would see right at the battery as well using your meter. If not, then turn your attention to the gage itself, or a bad connection associated with it. If easy to do, try swapping gages, or battery cables to it.

If the alternator is suspect, any or most auto parts stores check them for free.

As always, Good luck!

Belt tension is good. I will take a picture tomorrow. I will get soon readings around the alternator. I think the gauge is good. I got the same readings at the plug (socket) when I unplugged the gauge.

That is good news and bad news..... Gage is good, that is the good news, if your alternator is bad, that will cost a fair amount more than your gage....bad news. Hoping it is just a bad connection.

FYI i had the good fortune of having a bad VR in one of my alternators, Rather than replace the whole unit, I brought it to a local automotive electrical shop. They 'rebuilt' it (replaced only the regulator) and only charged me $20. Yippidee doo da, more gas $.

I stood there and watched him do it.... 20 minutes.....

Worth a try if you find it is the alternator(regulator).

My starboard battery reads 14.2 volts. When I start the motor it drops to 12.6 volts. when i raise the RPM's a little it goes to 14.5 volts and jumps around. The first pic shows the can.

Looks like an old style can capacitor.....seems like it is all charging correctly, judging by your numbers there.....I would take the numbers off that and the alt( motorola) and visit your local alternator rebuild guy and ask him what that is for and do you really need it.....maybe that is giving you the trouble ......does the other engine have the same setup??

The "can" is a capacitor. It's function is to take the noise out that a rotating type generator will produce. Without it you risk transferring that noise into radios as a whine that changes with rpms.

As you stated a connection could certainly be at fault. May be worth switching batteries just to rule out a bad one.

Try match marking the belt to the pulley just to be sure it is not slipping.

Correct on the capacitor, it also is supposed to act as an electronic snubber or shock absorber for preventing exactly what you are describing. (erratic voltage). Think of it as a small battery that helps hold the correct voltages but for only short periods of time. It could be that it is failing and not so much the alternator. Most all the newer alternators have built in voltage regulators and noise suppression. That way they can sell you a new or rebuilt alternator vs repair only the less expensive bad part. Like I said earlier, voltage regulators either work, or they dont 12-14.5 or 0.. I think you are getting warmer. Not sure of the age of yours, and even if a rebuilder will have the proper parts for it. I'd sure try.