Strange Engine Warning Light Issue

[ctjstr]

On our 97 Camano Troll, I've encountered a issue I'm having trouble working through. Thought the electrical brainiacs on here might be of help.

One of our warning lights is marked "battery". when it goes off, it flashes a light and buzzes a loud warning. It occurs at both steering station. At the same time, the tach drops down significantly and bounces around a bit.

Its random and really is not affecting the actual engine rpm. It might be reading 2500, and go down to 500, back to 1000, down to 800 and then back to 2500. All the while, the engine is running fine. At the same time, my volt meter is constant, showing around 14 volts. It might run fine for a couple hours and then start bouncing around again.

Since the symptoms are the same at either helm station, my guess is the problem is where things come up from the engine room, but hard to say for sure.

any thoughts?

FWIW, the engine is a volvo tamd41 with dual alternators. 4 house batteries and a single start.

thanks for any suggestions.

toni

 

[AusCan]

Your tachometer gets its reading from one of your alternators.

The problem could be a loose loose connection between the tach and alternator, or the alternator belt is slipping. Either problem would cause both the low voltage alarm and a drop in rpm reading.

 

[FF]

With a tach hooked to an alt , it is not unusual to get strange readings when the batts are fully charged and there is no electrical load.


Put on a fan or the running lights , should solve th hassle.

 

[crashley]

My tacho does the same when the batteries are fully charged try turning on a large load and see if it settles down

 

[ctjstr]

thanks both for the suggestion. I'll give it a try.

 

[ctjstr]

so here is the update. Just finished a month off while servicing or replacing all the coolers on the engine. So now back to the charging issue. I'm reasonably convinced that the problem is one of the alternators. Tach stopped working completely, and battery light on constantly. Problem is, which one? I know how to test an alternator, but as close as I can tell, when there are two hooked in what is essentially parallel, when you check voltage at either alternator, you would get the same reading, since the two positive leads are essentially connected to a common. At least that's what my simple brain comes up with. My thought is that the only way to be sure is to disconnect the alternators one at a time and check the output of each one at a time? Does that seem right?

thanks
toni

 

[porman]

I may be wrong, but on our 2005 Troll I thought one alternator (the low output one) was just for the start battery and the high output alternator was for the house bank. The low output alternator fed the tach. I don't remember which one was connected to the low voltage warning.

 

[gsholz]

You might have a shorted diode in your alternator which will screw up the AC signal your tachometer is looking for. It would also account for the low battery voltage warning.

When you disconnect one alternator make sure you also disconnect the field otherwise you'll fry the diodes.

 

[ctjstr]

thanks both for the replies. Anything electric is PFM to me, but I'll try and work through it. I'll check both again and see what I can figure out. Thanks for the tip on disconnecting the field. Hope that ship has not sailed for me yet.
toni