Alternator...or not?

[rpell1@msn.com]

Symptoms of alternator on Universal 25xp diesel:
Tachometer drops to zero for 15 - 20 seconds, engine sounds like load was removed, (revs a little higher). Then tach returns and engine sounds loaded, (revs a little lower). Voltmeter shows 13.5 normal and 12.5 when tack fails.
This condition repeats itself every 20 - 30 seconds, continuously.
I tried switching from one batter to the other and both with the same results. Both batteries are at 12.5v at rest.

When I go to the boat tomorrow, I will be checking connections from battery, battery switch, alternator to tach and alternator to battery in the hope that this is the problem.

If this does not resolve the problem, has anyone had any similar experience they will share so I don't replace the alternator unnecessarily?
Thanks and Happy Sailing....

 

[centerline]

assuming you have a stock charging system, you may have either a short in the boat electrical, OR, more probable is the regulator shorting within the alternator....

 

[Stu Jackson]

What alternator? Internal or external regulator?

 

[rpell1@msn.com]

Hi Stu,

This is the original alternator on my '91 Cat30.

 

[Stu Jackson]

Sounds like a regulator issue or most likely a loose wiring connection.

 

[Capt jgw]

The alternator on my Albin diesel did that. Tach was not alternator driven so that was not affected, but voltage would go up to charging voltage and the same time engine would slow slightly. Then voltage would drop and engine speed would increase slightly. It would cycle like that every few seconds just like the OP's. I thought it was a fuel problem til I noticed the RPM's changing opposite the voltage. Alternator failed completely shortly after. It was an internal short in the alt. New alternator did not cycle like that so it was definatly not fuel related.

 

[JohnShannon]

What I think is happening is the field the alternator for some reason stops charging. When the charging stops, the engine revs since there is less load. The tach stops since that works off alternator pulses and now there are no pulses. Finally the bus voltage drops to 12.5 (battery voltage) instead of 13. whatever (charging voltage)

So why? Sounds like a loose connection in the field winding circuit if I had to guess (which I just did). You could jiggle the key see if that brings it back.

 

[Benny17441]

Concur that it sounds like a regulator issue. Don't know about the alternators being used by Universal but would suggest removing it and taking it to an automotive electrical shop for a repair. I'm personally not too keen on aftermarket replacements.