Mercruiser 4.3L Alternator acting up

[flynnhart1]

I replaced the alternator in my 1997 Sea Ray 4.3L with the correct part last May. Everything has been fine, correct voltages at alternator output, battery, gauge, etc, until last week. I started and launched the boat, everything was fine until I heard a tinny, pinging sound coming from the engine compartment. I immediately recognized it as the same sound the bad alternator made before I replaced it. I looked at my volt gauge and it read zero volts. I shut the engine off and checked a few things (belt, wiring, etc). After sitting for a few minutes I re-started the engine and everything was fine for the rest of the day. Anyone have any ideas?

 

[Silvertip]

Have you used a voltmeter to measure engine off and engine running (1800 rpm) at the battery to prove or disprove the alternator is/is not working? It may be the voltmeter at the helm is not working because of the loose wire. With the engine running, voltage at the battery should be 13.6 - 14.5 give or take a bit. If the battery voltage measures the same with the engine running or not (12.6 volts or less) the battery is not charging. Note that I said the "battery" is not charging. That does not mean the alternator is at fault. We have no idea what the wiring situation is. A voltmeter that reads zero indicates a problem with the gauge because even an almost dead battery will show some voltage, engine running or not.

 

[bilge rat jim]

Start by taking apart and cleaning all electrical connections in the charge system, including the ones on the volt meter.

 

[flynnhart1]

I didn't have my DVM with me at the lake so i don't know what the voltages were when it acted up. I'll bring it with me next time I go to the lake. It was only a temporary thing when it happened. It acted up, I turned the engine off, checked a couple of things, re-started it and all seemed fine. Since I've been back home I removed the volt meter from the boat and hooked it to a stand alone power supply and it seems fine. I shook it, tapped it and could not get it to act up. I also measured resistance across the terminals and it is in spec at 130 ohms. I have the factory wiring diagram and I checked all of the wiring at the volt gauge (and all other gauges including the ignition switch) and the alternator and everything seems fine, no loose or dirty connections. My gut suspicion is that something is up with the alternator sense circuit. I am assuming the boat is still wired the same as it was from the factory and the way it is wired, the sense wire to the alternator comes from the + terminal on the volt gauge. If there was an intermittent short to ground on that wire it would make the volt gauge go to zero. Has anyone seen what an alternator does if the sense gets shorted to ground? Like I said, the alternator was making a tinny, pinging kind of sound when this acted up. It went away after I re-started the boat. Has anyone seen an alternator failure where the sense internally shorts to ground?

 

[Silvertip]

Smoke results from a sense wire being grounded. I question what you said about the sense wire wire coming from the POS terminal of the voltmeter. That's not necessary since the battery is much closer to the alternator than the voltmeter. electrically they are the same point but physically they would typically not be. The positive terminal of the voltmeter typically is a daisy chain connection between all POS terminals on all gauges -- as are the ground connections. The only "engine" related connections would be from senders like oil pressure, water temperature and fuel, The voltmeter is just that, two leads that measure system voltage and that can be done through the feed from the ignition switch. If the sense line were shorted, the voltmeter would read zero for the duration of the short. But if that short was longer than a few seconds you would have seen or at least smelled smoke and that wire would show signs of heating.

 

[wrench 3]

Silvertip: Since he's saying the wire comes from the voltmeter that is ignition controlled, I'm wondering if he's talking about the exciter wire.

flynnhart1: I'm questioning the fact that the voltmeter went to zero. I can't see that happening (other than a bad gauge) without the rest of the instruments going down also.

 

[flynnhart1]

My bad, thanks. I meant to say the field (exciter) circuit and an open. The factory manual shows the field wire coming from the ignition switch to the volt gauge and from the volt gauge to the alternator field input. I checked the wiring and the +12v from the ignition switch goes first to the Tach (fanned out from there to the other gauges) and then from the Tach to the alternator field input. I'm wondering if the alternator was temporarally putting out zero volts? The tinny, pinging sound it made was exactly the same sound the previous bad alternator made all of the time. The new alternator made that sound and the voltage gauge read zero but everything went back to 'normal' after I turned the engine off and re-started it.

 

[flynnhart1]

wrench 3:

I removed the gauge and connected to a bench power supply. It read almost exactly whatever voltage I applied to it. I shook it, tapped it, etc and could not make it fail. I ohm'd out the terminals and they were in spec (130 ohms).

Of note: In addition to the zero volts on the gauge, the alternator was making the same tinny, pinging sound the previous bad alternator made.

 

[wrench 3]

I'm sorry but I'm having a problem getting my head around this one. You say that the voltmeter feeds off of the tack supply and I imagine that the grounds are some what similar. So how does the voltmeter go to zero and the rest of the gauges keep working?
Also a tinny pinging noise sounds like a mechanical failure (bearings etc) in the alternator. Electrical failure (if they loose a phase or overload) usually gives off a wining sound if any.
If the alternator simply quit charging, the voltmeter would go to battery volts (proximately 12V) not zero.

 

[flynnhart1]

Yes, this is definitely a weird one. For now, I'm going to assume it will happen again. I'm bringing my DVM with me from now on and if/when it happens again I'll measure everything and maybe have more to go on then. I''ll post an update if I learn more in the future.