[SEMIJim]

Originally Posted by abillmann

Am I missing anything here?

Not that I can see. You only need to disconnect the negative side of the battery, tho.

Jim

[22empire]

Okay, so is it just me or does this sound like water in the fuel or a failing fuel pump.. It is possible that the week battery could actually cook an alternator if it was being asked to carry the full load. The engine stopping while at higher RPM can point to a fuel pump running at lower pressure than it is suppose to. At idle the low pressure wouldn't show up but as the rpm and the volume requirement of gas increases the low pressure situation can cause the motor to die as you bleed the fuel out of the primed line. Just my pennies but I have experienced a similar issue with a fuel pump and a plugged seperator..

[AirNavy]

A weak battery can't really kill an alternator, but a battery with shorted plates can over time burn up the rectifier diode inside the alternator (not related to this gripe). In normal operation, the battery is only used for starting. After that everything runs off of the alternator, along with it charging the battery. Now if the alternator is failing (bad brushes/windings/voltage regulator/rectifier), the battery is forced to step in to provide the current to run everything which means it is no longer charging. With everything working correctly, battery voltage with the engine running should be right around 14 volts (12.8 - 14.7 is the acceptable range). The range takes into account variables like heat (higher temps increase a battery's charge capacity, but also increases resistance in the alternator's windings which can drop voltage output).

[AirNavy]

Originally Posted by SEMIJim

I don't know how accurate they are (they're relatively worthless if they're not accurate, btw), but 11VDC is definitely well below healthy for a 12VDC lead-acid battery. That battery is effectively dead. But you already knew that .

It just occurred to me: These boats have no "idiot light" to warn you that the alternator isn't doing what it should. I regularly scan oil pressure and water temperature, but, in all honesty, pay little attention to that voltmeter. I think I'll add that to my list of gauges to scan regularly whilst under way.

I wonder what those battery idiot light circuits consist of...?

Jim

Depending on the complexity of the voltage monitoring, it can be as simple as a few resistors, a transistor and a lamp/LED. The easiest solution is a ready made voltage monitor/gauge. Here's a couple made for motorcycles, but should work just fine in a boat:

http://www.customdynamics.com/LED_battery_gauge.htm

Another option is an actual digital voltage gauge. I went this route for one of my bikes since I had one left over in the garage from a stereo install I did a few years ago (I had blue on hand, but they also make them in red):

[abillmann]

Originally Posted by 22empire

Okay, so is it just me or does this sound like water in the fuel or a failing fuel pump.. The engine stopping while at higher RPM can point to a fuel pump running at lower pressure than it is suppose to.

Excellent points. BUT... instead of a failing fuel pump, doesn't it make more sense that the pump (which is electric on my 4.3L Alpha One) isn't getting the voltage it needs to output a higher fuel volume? Or that other parts of the fuel delivery and/or ignition systems would sputter due to lack of adequate voltage?

Plus, a failing fuel pump wouldn't explain the dying battery very well either, although I suppose that is possible. And water in the fuel would cause problems across all RPM ranges. I haven't put a multimeter tester on it yet, but if the voltage gauge on the dash is even close to accurate (it's reading just shy of 11 volts regardless of the engine running or not) it would almost certainly point to the alternator, wouldn't it?

I'll have the new alternator in by tomorrow night, and I'll report back.

[22empire]

Its possible it is due to low voltage, the the motor should be set to run at 12V's so 11 aint that bad.. You should not pretty quick once the alternator is in...

[abillmann]

UPDATE:

Well, it looks like I have two concurrent problems.

First--bad alternator. I put the new one in tonight, and immediately, the running voltage returned to a normal range--13.0 to 14.0 or so. Previously, both the running voltage as well as the voltage at the battery when the engine was off was about 11. The new alternator is providing nice voltage and--*gasp*--actually keeping the battery charged! I used a multimeter to test this. I'm also pleased to report the voltage gauge on the dash is very accurate, too, and the new alternator is definitely a little more quiet than the old one.

Second--the bad news. The new alternator did not solve the problem of the engine stalling at 3000+ RPMs. Just like before, the engine starts easily, and runs fine at idle and slower speeds. Then, whether slowly accelerating or hammering the throttle, when it reaches 3000RPM or so, just after planing or just as it's about to plane, it dies -- the tach needle drops to zero pretty quickly. It sure feels fuel-related.

So I guess I was half right, and I'm certainly no worse off for replacing an 18-year old alternator that clearly was having problems. But man... I'm just DYING to hammer this thing and finally fly across the lake! I have a service appointment at an authorized repair place on Tuesday.

If the fuel pump turns out to be the culprit, I'll owe 22empire a nice 12-pack of quality brews for the accurate insight!

[dudders]

Do these motors have coils like car motors?
I had a similar sort of problem with a car I had once, it would rev up ok out of gear, then put it into gear it would go ok at idle but die if i put my foot down a bit.
Turned out the coil was pretty much had it, put a new one in and problem solved.
Just one idea for you mate.
Good luck.

[SEMIJim]

Originally Posted by dudders

Do these motors have coils like car motors?

They do. They're really not all that different from an automobile engine. Some marinized parts to resist the constant exposure to a hostile environment; backfire flame arrestor on the carburetor, instead of an air filter; raw (sea/lake) water pumped through the engine, instead of a closed radiator system with a fan (tho some do have a closed cooling loop, with the radiator cooled by raw water); and, on most boats, "used" cooling water mixed with the exhaust. If you can wrench on an automobile engine you can wrench on these.

Jim

[abillmann]

I'm having the Mercruiser techs look everything over on Tuesday, and I'll report back.

Place your bets:

A. Fuel pump
B. Coil
C. Something else

and

1. Less than $100
2. $101 to $499
3. $500 to $800
4. Pull the plug and turn it into a fresh water reef.