1987 Cuddy Cruiser Alternator Issue - Thoughts?

Thanks for all this info. I’m going to do as much investigating as I can.

Turns out my battery is a 2012, could that be the problem if it’s old?

I have changed it up with a new Noco Genius charger, maybe I’ll try it out.

Still sounds like an ignition (spark) problem because you describe it as no problem starting cold, but it shuts down on you when it gets hot.
 
Still sounds like an ignition (spark) problem because you describe it as no problem starting cold, but it shuts down on you when it gets hot.
I'm going to grab two brand new (Interstate) batteries 1000 cranking amps one starting and one that's both starting and deep cycle... Take it out for a drive and see if that solves the issue. If not, I'm dropping it off at the local marina and just getting them to diagnose the problem. Don't really have the knowledge to change parts and the boat has been sitting for a year now which I don't think is good for such an old engine.
 
Still sounds like an ignition (spark) problem because you describe it as no problem starting cold, but it shuts down on you when it gets hot.

So I had the local Marina take a look at the boat for some other things and the alternator is charging fine. They are believe the issue is the impeller causing the engine to overheat. Makes sense that it died when I was basically in neutral.
 
So I had the local Marina take a look at the boat for some other things and the alternator is charging fine. They are believe the issue is the impeller causing the engine to overheat. Makes sense that it died when I was basically in neutral.

Not sure I see the connection.
Usually a failing impeller in your alpha drive will fail to keep up with an engine as the RPM increases.
You were in neutral so I’m going to assume you were at idle or close to it.
Did your dash gauge show you overheating? Did you hear an alarm to alert you to overheating?
While shutting down an engine that is running hot is a good thing, your 87 was not originally equipped with the necessary sensors and control module to do that for you automatically. Unless a previous owner did some upgrades your boat doesn’t know to shut down when it overheats.
More likely it just came with an alarm buzzer wired to oil and temp senders to alert you when the boat loses oil pressure or starts to run hot so you can shut it down yourself to avoid a catastrophe.
If you turn your ignition key to the on position and hear a buzzer before you start the motor then you have one of these. If you don’t have one you may want to install one. The kits that contain a buzzer and the necessary senders are pretty cheap ($30.00?), are really easy to install and are great insurance.
What is possible is that a failing electronic component in your ignition (pickup in distributor or ignition control module) is getting too hot causing it to stop functioning properly causing the motor to stall and once it cools off it functions well enough to get you going again.
A malfunctioning overheating electronic component in your ignition doesn’t necessarily have to have anything to do with the operating temperature of your motor.
The ignition control module on your V8 Mercruiser, if a Thunderbolt original, will be a brick looking thing mounted to one of your risers. It could also be a triangle looking thing instead if it has been replaced by a previous owner because they stopped making the brick looking ones and the triangle was the replacement.
They are mounted with some insulating washers to keep them off the hot riser a bit so if the engine is overheating and that riser is getting too hot they won’t fail.
If the engine is overheating and that riser is getting too hot causing the module to shut down the ignition then either the module is failing or the insulating washers are missing.
There is also a pickup inside the distributor that I’ve seen exhibit your symptoms when they fail. That pickup is cheaper and easier to find than the ignition control module.
 
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It's my understanding that on an Alpha (Gen I or Gen II) the waterpump is driven by the driveshaft in the outdrive - the shaft spins all the time so the pump runs in F-N-R. If the impeller is damaged/worn it is not pumping enough volume at the low RPM in neutral. As you run the boat and the RPMs increase, the pump speeds up and pumps more volume. So long as I'm not in neutral for too long a period of time, it should be pumping enough water.

Nonetheless, I need to get the impeller replaced anyways.

There is no buzzer to indicate of overheating when starting to the best of my knowledge. We are sole owners.

Not sure I see the connection.

Usually a failing impeller in your alpha drive will fail to keep up with an engine as the RPM increases.

You were in neutral so I’m going to assume you were at idle or close to it.

Did your dash gauge show you overheating? Did you hear an alarm to alert you to overheating?

While shutting down an engine that is running hot is a good thing, your 87 was not originally equipped with the necessary sensors and control module to do that for you automatically. Unless a previous owner did some upgrades your boat doesn’t know to shut down when it overheats.

More likely it just came with an alarm buzzer wired to oil and temp senders to alert you when the boat loses oil pressure or starts to run hot so you can shut it down yourself to avoid a catastrophe.

If you turn your ignition key to the on position and hear a buzzer before you start the motor then you have one of these. If you don’t have one you may want to install one. The kits that contain a buzzer and the necessary senders are pretty cheap ($30.00?), are really easy to install and are great insurance.

What is possible is that a failing electronic component in your ignition (pickup in distributor or ignition control module) is getting too hot causing it to stop functioning properly causing the motor to stall and once it cools off it functions well enough to get you going again.

A malfunctioning overheating electronic component in your ignition doesn’t necessarily have to have anything to do with the operating temperature of your motor.

The ignition control module on your V8 Mercruiser, if a Thunderbolt original, will be a brick looking thing mounted to one of your risers. It could also be a triangle looking thing instead if it has been replaced by a previous owner because they stopped making the brick looking ones and the triangle was the replacement.

They are mounted with some insulating washers to keep them off the hot riser a bit so if the engine is overheating and that riser is getting too hot they won’t fail.

If the engine is overheating and that riser is getting too hot causing the module to shut down the ignition then either the module is failing or the insulating washers are missing.

There is also a pickup inside the distributor that I’ve seen exhibit your symptoms when they fail. That pickup is cheaper and easier to find than the ignition control module.
 
It's my understanding that on an Alpha (Gen I or Gen II) the waterpump is driven by the driveshaft in the outdrive - the shaft spins all the time so the pump runs in F-N-R. If the impeller is damaged/worn it is not pumping enough volume at the low RPM in neutral. As you run the boat and the RPMs increase, the pump speeds up and pumps more volume. So long as I'm not in neutral for too long a period of time, it should be pumping enough water.

Nonetheless, I need to get the impeller replaced anyways.

There is no buzzer to indicate of overheating when starting to the best of my knowledge. We are sole owners.

If the impeller is bad (missing a piece) or worn it won’t pump enough water to cool the motor when demand is higher when you raise the RPM.
Besides, if you overheated you would have seen it on your dash gauge.
It’s always good to change the impeller though if you’re unsure of its age or condition.
Do yourself a favor and get the buzzer alarm kits. They’re worth their weight in gold because they may save you a motor one day.
Here are some pictures of Thubderbolt IV ignition parts. It’s what my 87 Sea Ray had so I’m thinking yours is the same. Again though, there is no telling what’s been changed on a 32 year old boat.
The first one (brick) is what my original ignition control modules looked like, the second is the triangle shaped replacement. Do you have something that looks like either one bolted to the engine side of one of your risers?
When one of my original modules failed (exact same symptoms you describe) I was able to properly diagnose it pretty quickly by swapping between the two motors to see if the problem moved over. You don’t have that option, but maybe your mechanic has a good spare old one or something. Just be aware that the one you use has to be from/for another 5.0 motor because the module controls the timing advance and different motors have different advances. Also, if you have the brick and need to change to the newer style you will need a harness for it.
There used to be a flow chart out on the internet that tells you how to diagnose a bad ignition control module. It’s been many years since I dealt with the problem so I don’t have one but maybe you can find it with google.
The third image is the sensor (pickup) that goes in the distributor. I had one of those go bad on me too. They’re much cheaper and probably still available.
465296B4-4391-4921-AA8F-014AA30BAA4D.jpeg


4E065E9A-F878-4C8E-8F5E-A4D8315E1FDC.jpeg


E154B3AE-B90C-40B5-9B80-A0891BC02C10.jpeg
 
Looks like I have the original thunderbolt. I’ll definitely look into the buzzer.

I’m doubtful my mechanic will have another one from a 5.0 L motor. Where could I get a replacement like yours?
 

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Found one for you:

25830D1A-0996-4E2B-9B79-FE1720E87B40.jpeg

Looks like I have the original thunderbolt. I’ll definitely look into the buzzer.

I’m doubtful my mechanic will have another one from a 5.0 L motor. Where could I get a replacement like yours?

That’s the module!
First, absent a spare to swap for testing, I would use the trouble shooting chart to see if it needs replacing or if it is some other issue with the ignition system.
Is it the module, sensor in the distributor, coil?
Those ignition control modules aren’t cheap (I’m guessing $300?) and you’re going to have to find or make up an adapter harness to change to the triangle one.
The new modules used to come with a new harness but I think they stopped doing that.
If you do need it you’ll have to find out how many degrees of advance the 5.0 needs. Is it 22, 24 etc.? You may be able to find a marking on the old brick somewhere, the triangle units usually have it etched in to the part number on them.
After doing a little googling I think you may need a V8-22, but I would really want to make sure by either seeing what the old one is or gettin info from a really knowledgeable source.
Once you have that info you can google a part number for replacement and you should be able to find one.
Hopefully you’ll get lucky and it will be a coil or sensor.
Pretty sure Michigan Motorz has the sensors and the coils are pretty much everywhere.
If it does turn out to be the ignition control module and you have a problem locating a new one you may want to consider just changing to another ignition system (Delco Voyager or some other marine specific system). That may be cheaper and easier in the long run.
Either your mechanic, Michigan Motorz, or someone more knowledgeable than me should be able to point you towards a suitable and cost effective replacement.
 
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Here is a graphic of the numbers stamped in the old style and another showing how the numbers are stamped in the newer style:

F98ADF38-8D9B-4ED2-8FE8-1A869A25E90D.jpeg


9C003062-ECE5-484A-8EEE-90B0FDC2FADF.jpeg
 

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