One for the experts possibly.....

Replaced my burnt out alternator this week only to find that now if I push the throttle past 2500 rpm she starts backfiring through the carb every few seconds as if the timing is out, although i have never touched the timing and wondered if when the alternator was banging out 16 volts before it might have damaged a part ??? I did notice that one of the thick wires to the alternator was burnt to a crisp at its connection.
Any ideas ?

My definition of an expert is someone who knows more than I do......so pretty much everyone on here!


92 4.3l merc (T'bolt ign) alpha gen ll

Re: One for the experts possibly.....

Keep it simple.

Replace the cap and rotor, check the spark plug wires. Go from there.
The backfire is most likely not related to the alternator.

Crispy wire ends are normally caused by loose or corroded connections, check for others.

Re: One for the experts possibly.....

".....wondered if when the alternator was banging out 16 volts....."


Not sure how long it ran like that. Over-volting (technical term ) is not good for any electrical part, and the longer it was run the more chance of collateral damage. More volts means more amps which can quickly overload wiring and components that were designed to handle loads from a 12 volt source.

Re: One for the experts possibly.....

Normally I would agree with you, however all the ign parts including sensor and module have been changed in the last 8 months....along with a thorough check of fuel system.

Re: One for the experts possibly.....

I am not sure what symptoms prompted you to change those components, but you just changed the alternator last week, so the new components were subjected to the improper source voltage for a period of time, also the wiring harness was under duress as well during that time. Or am I missing something.....

Re: One for the experts possibly.....

neilmw said:

if I push the throttle past 2500 rpm she starts backfiring through the carb every few seconds as if the timing is out

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Hmmmm.... okay... so why don't ya check it???

maclin said:

More volts means more amps which can quickly overload wiring

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Hmmm.... I don't know which version of ohms law you know.... as far as I know there's only the one. And the one I know says when Voltage goes up, resistance goes up.. so current demand goes down.
High voltage can breakdown blocking diodes and scr's inside of the modules... will 16 volts do it? I don't know. Some ignition systems are more sensitive than others...

neilmw said:

however all the ign parts including sensor and module have been changed in the last 8 months....along with a thorough check of fuel system.

Click to expand...

Why??? where all the parts bad or are you throwing parts at it? And can you define what a thorough check of the fuel system is?

Personally. I'd shoot the timing first, its the easiest thing to do. Check both base timing and advanced.

Then go through this checklist... It was created by Don S. So credit goes to him.
Engine Won't Reach Operating RPM. Check

Fuel condition. Type and Octane possibly old fuel
Propeller pitch or diameter, damaged blades
Restricted fuel pickup tube or anti siphon valve Fuel System Test
Crankcase oil volume, high oil level can cause aerated oil and lifter collapse
Marine growth on hull and outdrive
Wrong gear ratio in outdrive
Restricted carburetor air intake (clogged flame arrestor)
Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive
Poor cylinder compression Compression Test
Carburetor defective, or wrong type.
Fuel pump pressure and vacuum
Boat overloaded, improperly loaded, or improperly trimmed.
Engine Overheating
Engine timing and ignition system operation
Remote control cables and linkage for proper travel to open throttle plates fully.


Your problem is in there somewhere.

Re: One for the experts possibly.....

captJason said:

Hmmm.... I don't know which version of ohms law you know.... as far as I know there's only the one. And the one I know says when Voltage goes up, Current demand goes down.

Click to expand...

Jason, yes, there is only one version of Ohms law. If the resistance remains constant and the voltage goes up, current goes up along with it.

I suggest you do a test. Suppy a light bulb with 12V and then supply it with 24V. See which voltage cause more current.

If the alternator really did have an overvoltage condition, I would be checking the ignition first.

Re: One for the experts possibly.....

Yup...Voltage is Potential. Current is what happens when the Potential is realized (used). The more Potential the higher the Crispy Wires factor becomes.

Re: One for the experts possibly.....

Bruceb58.....

That's the way it works on paper, but not in real life. I think your assuming that you CAN apply 24V across a 12V bulb.
You can't... sorta.
If you apply 24 volts to a 12 volt bulb you'll pop the bulb.
The biggest thing here is in an active circuit resistance will not remain constant, resistance goes up. Voltage and current cause heat which increases resistance. The resistance of a bulb measured at 50 degrees will not be the same when the bulb is 150 degrees.
So if resistance goes up... current goes down, if the voltage remains the same. But Voltage will not stay the same... If the resistance goes up, voltage will drop. Which causes current consumption to go up.
Watts law comes into play when using ohms law. You need watts law because it takes into account heat. Where ohms law does not.
Lets take the light bulb. When you buy one, you buy based on voltage and wattage. Lets use a headlamp as your bulb for instance. An everyday automotive headlight is 12V 55W.
Starting with watts law. a 12V 55W will have a resistance of roughly 2.5 ohms and it will draw 4.5 amps at 12V.
Now if you have a 24V 55W bulb. The bulb will have a resistance of roughly 10.5 ohms and it will draw about 2 1/4 amps at 24V.
When you go and buy a light bulb... you don't go and buy a 10 ohm bulb. You buy a bulb based on wattage.
This doesn't even take into account voltage drop, which causes current increase. The lower the voltage the larger the voltage drop across a line... ='s more current drawn. the higher the voltage the lower the voltage drop across a line... ='s less current drawn.

Why are big deisel trucks wired for 24 volts? Sure they need the extra current because of the increased load on the starter motor from increased compression. But.... lets say a starter motor draws 1000 amps at 24V... if i decrease the voltage to 12... i'm asking the battery to supply 2000 amps.

This should be it's own thread, it's getting a bit off topic... but since you hijacked it first

Re: One for the experts possibly.....

......

Re: One for the experts possibly.....

captJason said:

The lower the voltage the larger the voltage drop across a line... ='s more current drawn. the higher the voltage the lower the voltage drop across a line... ='s less current drawn.

Click to expand...

I see where you get confused. With an electric motor,if you drop the voltage, you can indeed get a bigger current draw becuase the motor will have a larger inductive load dues to spinning at a lower RPM. For a purely resistive load, such as the integrated circuit that is the subject of this thread, the more voltage that is sourced, the more current will also be sourced and converted into heat by the integrated circuit.

Re: One for the experts possibly.....

Bottom line for the original poster, the higher voltage more than likely caused other problems, both to components and wiring. If it did not matter how much voltage was supplied to the system then the alternator wire would not have crisped. Something could and was drawing more amps.

Re: One for the experts possibly.....

hmmm maybe we can start a discussion about this in the electrics section so all can learn.

Re: One for the experts possibly.....

Now it wasnt my intention to start a debate on ohms volts and amps mainly cos I understand none of it.......as for the original problem, the engine was running for around 20 minutes in total showing 16v on the guage before a I could smell burning plastic and the guage went down to under 12v
(I pressumed the alt had packed up all together)
The ignition parts were replaced due to coroded terminals on the module and service items. I have checked the fuel system as per Don's list before whilst trouble shooting for another problem a while ago.
I will first check the timing when i get back down there, I assumed as I had never touched it there would be no reason for it to have changed.