Help Me Understand This Wiring Diagram - MCM 260 Content

[San_Diego_SeaRay]

Hello All

Gonna rebuild my wiring harness on my 1981 MCM 260. Gonna splice everything into the cannon plug and that's all I'm keeping of the old harness.

But I want to make sure I understand how it works so as not to create an inferno on the water with my handy work. Specifically, I'm looking at the purpose of the circuit breaker in the attached circuit.

Here are my observations:

• The red/purple wire coming off the Alternator is the sense wire (correct?)
• It is wired to one side of the circuit breaker
• The circuit then continues to the starter terminal to which the BAT+ is also connected via a heavy gauge wire from the battery +

Here's my guess at what's happening:

• The sense wire receives it's required voltage "signal" from the BAT+ via the starter terminal (typically just over 14 volts)
• If the load on the battery increases, the Alternator responds by increasing amp output (and voltage output increases as well) to the "+" on the battery
• If the voltage at the BAT+ terminal on the battery is 14.2 volts or something similar, everything works fine
• However, if the voltage at "+" battery terminal increases, let's say to 18 volts, the voltage at the starter terminal also increases to 18 volts
• In turn, the voltage delivered to the sense wire is now 18 volts
• Per Ohm's law, any increase in voltage must produce a corresponding increase in amperage
• So the amperage to the sense wire circuit increases as well, which should trip the circuit breaker

Is this what's going on? This is just a wild guess on my part. Do alternators create higher voltages, or do they have some way of just creating higher amps w/o creating higher voltages?

I'll have some follow up questions but I need to first understand what's going on here.

Thanks,

JC

EDIT: OK so the alternator should never really produce more than 14.XX volts. So there goes my theory. Sigh.

 

[HT32BSX115]

Howdy,



Look at it this way.

The battery is a STARTING source of power (only) during normal operation.

When the engine is running, the battery is NOT providing any current to any load at all (since alternator voltage will normally always be higher than battery terminal voltage).

All current comes from the alternator unless it's defective or the engine is idling and the idle current capability of the alternator is exceeded.

Most alternators have a simple regulator built in that varies (field) current to the rotor which acts like a big rotating magnet.

The stator is wired in a 3 phase fashion so that you get 3 phase AC which is then rectified into pulsating DC (you've heard the alternator "whine" in some radios and stereos........mostly filtered by the battery)



The regulators are generally "fixed" such that they provide rated voltage (around 13.8-14.2v) up to a max (rated) current using battery voltage as the "sense" reference.

Try to draw more current than rated, and there'll be an internal voltage drop in the alternator stator, excessive heat will be generated in each phase of the stator and the diodes will get hotter than they should.



Older MCM and others used separate regulators but most nowadays have the regulator built right in and some have only 1 wire output (grounded through the metal frame/case)


If you EVER see 18, you have a bad ground or + battery connection, bad regulator, battery, or a combination of all and you need to fix it fast as most electronics are risking destruction at high voltages.


Regards,



Rick

Let me also add that the main CB will NOT protect against over-voltage the way you think. The CB MIGHT trip in an over voltage condition but it will likely be well after other solid state electronics have been damaged

 

[wrench 3]

The circuit breaker is to protect the entire electrical system in the event of a short circuit to ground. It will trip out instead of the wiring burning up.
By the way, you left out the resistance factor in ohms law. The sensor circuit in the alternator has extremely high resistance so there is almost no amperage flow in that circuit.

 

[Bt Doctur]

If you can wait until the private messaging is fixed you can pm me about a harness

 

[San_Diego_SeaRay]

If you can wait until the private messaging is fixed you can pm me about a harness

I'd love to but then I read your signature and it puts me in an endless loop. :lol:

Just kidding. Any idea about when the pm's are going to be fixed?

 

[Bt Doctur]

hopefully there going to get it fixed tonite during their maintance. It looks like the post counter is now working too.

 

[achris]

Answers in red...

Gonna rebuild my wiring harness on my 1981 MCM 260. Gonna splice everything into the cannon plug and that's all I'm keeping of the old harness.

But I want to make sure I understand how it works so as not to create an inferno on the water with my handy work. Specifically, I'm looking at the purpose of the circuit breaker in the attached circuit.

Here are my observations:

• The red/purple wire coming off the Alternator is the sense wire (correct?)yes
• It is wired to one side of the circuit breaker
• The circuit then continues to the starter terminal to which the BAT+ is also connected via a heavy gauge wire from the battery +

Here's my guess at what's happening:

• The sense wire receives it's required voltage "signal" from the BAT+ via the starter terminal (typically just over 14 volts)no, the sense wire senses battery voltage, typically between 11.8 and 13.2 volts)
• If the load on the battery increases, the Alternator responds by increasing amp output (and voltage output increases as well) to the "+" on the batterycurrent increases, but voltage should remain at regulator voltage (about 14v) until current required exceeds the alternator maximum
• If the voltage at the BAT+ terminal on the battery is 14.2 volts or something similar, everything works fine
• However, if the voltage at "+" battery terminal increases, let's say to 18 volts, the voltage at the starter terminal also increases to 18 volts
• In turn, the voltage delivered to the sense wire is now 18 volts
• Per Ohm's law, any increase in voltage must produce a corresponding increase in amperage But it seldom works that way in practice. Generally a serious voltage increase has a tendency to fry things. A rapid spike in current followed by no current!
• So the amperage to the sense wire circuit increases as well, which should trip the circuit breakerNo, the circuit breaker protects against excess current in the rest of the circuit.

Is this what's going on? This is just a wild guess on my part. Do alternators create higher voltages, or do they have some way of just creating higher amps w/o creating higher voltages?Alternators usually have regulators built in, and produce a fixed voltage and varying current depending on the load...

I'll have some follow up questions but I need to first understand what's going on here.

Thanks,

JC