Alternators, Fuses, and General Electrical Questions - MCM 260 Content

[San_Diego_SeaRay]

As you might know from my previous posts, I'm basically rebuilding a 1981 Mercruiser MCM 260. Was mostly dismantled when I bought her so I've been having to do some detective work on how to reassemble it. I'm now at the "electrical phase" and am a bit stumped about some of it. OK, I'm actually stumped at the very beginning of it so you might say that so far I'm stumped on all of it.

I bought an aftermarket marine AC Delco 10SI Alternator that has two wires and a "BAT" terminal and puts out 105 amps; significantly more than than the OEM alternator.

The attempt at my understanding (things I think I might have figured out):

• The purple wire that connects to the #5 terminal on the harness plug and is wired in parallel w. the "+" coil wire is the exciter wire (black wire on my alternator)
• The red/purple wire that is connected to the circuit breaker is the "sense" wire and ultimately detects voltage of the + side of the battery
• The "BAT" terminal is wired to the #4 terminal on the harness plug and....well...I don't know what it does

The confusion:

• Why would there be a circuit breaker in a dedicated sensing wire circuit? (It makes me think that this is not a sensing wire circuit)
• I thought that the "BAT" terminal supplied the amps to recharge the battery. Instead, the only wire at this terminal is a smallish orange wire that is routed to the harness plug. I would imagine there should be a heavy gauge wire that gets routed to the battery from this terminal. Definitely not a small wire into a harness plug.
• Is the alternator grounded to the engine block? If so, what is the purpose of routing a ground wire out of the alternator and putting it in a parallel junction w. harness plug terminal #1, the shift interrupter, and the starter relay (which are already tapped into the engine ground)? The reason I ask is because I'm not sure my alternator has a ground terminal per se and I'd rather avoid the subject if I can.
• I'm also wondering about the ramifications of buying an "oversized" alternator.
• Why does my tech manual mention diagram models with and without a 90 amp fuse? (My engine does not have a 90 amp fuse). Is this a safety concern?
• I was thinking of installing an ammeter for the enjoyment of watching another needle on another gauge. Would this facilitate installing a 90 amp fuse at the same time if advisable?

Thanks in advance -

JC in SD

 

[alldodge]

[FONT=&quot]

As you might know from my previous posts, I'm basically rebuilding a 1981 Mercruiser MCM 260. Was mostly dismantled when I bought her so I've been having to do some detective work on how to reassemble it. I'm now at the "electrical phase" and am a bit stumped about some of it. OK, I'm actually stumped at the very beginning of it so you might say that so far I'm stumped on all of it.

I bought an aftermarket marine AC Delco 10SI Alternator that has two wires and a "BAT" terminal and puts out 105 amps; significantly more than than the OEM alternator.

The attempt at my understanding (things I think I might have figured out):

• The purple wire that connects to the #5 terminal on the harness plug and is wired in parallel w. the "+" coil wire is the exciter wire (black wire on my alternator)
• The red/purple wire that is connected to the circuit breaker is the "sense" wire and ultimately detects voltage of the + side of the battery
• The "BAT" terminal is wired to the #4 terminal on the harness plug and....well...I don't know what it does

The confusion:

[*]Why would there be a circuit breaker in a dedicated sensing wire circuit? (It makes me think that this is not a sensing wire circuit)

So if the breaker opened up it would shut down the ALT

[*]I thought that the "BAT" terminal supplied the amps to recharge the battery. Instead, the only wire at this terminal is a smallish orange wire that is routed to the harness plug. I would imagine there should be a heavy gauge wire that gets routed to the battery from this terminal. Definitely not a small wire into a harness plug.

Back when your engine was made this is how they did it. The ALT power went back via the connector. Today they run it to the starter main post going through the 90AMP fuse

[*]Is the alternator grounded to the engine block? If so, what is the purpose of routing a ground wire out of the alternator and putting it in a parallel junction w. harness plug terminal #1, the shift interrupter, and the starter relay (which are already tapped into the engine ground)? The reason I ask is because I'm not sure my alternator has a ground terminal per se and I'd rather avoid the subject if I can.

The ground wire should be there. While the ALT is grounded to the block by means of the bolts holding it, copper wires carry voltage better then steel and corrosion can start over time and degrade the connection. Pull one of the long skinny bolts that hold the ALT case together and add a terminal lug.

[*]I'm also wondering about the ramifications of buying an "oversized" alternator.

Batteries can only take current so fast. Adding more batteries will divid up the output. That said the ALT will only put out the current requested by the sense lead. Also if you add the higher output ALT you will need a larger fuse (Need to install one maybe)

[*]Why does my tech manual mention diagram models with and without a 90 amp fuse? (My engine does not have a 90 amp fuse). Is this a safety concern?

As before your engine didn't have one.

[*]I was thinking of installing an ammeter for the enjoyment of watching another needle on another gauge. Would this facilitate installing a 90 amp fuse at the same time if advisable?

You can but I would not recommend because it would be mush harder to do. An amp meter requires a shut for the meter to measure across. Most amp meters have the shunt built into the meter. So you would need to run the ALT output lead up to the helm and back if you cannot find one which has it separate. Would be easier to just use a volt meter.

Thanks in advance -

JC in SD

Would post a pic of the ALT wiring but the site here is having issues today[/FONT]

 

[alldodge]

Man did this post ever turn out hosed. Trying to edit it but won't let me in

 

[Bt Doctur]

Alternators, Fuses, and General Electrical Questions , cannot access original

use this one

 

[San_Diego_SeaRay]

Man did this post ever turn out hosed. Trying to edit it but won't let me in

No sweat. I can still read it and I appreciate the answers.

Question: Why would there be a circuit breaker in a dedicated sensing wire circuit? (It makes me think that this is not a sensing wire circuit)




Answer: So if the breaker opened up it would shut down the ALT

OK. So if I'm following you, if the Alternator started to go haywire and produce, say 30 volts, this Circuit Breaker would trip, break the voltage sensing circuit, and the Alternator will not function without a "sensing" signal? So this is the sensing circuit?

[FONT=&quot]Back when your engine was made this is how they did it. The ALT power went back via the connector. Today they run it to the starter main post going through the 90AMP fuse[/FONT]

I'm thinking I should add a 120 AMP fuse (considering my ALT puts out 105 AMPs). If Mercruiser "upgraded" their design by doing so, maybe I should as well. However, I also imagine that a lot of things will start smoking before this fuse ever gets blown

[FONT=&quot]Back when your engine was made this is how they did it. The ALT power went back via the connector. Today they run it to the starter main post going through the 90AMP fuse[/FONT]

OK this makes a lot of sense. And BT Doctur's posted image clearly shows the Orange wire now going directly to the starter and not to the harness. The confusing thing was, if you look at the recommended wire gauge for 60 amps (the appx amps of the original alternator) going 15 feet, you're looking a size 4 gauge wire. That little orange wire hardly seems big enough. The wire I expect will replace it will be more like a cable than a wire...

[FONT=&quot]The ground wire should be there. While the ALT is grounded to the block by means of the bolts holding it, copper wires carry voltage better then steel and corrosion can start over time and degrade the connection. Pull one of the long skinny bolts that hold the ALT case together and add a terminal lug.[/FONT]

OK that makes sense, but then brings up another question: Should the ground wire going back to the engine block be as heavy gauge as the power wire going to the battery (via the starter)?

[FONT=&quot]You can but I would not recommend because it would be mush harder to do. An amp meter requires a shut for the meter to measure across. Most amp meters have the shunt built into the meter. So you would need to run the ALT output lead up to the helm and back if you cannot find one which has it separate. Would be easier to just use a volt meter.[/FONT]

OK got it. I thought it would be possible to route a low amp "remote signal" from the ammeter to the helm but it doesn't sound like that's the way ammeters work. I'll stick to the volt meter.

use this one

Thanks, the picture helps!

 

[alldodge]

OK. So if I'm following you, if the Alternator started to go haywire and produce, say 30 volts, this Circuit Breaker would trip, break the voltage sensing circuit, and the Alternator will not function without a "sensing" signal? So this is the sensing circuit?

If you notice the red/purple wire comes from the battery via the battery. Merc later just moved it to the starter. The 30V thing wouldn't trip the breaker only amps trip the breaker, but if it did trip the engine would die and the ALT along with it.

I'm thinking I should add a 120 AMP fuse (considering my ALT puts out 105 AMPs). If Mercruiser "upgraded" their design by doing so, maybe I should as well. However, I also imagine that a lot of things will start smoking before this fuse ever gets blown

Probably the way to go but I didn't when I installed the same ALT. My issue for installing it was an attempt to make my batteries last long. I'm not going to get into everything just that I have the exact same ALT (bought from ebay) sitting back on a shelf in the shop. I replaced the 10 gauge orange wire with a 6 gauge wire. Used the new ALT for a year and saw nothing changed in the overall health of my batteries and the higher output did not give me anything I didn't already have with the standard ALT.

 

[San_Diego_SeaRay]

Probably the way to go but I didn't when I installed the same ALT. My issue for installing it was an attempt to make my batteries last long. I'm not going to get into everything just that I have the exact same ALT (bought from ebay) sitting back on a shelf in the shop. I replaced the 10 gauge orange wire with a 6 gauge wire. Used the new ALT for a year and saw nothing changed in the overall health of my batteries and the higher output did not give me anything I didn't already have with the standard ALT.

The reason for my higher amp ALT will be for a windlass that I expect might get some strain with all the kelp that grows off our coast. Thanks for the help!

 

[alldodge]

The reason for my higher amp ALT will be for a windlass that I expect might get some strain with all the kelp that grows off our coast. Thanks for the help!

I have a Maxwell 500 winless, 125 feet of 1/4 chain and a 30 pound Bruce anchor and have had no issues