Questions about the nature of fuses

[RobH2]

There are a lot of threads that talk about blown fuses and checking fuses after an abrupt engine failure but I have a question about fuses in general.

Does a blown fuse always indicate a looming problem or the beginning of a chronic problem? I know it sounds like a dumb question but during my life I've only had 3 fuses ever blow on me in over 35 years in both cars and boats. Two were in a car and one today in my boat.

The car had a bad DC adapter plug shorting out. The boat died abruptly today and the first thing I checked was the 4 fuse holders that are wired to various gauges behind my instrument panel. The one that blew as a 20A fuse to the starter key switch.

I replaced the fuse and all was well for 30 minutes until I got back into the slip. I didn't want to risk a longer test as it was getting dark and I didn't' want to get stranded out all night.

I did a quick check of all wiring and it was all tight and looked good. I didn't see any signs of failure, corrosion or anything else.

Do fuses just blow sometimes and then all is well? Or, do I need to go on a major hunt for a culprit? I could just wait till it happens again. If it doesn't happen in the next, say, 20 hours, then I'm inclined to think it was just a random occurrence.

Is that naive?

[Dave Neptune]

Just fuses

Rob, fuses are just fuses and they are rated at a specific draw of power. When it blows all that has happened is the draw exceeded to rating of the fuse. Now what caused the additional draw a short is the most common and can cause intermitant headaches until the short is found.
Another common cause is the intermitant blowing due to poor connections which can vary the draw from vibration and temp variations and each bit of resistance adds to the draw until you blow another one. Often in this case you will see people add a bigger fuse and the problem will abate for a while then come back, this is why going bigger than the rating recomended is only a patch and the patch endangers the equipment and wiring you are trying to protect with the fuse. You can check the voltage at the fuse and again at the piese it is protecting for a voltage drop which indicates poor connections and/or to much resistance causing more of a load for the fuse.

Hope this helps a bit, I'm not really an electrical guru however this is just basics.

Dave Neptune

[RobH2]

Dave,

Yea, I know all of that intellectually. I'm pretty good with electricity. I was just hoping someone would say, "yea, Atomics are fluky sometimes and if the new fuse works you are probably OK." I'll throw a clamp around that wire and see how much it's drawing and trace out the problem. Something has corroded or vibrated loose I guess.

And yes, I know better than to replace a blown fuse with a larger one. Sometimes people even jump fuses. That's scary as you are asking for a fire. The fuse I had blow was a 20A one to the starter key. I don't even know if that's the right size. I don't entirely trust my wiring as I found some speaker wire running to my instruments when I bought the boat. I replaced that immediately but now wonder what other shortucts might have been made. What I don't know though is if my harness is fused correctly. I really should find a chart somewhere that shows what and how much fusing should be used. I read a post that mentioned a 10A fuse from the oil pressure sending switch to the fuel pump. I don't have that fuse at all. I'll add that today.

I only have 150 hours of experience with an Atomic at this point two years after buying my boat so I'm still getting used to it. Shame on me for for blaming it and not realizing "it's a finely tuned machine" and would never just throw out some extra current and blow a fuse for fun.

We all know, whether car, boat, house or stereo...electrical problems are the worst and most illusive problems to track down. Sometimes it's just easier to run a new wire and be done with it.

Thanks for the feedback/corroboration.

[roadnsky]

The fuse I had blow was a 20A one to the starter key. I don't even know if that's the right size. I don't entirely trust my wiring as I found some speaker wire running to my instruments when I bought the boat. I replaced that immediately but now wonder what other shortucts might have been made. What I don't know though is if my harness is fused correctly. I really should find a chart somewhere that shows what and how much fusing should be used.

Rob-
Yes a 20A is correct for the ignition.
But I think you want to trace that wire to be certain of where it starts as well as where it terminates.

Here's the MMI drawing to give you a start.
There are other drawings and threads on the site too.
Do a search for DC Wiring.

There are also some nice drawings and discussion on this thread...

http://www.moyermarine.com/forums/showthread.php?t=5580&highlight=Wiring

[Mo]

+1 what Dave said. You mentioned it when you turned the key and you have checked the wiring and terminals. Could be in the ignition key switch itself, especially if corroded /dirty.

[RobH2]

Jerry, excellent chart. I hadn't searched for one yet. Thanks for going ahead and doing that. I'll get a lot of use out of it.

Maurice, good thought. It's an old switch and it's in a funny spot. I've bent a few keys in it so it's had some impact and wear. Glad to know it's the correct amperage but maybe I'll just go ahead and replace it. Pre-emptive strike right?

[RobH2]

Jerry, looks like that's Neil's E-2 chart. Do you have a larger version that I can get to print out?The one attached is very hard to read.

[roadnsky]

Jerry, looks like that's Neil's E-2 chart. Do you have a larger version that I can get to print out?The one attached is very hard to read.

No, sorry I don't.
Neil will be along later after work (West Coast time) and will certainly give you better direction and can possibly share some of his drawings.

We're all very lucky to have him here, in that it's actually his profession.

[RobH2]

Yea, I put that together once I looked over the chart's info panels. Really nice to his expertise for sure.

[roadnsky]

Rob-
While you're waiting for Neil and others with more knowledge,
check this thread out in the DRAWINGS & SCHEMATICS section.
It's the beginning of the other thread I gave you...

http://www.moyermarine.com/forums/showthread.php?t=5560

As I like to say here sometimes, that DRAWINGS & SCHEMATICS section is a goldmine.

[RobH2]

Yea, I had found that thread. It's a good one.

Something I was unfamiliar with was "ACR." Had to look it up. I've seen almost every inch of the boat by now and I don't recall seeing anything that could be an "Automatic Charging Relay." I need to find that. No idea what it might look like or how big it might be.

I have these monster copper blades (what look like fuses) mounted on a bulkhead near my batts. I'm not sure exactly what they are but I'd assume fuses. I'm sure they are not an ACR as they are just 1/2 inch wide copper blades.

I need to get in there and so some serious wire tracing, marking and refitting.

[RobH2]

Just found Neil's E-3 Schematic (in this thread... http://www.moyermarine.com/forums/showthread.php?t=5560). It's a nice one too. Nothing like a clean and professional drawing. It makes me chuckle though because when I look at it I see a handful of organized wires. When I look at my harness around my engine I see 100's of wires it seems and can't imagine where they all go and what they feed. I'll be finding that out starting today.

[ndutton]

Rob (et al)
If there's a drawing of mine you find useful and need a bigger version you need only ask. I have them all in CAD but due to file type restrictions within this forum software I've posted them in smaller, lower resolution versions, essentially large thumbnails.

Without a picture I can't be certain but I believe your 1/2" wide copper blades are battery disconnects. Also, not all battery wiring systems have ACR's and believe it or not, work just fine. In fact, some of us (one of us?) would rather not have them.

[roadnsky]

...In fact, some of us (one of us?) would rather not have them.

Uhhh... two of us.

[ndutton]

[re: aversion to ACR's] Uhhh... two of us.

Good, I'm not completely alone then.

Hey wait a minute, aren't you a PCV disciple? Oh well, I guess nobody's perfect OK, here's a shot at redemption: your regulator, fixed point or adjustable?