Blown engine fuse

[Anthony]

[ QUOTE ]
there are two ways a fuse blows the first is when the positive connects straight to earth and blows the fuse the second when the circuit is over loaded and you'll see the fuse wire melt

[/ QUOTE ]

Technically there is only one way a fuse actually 'blows', and that is when it is overloaded beyond its rating causing it to fail. The positive is always connected to negative in any live DC circuit, its just a question of how much resistance is in the circuit as to how much current flows (ohms law). Its just that a short circuit is a very low resistance connection of positive to negative so a large amount of current flows.

Dependig upon the alternator, it is possible that the fuse fractured due to age (I have seen this on Volvos), however it may have had the knock on effect of damaging the alternator due to there no longer being a load on it, and now that damaged alternator is causing the new fuses to blow.

How long after startup dies it blow, have you tried putting a DVM on the alternator to see what the voltage gets to when running?

Anthony

 

[windlipper]

Thanks for that, the next stage will have to be an alternator test. I know the loom is ok as is the analyser. Seems that it can now only be either the alternator or an exposed wire somewhere.

The joys of boating!!

 

[VicS]

[ QUOTE ]
Dependig upon the alternator, it is possible that the fuse fractured due to age (I have seen this on Volvos), however it may have had the knock on effect of damaging the alternator due to there no longer being a load on it, and now that damaged alternator is causing the new fuses to blow

[/ QUOTE ] The fuse is not in the alternator output, that just goes to the terminal on the starter solenoid and then to the battery via the isolating switch. The fuse is just in the supply to the panel. If it blows it will not damage the alternator.

Look at the wiring diagram in the owners manual!

If the alternator is faulty it is a separate issue.

 

[pvb]

[ QUOTE ]
Thanks for that, the next stage will have to be an alternator test.

[/ QUOTE ]Anthony's advice is wrong - a blown fuse won't damage the alternator.

 

[windlipper]

I suppose a fault on the alternator could cause a power surge, which may be causing the fuses to blow, especially as it starts ok and then the fuse blows.

 

[pvb]

Even if it's faulty, the alternator can't produce a "power surge" as such - all it can do is raise its output voltage. A higher voltage won't blow the fuse. It's a 15A fuse and, as has been said before, a short circuit is the most likely cause of it blowing.

 

[windlipper]

Thanks, will post on here when the problem's been identified

 

[Anthony]

Hi, the damadged alternator was just a theory, I do not know the exact wiring configuration of this particular engine, in the volvos that I had seen a blown fuse on the fuse did sit in series with the alternator, however it sounds as though this setup maybe different. If indeed the blown fuse does not disconnect the alternator output on this engine then no, it is unlikley to have caused any damage to the alternator by failing. Apologies for any confusion caused by my suggestion.

Its unclar so far as to what point the fuse blows, is it a freaction of a second after starting, or a notable time? Does it vary? If it is totally random that would indeed suggest a short circuit. If it is a short period of time then it would suggest that there is a constant fault that is causing an overcurrent condition that just takes a little while to actually burnt he fuse out.

Alos you mentioned:

'all four have now gone '

Do you mean there are 4 seperate fuses and all have now blown, or that you have replaced a single fuse 4 times, and it had failed on each occasion?

Anthony

Anthony

 

[windlipper]

There is a fuse "box" with 4 connections, three are unused, presumably spares. They have all blown because it was necessary to move the connection each time in order to start the engine to get the boat home. As I said the engine started each time it was connected to a fresh connection, of course all four have now blown.

 

[VicS]

[ QUOTE ]
I do not know the exact wiring configuration of this particular engine,

[/ QUOTE ] Then down load an owners manual from volvo penta website and study the wiring diagram.

Some of the older mauals, although not this paricular one, also give a circuit diagram for the "electronic module" in the control panel.

Worth seeking out one of those and studying that in case there is something obscure happening in there but I think if there was the fuse would blow without starting the engine.

 

[windlipper]

Thanks to all for the comments. Problem now solved, would you believe loose wire on the light to the fuel gauge causing earthing!!!!!

 

[pvb]

Well done!