boat electrics & amp meter dashboard wiring oddity

[mightymouse]

ello.
Gotta electrical issue here that we can't figure out.
The wiring loom in my boat includes an ammeter on the dash board. Fine.
The alternator sends current up to the amp meter via a decent sized cable (4mm) whilst another decent sized cable returns from the second post of the amp meter back to the engine bay where it connects to the positive terminal of the battery.
This all seems above board as the amp meter has to be 'in line' between the charging source and the battery to give a valid readout.

Another decent sized wire runs from the positive terminal of the battery up to the dash where it terminates at post 1 of the ignition switch (3 pole).

The ignition switch has switched electrics on pole 2 and the starter motor on a sprung return 3rd pole.
what i can't understand is there is a fused link between the live terminal of the amp meter and pole 2 of the ignition switch (for switched electrics).
This would make a second link between the charging alternator current and the battery current, even though the battery current is already present and is topped up by the alternator via its amp meter loop.
image is attached with text and two larger images can be found at the right of the top row of images at:
http://www.tribidium.com/matt/boatpics.html

thanks

 

[sarabande]

anything to do with the alternator exciter circuit ?

 

[Wiggo]

OK, so what the ammeter reads is not actually the current flowing. Somewhere on either the battery +ve or -ve post will be a shunt. It will be a small block attached to one post or other and the two wires to the ammeter will connect to either side of it. In simple terms, it 'infers' what current is flowing by the voltage drop across the shunt. The starter motor will normally be connected directly to the battery post, but everything else will be connected to the other side of the shunt. This way, you get an ammeter that reads sensible values for normal usage (e.g. +/- 20-30 amps or so) but ignores the 400 Amps or so that cranking draws.

If this isn't clear, I can try to draw a schematic.

 

[VicS]

No I dont understand it either.

Wired as you describe:
The ammeter will never give a reading as both terminals are effectively connected together.
The circuits that should switched on and off via the ignition switch cannot be switched off.

All would work if that fused link was removed but the ammeter would only register current flowing to the battery.

If the link from the battery to the ignition switch were removed and that fused link connected to terminal 1 of the switch in its place it would all work and the ammeter would also show any drain by the switched circuits and the starter solenoid.

Maybe that fused link is intended to supply circuits that are not controlled by the ignition switch, but are there any?

Thats my take on it anyway but perhaps I have missed something.

Maybe Wiggo is right but I cannot follow what he is suggesting. I did wonder if there was ashunt somehere but still couldnt figure it out.

 

[NorthUp]

Re: boat electrics & amp meter dashboard wiring oddity

I think that its one of the old Lucas ammeters with an internal shunt. The rest just looks like very poor wiring! Rip it all out!

 

[halcyon]

Re: boat electrics & amp meter dashboard wiring oddity

Either the switch is faulty, and the fused wire is a link, or it is a link to provide power with the switch off.

Brian