Baffling electrical problem - any ideas?

[Zippysigma]

I have between 9 and 12V on the "wrong" side of the on/off switch.

This is the story so far: I started the engine with the mains battery charger on, ran it for a about 20 minutes to warm up, then cast off and went for a short motor. I moored up, stopped the engine with the fuel shut off, but no alarm sound.

I found that the 15A main fuse had blown. Replacing this, the instrument lights glowed (flickered) and the fuel gauge was working, even with the electrical on/off switched to off. Measuring the voltage, there was 9V DC. compared to 12.6 on the other side of the switch. (it was enough to still start the engine).

Even disconnecting the on/off switch, the starter and glow plug relays, I still had the voltage. After a while this had gone up to 12V. By now I was convinced a diode had blown in the alternator so took it off for testing. The Auto electrician ran it up on a test bed and could find no fault.

So, I am at a loss.

Any ideas anyone?

BTW, Beneteau 311 with Volvo MD2020, 2 battery setup.

 

[roly_voya]

I would suspect an earth fault. Starting from the batt neg terminal check resistance at each junction through to the control panel. The voltage you see may be comming from a partial short to the neg side further up than the earth fault, especially if the wiring is damp/oily in places

 

[halcyon]

Have you tried disconnecting the mains charger, you may have blown a diode in the charger open circuit, and you are backfeeding the engine cicuit. This will depend of cause on how the boat is wired.

 

[Zippysigma]

Thanks for both ideas, I will check them out on Monday.

Ref the earth leakage, The wiring is all pretty clean, but I did notice that there was no instrument light on when the alternator was disconnected and the -ve lead is common to both.

I guess a fault in the charger could explain the initial blown fuse, but no further occurence. mmmmmm thoughts to ponder.

 

[bedouin]

Do you have indicator lights on your switches?

If so a fault with the earth there would cause the positive voltage you notice.

 

[VicMallows]

Or a wind-up ? /forums/images/graemlins/smirk.gif

Vic

 

[qetoo]

It might be a faulty alternator regulator feeding some current back to the load side of the switch.

 

[narooma]

Is it not bad practice to have a mains fed battery charger working at the same time as the alternators?

 

[pampas]

At a guess I would think that somewhere you have a faulty earth, not knowing your wiring system for your type of vessel, that you check as mentioned the earth from battery Neg to engine for contact and tightness then up to the instrument panel. initial testing with power off then do voltdrop test across each cable and joint.

 

[hightech]

Disconnect the switch and check the resistance between the terminals, preferably with a megger. I reckon the switch is carboned up and giving a partial short when in circuit. While it is disconnected, check if the voltage is still there or not.

 

[Zippysigma]

Thanks for all the suggestions.

The mains charger and earth fault look the favourites as I have removed the alternator and got that tested ok and the on/off switch has been ruled out as I still have the fault when this switch is removed from the circuit.

I will be going onto the boat on Wednesday to try these out.

Thanks again

 

[Zippysigma]

Ok, tried all the suggestions and still there. I removed everything connected and no joy, so back to the shorted wire theory.