Ground fault confusion

[sebastiannr]

Hello,

I have been testing the electrical system on my boat as i believe there may be a minor current leak in the system. Following Calder's classic book i began checking for ground leaks. He suggests that all equipment be switched off, the battery isolator switch be left on, the positive terminal be disconnected, and the voltage checked between the battery post and the terminal: if the meter reads 12v, there is a ground leak. Sure enough my meter reads 12v, and i have identified the starter motor cable as being the cause. But is this not because the starter motor cables completes the circuit as it is bonded to the engine, which is a common ground?
I apologize if the answer is obvious, but thinking about electrics just seems to make my head hurt!
Thanks, seb

 

[VicS]

If all the equipment is off you should not have got a reading.

You have a problem somewhere...... although there might be an innocent explanation .. but I havent thought of it yet!

 

[sebastiannr]

When removing the starter motor cable, there was no reading.

 

[Barry Jones]

Not familiar with the book but I understand what he is saying (I think).

The starter motor cable shouldn't be drawing current because the contactor in the starter solenoid is open circuit.

What is the nature of your problem - can you give a bit more background ??

 

[sebastiannr]

Sorry, to clarify, when the starter motor cable was removed from the battery isolator switch, there was no reading

 

[VicS]

When removing the starter motor cable, there was no reading.

When you say the starter motor cable, you mean , the positive from the battery isolator to the solenoid. ( yep alreday clarified)

I have not thought it through but "dodgy" diode in the alternator springs to mind.

Any fancy charging systems ? Any other charging systems?

 

[sebastiannr]

The problem is that the battery drains to around 12.4v without any equipment on. The wind gen charges the battery, but it quickly returns to 12.4v when its not charging. I have tried a different battery with the same results.

 

[Barry Jones]

Look at the positive terminal on the starter housing.

Remove all the other wires apart from the thick one from the battery isolator.

Try the test again and advise back.

 

[sebastiannr]

Yes vics there is a wind genny. But aside from that nothing fancy at all.

 

[VicS]

The problem is that the battery drains to around 12.4v without any equipment on. The wind gen charges the battery, but it quickly returns to 12.4v when its not charging. I have tried a different battery with the same results.

You have to systematically check circuits to find where the current is flowing.

You are already a long way down that route. It must be engine related... I think.

I'd check for small currents with an ammeter rather than using a digital voltmeter. They have such a high impedance that they will almost measure the volts through the air.

Just to be on the safe side disconnect the wind genny and any controller. ( but follow any special instructions when reconnecting it)

 

[sebastiannr]

I just checked the resistance between the disconnected positive starter solenoid cable and the negative battery terminal (with starter cable connected). The reading was 186 ohms (with ohmmeter set at 200). So there is a closed circuit through the starter cables, i presume this is a problem

 

[Cymrogwyllt]

To me this makes no sense. With the battery + disconnected everything on the boat is effectively connected to 0V. Any connection between the battery and boat wiring will show 12V on the meter

 

[sebastiannr]

I believe that if all equipment is turned off, all CBs off, there should be no closed circuits, and so no voltage between +terminal and +cable. That's my understanding of it anyway

 

[Barry Jones]

Presumably you mean disconnected from the battery isolator ?

So you are measuring resistance from the end of the cable through the starter feed and the starter windings down on the earth strap to the hull and then back via a steel hull (or cable) to the negative post of the battery ??

is that the circuit ?

If this is the case, just disconnect the cable from the negative terminal and try again - same answer ???

Did you disconnect the other smaller cables on the positive starter feed like I suggested previously ?

I just checked the resistance between the disconnected positive starter solenoid cable and the negative battery terminal (with starter cable connected). The reading was 186 ohms (with ohmmeter set at 200). So there is a closed circuit through the starter cables, i presume this is a problem

 

[VicS]

I just checked the resistance between the disconnected positive starter solenoid cable and the negative battery terminal (with starter cable connected). The reading was 186 ohms (with ohmmeter set at 200). So there is a closed circuit through the starter cables, i presume this is a problem

Yes Alternator would be the first thing I would consider.

You sure there's not something somewhere switched on. If it was car I'd suggest the light in the boot ... got the tee shirt for that one!

 

[sebastiannr]

I'm quite sure nothing is swithed on. I completely isolated the +starter cable and tested v between that and +battery terminal and reading was system voltage. Thanks for all your help everyone, i think i will have to carry on with this one in the morning. Barry, yes i disconnected all the small cables going to the engine ignition panel, and still reading 12v. Thanks again for all the advice, its very much appreciated
seb

 

[pampas]

go to starter,with battery+ disconnected. remove All wiring from starter, as though you are about to remove, make sure starter+ is not touching anything, reconnect battery+, place voltmeter from live starter feed and test to remaining wires and also the starter connection . This will tell you which circuit is at fault. IF the voltage low on the main cable to the negative, then it will be a leakage in the main cable.

But be carefull during this testing you do not get a short to the Negative, for safety I would temporaily connect at the battery I/ 2 amp fuse before starting.

 

[AHoy2]

Do you have a broadcast radio receiver (car radio) fitted and fed with a keep-alive 12v. If so check where that feed is coming from and check that it is also disconnected for your test.