Testing the 12V Earth on a boat use my my meter to measure the

RichardS said:
If the connection is "rather poor" then it will fail the continuity test and show up as having a resistance which is what you need to know.

I've never suggested measuring current or voltage .... I've only ever suggested a multimeter continuity test which would use the ohms function, which is why I'm asking the question and being told to read previous and next emails which are totally irrelevant to my question. ;)

Richard
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I didn't suggest you mentioned measuring current or voltage but your multimeter must have some way of measuring continuity. If the way it measures continuity does not accurately expose the problem then it is a bit pointless. i.e. continuity will register as fine for your multimeter but won't be fine for a big load. That is my understanding anyway but then I still think the pixie blockage is the problem. I understood Ohms law to mean that the resistance is relative to the current and the voltage and so a measure of resistance using a small current and small voltage might not be accurate enough for a greater current and greater voltage.
 
pmagowan said:
I read somewhere that electronics run on smoke and the trick is not to let the smoke out! I was working on a little buck converter yesterday for an arduino project for the boat and overexcited the pixies and they let the smoke out. Very odd smell. Lucky I bought 3 of the little units on ebay. Note to self: don't solder while something is connected to power!

I find the conventions in electrics to be confusing. I would assume that the +ve is where all the little pixies are packed in and when released they will run to the -ve to get room but instead we must only imagine this is happening while they are actualy running the other way!
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It was a 50-50 chance and they got it wrong. +Ve was picked out of a hat before the actual charge carriers were nailed down.
But why fret which way the pixies dress?
Consider the argument why the following reality is not where we live:
Batteries when completely charged are empty, if you connect one to an electric dark bulb the device will suck all the dark particles out of the air and fill the battery, leaving the room brighter for a while. When the battery is full the dark bulb stops doing its thing and darkness again fills the room.
 
Thanks for all the information guys.The engine is a Lombardini LDW 702M. The wiring diagram shows an earth symbol attached to the starter motor and alternator so I suppose the negative connection is through the engine and from there back to the battery with a fairly thick cable.I will identify this and clean the connections.
 
RichardS said:
My multimeter is very accurate but if it's not accurate enough then a meter with even more accuracy would be required, so "extreme accuracy" seemed a fair description. :)

However, your attempt to move the subject away from the issue under discussion tells me all I need to know .... which is that using my digital multimeter to determine continuity was, and will continue to be, a perfectly acceptable technique. :encouragement:

Bye. ;)

Richard
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I am sure your multimeter is very very accurate We would not expect you to have anything that wasn't , but the accuracy is not really the issue. A £5 Maplin's cheapie with an accuracy of 1% would be accurate enough but simply not have an adequate resolution to measure the very small resistances which would cause problems in a starter motor circuit. I though I had explained that adequately but perhaps the arithmetic ( 100 x 0.1 = 100) is a bit too advanced for you..
One needs a meter with a resolution of 0.01 ohms or better. Perhaps you have a high precision milli-ohm meter if so that's different but it's not the sort of thing most of us would have. The ordinary multi-meters that most of us have do not, never have and never will offer an acceptable techique

Moreover I am not attempting to move the subject away from the original issue which was checking the battery wiring on Crown22 boat. Merely correcting some of the misinformation you have introduced

RichardS said:
If the connection is "rather poor" then it will fail the continuity test and show up as having a resistance which is what you need to know.

I've never suggested measuring current or voltage .... I've only ever suggested a multimeter continuity test .........................
Richard
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The fact is that you need to be able to measure a lower resistance that is possible with an ordinary multi-meter. That is why your suggestion is unlikely to be helpful. It'll find a very poor connection, ie one with several hundred milli-ohms resistance but not one with a only few tens of milli-ohms resistance but enough to affect the operation of a starter motor.
 
crown22 said:
Thanks for all the information guys.The engine is a Lombardini LDW 702M. The wiring diagram shows an earth symbol attached to the starter motor and alternator so I suppose the negative connection is through the engine and from there back to the battery with a fairly thick cable.I will identify this and clean the connections.
Click to expand...

Probably but a version of the engine is available with an isolated negative return. It will be easily recognised by negative connections to the starter motor and alternator and by the fact that sensors, such as the low oil pressure and high temperature senors, will be a " two wire" type and of course no negative connection to the block.

The isolated negative version might be used if the boat had a sail drive rather than shaft drive.
 
RichardS said:
so that's clearly another debate lost,
Richard
Click to expand...

I am amazed you have conceded defeat so soon. You usually twist posts and facts around until they resemble the "three twisted twist that the twister of twists once twisted" .
 
I used to use in the aeroplane radio business a low resistance ohm meter. The demand was for a resistance of less than .006 ohm from radio mount tray to airframe. Now the probes were pretty bulky like 10mm diameter spike. The easiest way to get a ground to the airframe was via the holes in the seat tracks. About 8mm in diameter in bare and worn aluminium. The contact to the tray was more difficult but you could get it with probing and pushing. OK the meter itself measured a voltage from probe to probe with a set current flowing from probe to probe. This required a wire down to each probe separate from the current carrying wire. Simply because you did not want to measure the resistance of the probe lead.
This particular meter used a moving coil analogue meter measuring up to 1 millivolt (1000ohm coil) the current was set up from a 1.5v dry cell. A potentiometer set the current. I just forget the details.
Anyway with any standard cheap Digital volt meter on ohms when you press the 2 probes together you will typically see a reading of .2 ohm if it has a 0 to 200 ohm scale. This means that the lowest resistance you can measure is what is seen as on top of the .2 ohm residual. So even really optimistic you may measure or see roughly .1 ohm.A cheap DVM might have a low ohms scale of 0 to 2000 ohms. So shows zero when leads are shorted together but with a resolution of 1 ohm again not much use. As already pointed out .1 ohm resistance is way too high for a starter motor circuit. Now if you are checking wiring for a lamp say 1 amp then .1 ohm measurement will be fine. But for a 10 amp load .1 ohm would represent a 1volt drop. Significant.
So that is why in the situation first described a DVM on ohms scale is no use.
You can make up a circuit with a LM317 wired as a constant current supply from a decent sized (current) battery. You set it for a current of 1amp then measure the voltage drop with your DVM on 200mv scale gives a measure of 0 to .2 ohm possibly useful. However as said easier to use the starter motor cranking current and measure volt drop across both the -ve and +ve connection.
ol'will
 
If not already mentioned, you cannot find faults chasing round a boat measuring various points. Voltages varies in both circuits, you need to take all your readings from one base point, battery neg.

1 get your self 10 metre of any 1-2 sq mm multi-strand cable, strip 50 mm from one end, divide strands into two and twist to form a Y. Place neg multi meter probe in V and twist wires tight around probe, rap in insulation tape.

2 strip back other end 30 mm, remove negative battery terminal, place stripped end of cable through terminal hole, re-fit over terminal and clamp.

3 you can now move anywhere on the boat and check voltage from a known point, but more important you can measure pos and neg to a common point.

4 make a note of battery positive and battery negative terminal voltages using positive probe,

5 work out from battery, to isolator switch, if the voltage drops in the positive cable you have a fault between were you are and battery. If in doubt turn lights or similar on, this will increase volt drop.

6 try positive probe on engine earth, if the voltage rises, 1 volt you have a negative fault, this will lie between the high reading and low reading is your fault.

Brian
 
halcyon said:
If not already mentioned, you cannot find faults chasing round a boat measuring various points. Voltages varies in both circuits, you need to take all your readings from one base point, battery neg.

1 get your self 10 metre of any 1-2 sq mm multi-strand cable, strip 50 mm from one end, divide strands into two and twist to form a Y. Place neg multi meter probe in V and twist wires tight around probe, rap in insulation tape.

2 strip back other end 30 mm, remove negative battery terminal, place stripped end of cable through terminal hole, re-fit over terminal and clamp.

3 you can now move anywhere on the boat and check voltage from a known point, but more important you can measure pos and neg to a common point.

4 make a note of battery positive and battery negative terminal voltages using positive probe,

5 work out from battery, to isolator switch, if the voltage drops in the positive cable you have a fault between were you are and battery. If in doubt turn lights or similar on, this will increase volt drop.

6 try positive probe on engine earth, if the voltage rises, 1 volt you have a negative fault, this will lie between the high reading and low reading is your fault.

Brian
Click to expand...

If the negative connection to the engine is poor, measuring with a multimeter is a waste of time, as said earlier.
 
halcyon said:
Why ?

Brian
Click to expand...

If the connection is poor, a digital multimeter can show the correct battery voltage or pass a continuity test, whilst being unable to pass any meaningful current. As soon as you hit the starter button the connection cannot carry enough current to even energise the solenoid. In some cases, the starter will actually engage and start the engine, but the alternator won't work.

The OP already knows that the starter and alternator are both good, so his problem has to be something that both things have in common. If i was there now, looking at his engine, i would clean the terminals that i said earlier. I would be pretty confident that doing so would cure the issues.
 
PaulRainbow said:
If the connection is poor, a digital multimeter can show the correct battery voltage or pass a continuity test, whilst being unable to pass any meaningful current. As soon as you hit the starter button the connection cannot carry enough current to even energise the solenoid. In some cases, the starter will actually engage and start the engine, but the alternator won't work.

The OP already knows that the starter and alternator are both good, so his problem has to be something that both things have in common. If i was there now, looking at his engine, i would clean the terminals that i said earlier. I would be pretty confident that doing so would cure the issues.
Click to expand...

You are looking for a volt drop in cabling, there is no starter current passing through multi-meter.

By anchoring the multi-meter neg to battery neg, any positive or negative volt drops can be measured relative to a fixed known zero. So voltage changes are relative to a common zero point, they are track able, you can measure both positive and negative voltages changes without altering meter, even tells you if it's a positive or negative connection.

Brian
 
halcyon said:
You are looking for a volt drop in cabling, there is no starter current passing through multi-meter.

By anchoring the multi-meter neg to battery neg, any positive or negative volt drops can be measured relative to a fixed known zero. So voltage changes are relative to a common zero point, they are track able, you can measure both positive and negative voltages changes without altering meter, even tells you if it's a positive or negative connection.

Brian
Click to expand...

If there is a bad connection the digital voltmeter can still read the correct battery voltage, the voltage drop won't occur until a load is imposed on the circuit, ie the starter motor. So measuring with a digital multimeter in this instance is so unreliable as to be a pointless test.

The starter is intermittent and the alternator won't charge, but both are known to be good. The fault has to be with something shared by both, almost always this will be the negative connection, more often than not where it bolts to the block. This is why the auto electrician said to connect a jump lead between the battery negative and the block, if it all works, the fault is proven to be the negative connection.
 
PaulRainbow said:
If there is a bad connection the digital voltmeter can still read the correct battery voltage, the voltage drop won't occur until a load is imposed on the circuit, ie the starter motor. So measuring with a digital multimeter in this instance is so unreliable as to be a pointless test.

The starter is intermittent and the alternator won't charge, but both are known to be good. The fault has to be with something shared by both, almost always this will be the negative connection, more often than not where it bolts to the block. This is why the auto electrician said to connect a jump lead between the battery negative and the block, if it all works, the fault is proven to be the negative connection.
Click to expand...

Why do people bounce between it could be here, or here or there or this, work through the problem, it's cheaper.

I did give the operator the gumption that he / she would need power on to see the fault, even think I said so.

In this case the alternator is working, what is the running voltage, pos and neg at alternator, if neg is zero then probably okay, if 2 volt then bad connection, work back to battery till voltage falls to near zero, simple.

Brian
 
If I may throw another spanner into the work guys without it getting too complicated.
My FP generator uses the same battery as the boat engine 50hp to start .
The boat engine start no problem .
The generator on the our hand will show 12.7v at the panel ,
as it goes to start the volts drop to 6.8 and it Struggle to turn over , once it start the volts goes back up to 17.2v
I cleaned both leads at the battery , started motor and on the generator body .
What else can I do to find where the Voltages drop is ,
I take it in one of the leads .
I can't use a jump leads as they won't reach from the Generator to the battery bank .
Please keep it simple .
 
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