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

[crown22]

Testing the 12V Earth on a boat

Hello new to me boat and the alternator was working not at all and the starter intermittently.Removed them and took them to
a specialist who says they both work perfectly on the bench.He says that I should have tested the earth before removal.A couple
of suggestions firstly connect a car jump lead from the negative battery terminal to the engine and see if the alternator works and secondly use a meter to test voltage between positive terminal of the battery and the engine casing.Any advice or suggestions gratefully received.

 

[RichardS]

Jump lead would work .... but of you have a multimeter simply test to check that the battery negative terminal and the engine are directly connected. I guess that you might have an engine where the block is not earthed but you should be able to follow the thick cable (probably black) from the alternator to an earth bolt on the engine or look at the starter motor and see if it has a thick negative cable, rather than being earthed through the block, as well as the usual positive (possibly red) one.

Richard

 

[PaulRainbow]

A digital meter is unlikely to be helpful in this case. Remove, carefully clean and replace both battery terminals and the negative cable where it bolts to the engine block (assuming it does bolt onto the engine block)

Might be useful to say what the engine is.

Minor point, there is no earth in your 12v system. Just negative and positive.

 

[VicS]

A digital meter is unlikely to be helpful in this case. Remove, carefully clean and replace both battery terminals and the negative cable where it bolts to the engine block (assuming it does bolt onto the engine block)

Might be useful to say what the engine is.

Minor point, there is no earth in your 12v system. Just negative and positive.

+ 1 on the comment about a digital meter. Perhaps one should explain that digital voltmeters take so little current that they can ( and do ) give noraml readings through a bad connection which is guaranteed to make a fool of you ............. got the Tee shirt for that one!

Engine in question is, I believe, a Lombardini LDW 702 M, which could have an isolated electrical system as an optional "extra"

 

[Lon nan Gruagach]

You can use any meter (with range suitable for 12V) .
Measure the voltage between battery -ve post and engine block while the equipment is meant to be operating.
For the starter you should have no more than a few volts,for the alternator then more than a volt should be cause for concern.

 

[William_H]

You can use any meter (with range suitable for 12V) .
Measure the voltage between battery -ve post and engine block while the equipment is meant to be operating.
For the starter you should have no more than a few volts,for the alternator then more than a volt should be cause for concern.

You are quite right suggesting measure the volts between the battery negative terminal ( the battery post) and the engine block or starter motor body or alternator body. However volt drop should be a lot less than you suggest. Not measurable for alternator and less than .1volt for starter. Any more should give concern. But also measure volt drop from positive battery terminal (the actual post) and starter motor positive terminal. This also should be very low when cranking engine. ol'will

 

[RichardS]

A digital meter is unlikely to be helpful in this case. Remove, carefully clean and replace both battery terminals and the negative cable where it bolts to the engine block (assuming it does bolt onto the engine block)

Why would a digital meter not be helpful for testing for continuity between the -ve battery terminal and the engine block?

Richard

 

[pmagowan]

Why would a digital meter not be helpful for testing for continuity between the -ve battery terminal and the engine block?

Richard

read the next post

 

[VicS]

Why would a digital meter not be helpful for testing for continuity between the -ve battery terminal and the engine block?

Richard

because one needs to be able to measure a lower resistance than one can do reliably with an ordinary digital multi-meter.

Dougal's suggestion is the way to do it , but one needs to be able to crank the engine for a few seconds without it starting. ( easy to do with SI engines , less so with most modern CI engines)

 

[RichardS]

read the next post

You read the next post.

Richard

 

[pmagowan]

You read the next post.

Richard

Read the previous post!

 

[RichardS]

because one needs to be able to measure a lower resistance than one can do reliably with an ordinary digital multi-meter.

Why do you need such extreme accuracy just to measure continuity? I've always found my multimeter to be accurate enough to show whether there is a problem with undue resistance across a connection.

Richard

 

[RichardS]

Read the previous post!

Read the last post. :encouragement:

(before this one)

Richard

 

[VicS]

Why do you need such extreme accuracy just to measure continuity? I've always found my multimeter to be accurate enough to show whether there is a problem with undue resistance across a connection.

Richard

Who said anything about extreme accuracy ?

 

[pmagowan]

If I read it right essentially a normal cheap multimeter of the type we are likely to have will essentially say there is sufficient continuity even if the connection is rather poor. When you require higher current through the connection there is a problem that the multimeter, with its low current, wont detect.

enough of the pixies get through to satisfy the multimeters pixie counting device when you send one through at a time but when you try to squeeze a lot through they get bunged up such that no work can get done!

 

[VicS]

If I read it right essentially a normal cheap multimeter of the type we are likely to have will essentially say there is sufficient continuity even if the connection is rather poor. When you require higher current through the connection there is a problem that the multimeter, with its low current, wont detect.

enough of the pixies get through to satisfy the multimeters pixie counting device when you send one through at a time but when you try to squeeze a lot through they get bunged up such that no work can get done!

Almost right

The comments about "low current" apply to using a digital multimeter on a volts range to check that there is a power supply. Enough pixies get through a bad connection to give a full 12.x volts reading as you rightly deduce.

The discussion has moved on to checking continuity and measuring any resistance there may be in the connections. A typical inexpensive multi-meter is likely to have a resolution of 0.1 ohms and an accuracy of 1% on its lowest resitance range
The accuracy is more than good enough but the resolution is not. A starter motor pulling 100 amps would create a potential difference of 10 volts across a resistance of 0.1 ohms therefore you need to be able to measure ( but not to " extreme accuracy") somewhat smaller resistances. 0.01 ohms or lower)

Also the clips, prods and meter plugs and sockets are likely make even a rough estimate of circuit resistance difficult.

Meters do exist of course, with much better resolution ( and accuracy) but cost rather more than the few quid of the average inexpensive multi meter and are not the type of thing most DIYers will have in their tool bags.

I dont know for sure but I suspect they measure the PD across the circuit when a significant current ( 10amps or more )s is passed through it.
This should suggest a way forward, Put a significant load on the circuit ( glow plugs or a couple of head lamp bulbs for example ) and measure the volts drop.......... Pretty much the same as Dougal suggested in #5.

 

[RichardS]

Who said anything about extreme accuracy ?

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

 

[RichardS]

If I read it right essentially a normal cheap multimeter of the type we are likely to have will essentially say there is sufficient continuity even if the connection is rather poor. When you require higher current through the connection there is a problem that the multimeter, with its low current, wont detect.

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

 

[Lon nan Gruagach]

If I read it right essentially a normal cheap multimeter of the type we are likely to have will essentially say there is sufficient continuity even if the connection is rather poor. When you require higher current through the connection there is a problem that the multimeter, with its low current, wont detect.

enough of the pixies get through to satisfy the multimeters pixie counting device when you send one through at a time but when you try to squeeze a lot through they get bunged up such that no work can get done!

Almost right.

You must admit though, thats a fairly neat description of the semiconductor junction presented by a rusty terminal.

 

[pmagowan]

You must admit though, thats a fairly neat description of the semiconductor junction presented by a rusty terminal.

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!