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

[lw395]

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 .

Is the generator starter dragging the battery down to 6.8V?
It could be, as a result of a faulty starter motor. Or the batery is on its last legs and the generator needs more current to start than the main engine.
So measure across the battery terminals while cranking. Or just try a known good battery.

If the battery is at say 11 or 12 V while cranking, and the starter is only getting 6.8, then measure the difference between battery -ve and the starter body while cranking. If it's more than say 0.5 V, the fault is between these points in the -ve wiring. Otherwise, it's in the +ve wiring.

It's useful to have a meter with min/max functions, you can see the min voltage the battery collapses to and the max voltage along the cables.
I would also use a DC Clamp ammeter to see what current the starter was drawing.

 

[VicS]

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 .

17.2 is presumably a typo

Assuming the battery is good, somewhere between the battery and where generator panel voltmeter is connected there is a bad connection or connections. It may be in the positive or the negative

If you have cleaned all the connections it may be due to a badly crimped on terminal or terminals .... I once had to solder all the crimps on the battery leads and starter motor leads on a car, each one was poor although individually not bad enough to have caused a problem . Collectively they caused a big enough volts drop to make starting difficult.

I detected the cause of my bad starting by putting a good load on the wiring in the form of a couple of headlamp bulbs and measuring the volts across various sections of the wiring, the ohms range of my meter not being sensitive enough to measure the resistances directly. I found measurable voltages between the crimps on the ends of each cable.

 

[sailaboutvic]

17.2 is presumably a typo

Assuming the battery is good, somewhere between the battery and where generator panel voltmeter is connected there is a bad connection or connections. It may be in the positive or the negative

If you have cleaned all the connections it may be due to a badly crimped on terminal or terminals .... I once had to solder all the crimps on the battery leads and starter motor leads on a car, each one was poor although individually not bad enough to have caused a problem . Collectively they caused a big enough volts drop to make starting difficult.

I detected the cause of my bad starting by putting a good load on the wiring in the form of a couple of headlamp bulbs and measuring the volts across various sections of the wiring, the ohms range of my meter not being sensitive enough to measure the resistances directly. I found measurable voltages between the crimps on the ends of each cable.

Yes sorry typo ,

 

[sailaboutvic]

Is the generator starter dragging the battery down to 6.8V?
It could be, as a result of a faulty starter motor. Or the batery is on its last legs and the generator needs more current to start than the main engine.
So measure across the battery terminals while cranking. Or just try a known good battery.

If the battery is at say 11 or 12 V while cranking, and the starter is only getting 6.8, then measure the difference between battery -ve and the starter body while cranking. If it's more than say 0.5 V, the fault is between these points in the -ve wiring. Otherwise, it's in the +ve wiring.

It's useful to have a meter with min/max functions, you can see the min voltage the battery collapses to and the max voltage along the cables.
I would also use a DC Clamp ammeter to see what current the starter was drawing.

@battery when starting it goes down to 11v but the panel shows 6.8v
The Gen is a small 4.5kw single cyl so I wouldn't thro it need any thing to turn it over as the 50hp boat engine
Current drawn 45 amps
Edit
I guess the gen panel is wired to show what volts are reaching the starter at it showing the right volts before starting at the panel as the battery .

 

[PaulRainbow]

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

I haven't bounced around, i have consistently said it's almost certainly a negative fault. Checking with a jump lead would confirm this, cleaning the connections would fix it.

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

Doesn't matter how many times you say it, using a digital multimeter for this job is wrong, for the reasons both myself and Vic have stated.

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

The alternator is not working, at all, the starter is intermittent. Once again, measuring voltage at the alternator is pointless, the multimeter will likely show battery voltage, even if the connection is not good enough to carry more than a few milliamps. Just because the connection is bad does not mean there will be a voltage drop.

 

[Lon nan Gruagach]

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 .

Use the test I suggested way back in this thread.
Measure the voltage between the ends of each cable from the battery while the generator is starting.
You will find that one or both of the positive and negative wires drops the 12.7V battery to 6.8V at the generator while starting.
If it is just 1 wire then check any connections and joins in the cable, also check if the cable has been damaged.
If both wires cause the drop by about the same amount then it is possible that the wires are too thin.
Often people think that a generator is only a small engine and can use much thinner wire, this is a mistake.

 

[lw395]

@battery when starting it goes down to 11v but the panel shows 6.8v
The Gen is a small 4.5kw single cyl so I wouldn't thro it need any thing to turn it over as the 50hp boat engine
Current drawn 45 amps
Edit
I guess the gen panel is wired to show what volts are reaching the starter at it showing the right volts before starting at the panel as the battery .

So you are losing a lot of volts between battery and panel.
But the fact that the battery voltage is pulled down to 11 suggests quite a lot of current is flowing.
There is 4V being dropped between battery and panel.
That should be easy to see with a multimeter.
It's either in the +, the - or split between the two.

That 4V is about 3V too much (someone will be along to quibble detail directly). 3V times 100A is going to make the bad connection warm unless it's spread over a big mass of copper. Check the solenoid contactor is not geting hot. Check all connections.

It might be worth looking at the battery volts while cranking the main engine. If they don't dip so far, the generator is pulling more current and may have a dying starter motor.
Don't crank the starter for more than a few seconds in this state or you may kill it.

It might also be worth doing the maths on the cable sze, it could be the installation is marginal, only works with perfect starter and excellent battery. Good to rule that out.

Lastly, it is possible that the problem is mechanical, the starter is worn to the point that there is ridiculous friction or the generator is almost seized or otherwise hard to turn.

Turning over a 500cc single is nearly as hard as turning over a 2litre four. It's the compression strokes.
Can you decompress the genny? Does it spin over as you'd expect?

 

[halcyon]

I haven't bounced around, i have consistently said it's almost certainly a negative fault. Checking with a jump lead would confirm this, cleaning the connections would fix it.



Doesn't matter how many times you say it, using a digital multimeter for this job is wrong, for the reasons both myself and Vic have stated.



The alternator is not working, at all, the starter is intermittent. Once again, measuring voltage at the alternator is pointless, the multimeter will likely show battery voltage, even if the connection is not good enough to carry more than a few milliamps. Just because the connection is bad does not mean there will be a voltage drop.

I give up, spent 40 years designing electronic circuits for our VSR's. mains chargers and switch panels, just feel sorry for the OP.

Brian

 

[PaulRainbow]

I give up, spent 40 years designing electronic circuits for our VSR's. mains chargers and switch panels, just feel sorry for the OP.

Brian

Sorry that you can't grasp why the multimeter measuring voltage along cables won't work.

No need to feel sorry for the OP, if he cleans the three terminals like i said, he'll most likely be good to go :encouragement:

 

[sailaboutvic]

Use the test I suggested way back in this thread.
Measure the voltage between the ends of each cable from the battery while the generator is starting.
You will find that one or both of the positive and negative wires drops the 12.7V battery to 6.8V at the generator while starting.
If it is just 1 wire then check any connections and joins in the cable, also check if the cable has been damaged.
If both wires cause the drop by about the same amount then it is possible that the wires are too thin.
Often people think that a generator is only a small engine and can use much thinner wire, this is a mistake.

Hi ,
I haven't don't your test because I am getting some strange reading I don't under stand , and I like to understand then before I do any other test .
Meter set at 20v
Battery read 12.8 over the + - terminal
Same if I used the meter over the starter Motor and the - on the engine body with out starting it .
Now here where it's strange.
One end of the meter on the battery - ve and if I touch the other end to the -ve on the where it is connected to the engine body I get a reading of nothing but if I touch it to the start motor + I get -12 .8

Put the meter on the + on the battery end and on the starter I get nothing
On the body I get 12.8

This doesn't make sense to me surly i should be getting reading - to body and + to the starter ?
Or am I missing some thing

 

[VicS]

Hi ,
I haven't don't your test because I am getting some strange reading I don't under stand , and I like to understand then before I do any other test .
Meter set at 20v
Battery read 12.8 over the + - terminal
Same if I used the meter over the starter Motor and the - on the engine body with out starting it .
Now here where it's strange.
One end of the meter on the battery - ve and if I touch the other end to the -ve on the where it is connected to the engine body I get a reading of nothing but if I touch it to the start motor + I get -12 .8

Put the meter on the + on the battery end and on the starter I get nothing
On the body I get 12.8

This doesn't make sense to me surly i should be getting reading - to body and + to the starter ?
Or am I missing some thing

I reckon you must have the meter connected "ass about face"

 

[sailaboutvic]

I reckon you must have the meter connected "ass about face"

I was being a ass Vic's

There a 1.7 drop on both Leads .

 

[VicS]

I was being a ass Vic's

There a 1.7 drop on both Leads .

That's too much but the resistances causing it are probably less than you can detect with an ordinary multi-meter on its lowest ohms range.
If you have cleaned the connections it may well be caused by bad crimped joints.