Quote:

A fuse can be thought of as a resistor. Therefore if current is flowing thru it there will be a small voltage drop across it. And that is what you are measuring. That's the theory anyway.

Hi, theory well understood... however the resistance of the fuse will be negligible, especially as measured with a multi meter, who's leads will have a much higher resistance than the fuse

Basically in practical terms, the resistance of the fuse will be so small, coupled with the tiny parasitic current passing though the fuse in this condition, the volts drop will be sod all... in the area of micro volts, ie not measurable with a DVM.

Additionally to be able to calculate the current in the circuit by measuring the volts drop across the fuse, you would have to know the actual resistance of the fuse, which you don't know.

I=V/R

Mat

Trucks wrote:

Click image to enlarge

I thank you all for your thoughts and advice which truly will be taken on board
The system of fuse removal surely would be fine if the system is set up correctly ie disconnect the battery earth put ,two jumper cables going to the battery and battery clamp (obviously not touching each other )attach your multimeter lock your car which you can as mentioned with the door locks pushed across with a screwdriver and bonnet catches closed( I will try again to remove the bonnet wire and short it across as advised and will use one click of the fob) wait for the car to go to sleep then remove the jumper cables from one end either the battery terminal or the battery clamp makes no difference then all the battery use is going through the multimeter which should be negligible if there is a high reading which could be intermittent by the way (so take that on board when diagnosing these things a high reading then nothing, five minutes later or less or even longer there is a high reading again).
Then by removing the fuse that is drawing the current you will see the reading drop immediately then locate what that fuse is .
I know you will say that removing the fuse could wake things up but as the car is sound asleep or should be it won't even know you are there tinkering with it, apparently this was the correct procedure before the introduction of the multi use clamp.
All this is not my thoughts but what I have learned being laid up for two months from days of reading and utube vids and believe me there us some useless and totally constradictive information out there.
After all said the meter clamp is the way forward and I could not get any readings via my multimeter by testing across the fuses .
As mentioned perhaps I need to sort my alarm out first but God knows how to do that ,I thought the first press was the main alarm and was set to feel any pressure drop in the car so would set itself off and the second press was to disarm that sensor and just relied door being locked.
I also take on board the use of the multimeter being used on the cable to the fuse box and using an earth on the car..
Also as for closing all the doors I could not access the fuse box on the passenger side hence leaving the door open but the catch closed and the door locked .
I do like the long testing leads for the fuse box on the passenger side bloody stupid place to have them there anyway.
As regards to the readings I must admit I am a bit wayward on the actual reading like 0.02 and 0.01 also clamp meter .055 I thought they were mlliamps and the recommended was 30 to 50 m/a on the D3 so I thought wrongly as it sems that I was just outside the recommended allowance by the clamp meter and inside with the multimeter.
Need to learn about basic car electrics I guess learnt loads already and still learning .
Lastly (I know blah blah blah)
Today I was at it again with the meteres and see the attached pictures they all read different readings doing the same task a simple battery check,just makes me wonder about all my test what the Tru results are.
Two readings are close but one is wa y off so I guess the the meteres which read the closest are the most reliable ,is there such a thing as a multimeter, multimeter tester ?



Thank you all.

  

matgriff wrote:

Hi, I would expect those relatively cheap multimeters to have an accuracy of less than 1% on that 20v range, ie +-0.2v so the results you're seeing are probably within the tolerance of the meters.

Regarding your comment on the fact you couldn't measure any voltage accross the fuse, see my previous post.

Also, if you're trying to measure very small currents, down in the 20-30ma range, then that clamp on meter won't be much use. Use the multimeter in series with the battery lead on the correct range, note that if you try and pull any significant load on the circuit when the meter is connected you'll at the very least blow the fuse in the meter (if it has one) or worst blow the meter up.


Mat

Hi

Did notice the op has the meters set to volts, personally when I done mine I set it to dc amps then went across the fuses

Took ages to finally finding the draw, I had fitted some elec seats and found the memory module was the culprit , bought a non memory seat switch and made up a completely new seat loom and fixed the issue

Don’t know why but my meters in series wouldn’t work, couldn’t get any reading

Wasn’t a 5 x min job that’s for sure

  

Quote:

Just quickly , u want ur multimeter set to DC amps not volts , then use ur meter probes across the fuses

Ref multimeter in-line for some reason I couldn’t get it to work like that

If you put your meter on the current range (Amps) and put it across a fuse that isn't blown, you'll measure nothing.. the current will flow through the fuse and not through the parallel meter. If you were to put the meter across a blown fuse, (with the fuse body still in situ) then you are in effect completing the circuit with the meter and you'll measure the current drawn by the circuit...but beware, you now have no fuse, so any fault current will blow the meter fuse or meter .

By the way, I'm not trying to be a tw**t here, or a smart ass... I'm just trying to help, I'm a power electronics design engineer, this is the type of stuff I do every day I'd like to pay back some of the help & advice I've got from others on the forum

Cheap meters are absolutely fine for general automotive use, just don't pay too much attention to the absolute accuracy, you don't need it anyway.

matgriff wrote:

Quote:

A fuse can be thought of as a resistor. Therefore if current is flowing thru it there will be a small voltage drop across it. And that is what you are measuring. That's the theory anyway.

Hi, theory well understood... however the resistance of the fuse will be negligible, especially as measured with a multi meter, who's leads will have a much higher resistance than the fuse

Basically in practical terms, the resistance of the fuse will be so small, coupled with the tiny parasitic current passing though the fuse in this condition, the volts drop will be sod all... in the area of micro volts, ie not measurable with a DVM.

Additionally to be able to calculate the current in the circuit by measuring the volts drop across the fuse, you would have to know the actual resistance of the fuse, which you don't know.

I=V/R

Mat

If you goto the link provided by gstuart, there is a table, where someone has gone to the trouble of working out the resistance and then what every volt translates to in milli amps for every value of fuse!

Click image to enlarge

A clamp meter is sufficiently accurate to show the difference between a good car and one which flattens batteries within a few days. Remember your lookin for a significant difference. Best to always zero the clamp before starting.

I wouldn't recommend using a meter in series with the battery on these cars. These car will draw 10A at start up and a lot of meters are only rated to 10A. (some 20)

Use the clamp meter.