fault finding (with a multimeter)

[jonnyboy]

Hi all

How would one go about using a multimeter to find out what is drawing current from a battery?
My 129 SL is draining. I've just replaced the C/L pump which has fixed the locking issue (and it might have been that anyway, time will tell, battery on charge at moment). Its a new battery.
I guess its the procedure I am hazy on
Thanks

 

[verytalldave]

Trying to isolate a problem like this can be a real headache. Specially on a car as complex as a modern Mercedes. I would suggest setting the MM to check for AMPS with a maximum setting of say, 5 or 10 amps. Make sure the battery in the MM is OK first!
Then just systematically go round the car connecting the MM probes to either side of the terminals of the device you want to measure.
Dont forget its possible the car may have one or two devices which may always draw a small amount of current. Things like clocks and some ant-theft devices spring to mind.
If it was me, I would take it to a proper dedicated auto electrician who has far more sophisticated devices than a simple multimeter to detect where a potential problem is located.

 

[Ted]

Switch the multimeter to 'amps' or 'milliamps' - preferably autoranging if possible.
Ensure the multimeter wires are in the 'current' sockets
Remove the negative lead from the battery *If there is any electrical equipment with a memory carefully put the meter in circuit, ensuring the battery does not become disconnected*
Now put the meter in series with the battery cable - i.e. one wire on the battery post, the other on the battery cable.
Measure the current draw.

DO NOT try to start the engine or turn anything on with a high current draw while you are doing this, as you will fry the multimeter.

 

[BlackC55]

and make sure the car is locked and the boot catch is locked over

 

[philimayfield]

Maplin sell an auto current tester for £9.99 which plugs into the blade fuse holders. This will indicate the circuit drawing the current and show the amps.

Phil

 

[grober]

Maplin sell an auto current tester for £9.99 which plugs into the blade fuse holders. This will indicate the circuit drawing the current and show the amps.

Phil

Looks a very interesting product. Automotive Current Tester : Test Accs : Maplin Not sure if it would adapt/fit to the older glass cartridge type mercedes fuse holders tho.

 

[Stratman]

Trying to isolate a problem like this can be a real headache. Specially on a car as complex as a modern Mercedes. I would suggest setting the MM to check for AMPS with a maximum setting of say, 5 or 10 amps. Make sure the battery in the MM is OK first!
Then just systematically go round the car connecting the MM probes to either side of the terminals of the device you want to measure.
Dont forget its possible the car may have one or two devices which may always draw a small amount of current. Things like clocks and some ant-theft devices spring to mind.
If it was me, I would take it to a proper dedicated auto electrician who has far more sophisticated devices than a simple multimeter to detect where a potential problem is located.

STOP!!!!!!!

When measuring current, as here, the ammeter should be in series with the device. What is described in red above puts the meter in parallel with it, effectively putting a low resistance path across the battery. This can have some interesting side effects ranging from blowing a fuse in the car's fusebox to an early firework dispaly in the palm of your hand.

To measure the current drawn by a device it is necessary to disconnect one of the supply wires (either the live feed or the earth) and connect one of the ammeter terminals to the wire and the other to the device terminal, observing the correct polarity. Ammeter + to the live side, ammeter - to the earthy side.

 

[alexander patie]

interesting thread, an easy question to ask, but a hard one to answer.

measuring amperage with a meter is much trickier than checking voltage or resistance, as has been said. the place to start is at the battery, and with everything off/locked and the boot or door pin/latch rigged to kid the car it is closed - as has also been said. the next bit is the tricky bit, note that whenever you disconnect and reconnect the battery there will be a strog spark. this is a sign of a momentary high current draw, this can damage meters, or if they have them, blow meter fuses. the trick is to connect the meter in a way to avoid this surge. one way is to disconnect the earth lead, then touch it firmly to the side of the terminal and hold it there, now connect the meter clamps to the terminal and the lead, now you can move the lead from the side of the terminal without the current surge - hope that makes sense!!

now the meter is connected and measuring, wiat. some cars can take up to 30mins to settle down to their true idle current draw. last thing now, you need to see around 50ma - 0.050 amps - anymore than that and you do have a problem. good luck.

 

[Sp!ke]

I had to do this myself recently and didnt find it too hard at all.

Initially put the ammeter in series on earth battery terminal and remove fuses one by one until you spot which circuit makes the biggest drop.

Once you know the circuit you need to look at you're half way there.

Go along that entire circuit measuring the resistance between live and earth, disconnecting appliances one by one until you find the culprit area or device. Its a simple enough task and quite satisfying when you trace the problem.

What is the problem by the way? Battery going flat overnight or something? I take it you've looked at the common causes already?