How To Find A Short?

[Imtjnotu 5,000+ posts]

ok guys i have googled but ehow really makes no sense or anything....

i went to start up my car today and it was dead so i check the batt and its only reading 2.8volts down from the 12.9 2 days ago...i didnt drive it yesterday to work i took my other car but i got in it today and none of the power locks worked or the alarm

how do u find what is still drawing power from the battery?

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[bhsdriller 10+ year member]

good question....would like to know also

 

[bubbagumper6 5,000+ posts]

One method that I've found helpful in the past is disconnect the positive battery terminal and put a DMM on it to measure the current. Then start pulling fuses in the fuse box, when the current has a significant drop you've at least narrowed down your search. This doesn't work in all cases obviously but it's helped me. Also a ghetto fix for this if you do find a circuit that's drawing current with the car off is to break the fuse and wire in a relay to break that circuit when the car is off...

 

[Imtjnotu 5,000+ posts]

One method that I've found helpful in the past is disconnect the positive battery terminal and put a DMM on it to measure the current. Then start pulling fuses in the fuse box, when the current has a significant drop you've at least narrowed down your search. This doesn't work in all cases obviously but it's helped me. Also a ghetto fix for this if you do find a circuit that's drawing current with the car off is to break the fuse and wire in a relay to break that circuit when the car is off...

i will give this a shot....only ****** thing is i have a 70 pound batt and no way of getting to autozone.....and have work in an hour

 

[CHEMMINS 10+ year member]

For now......drive to where ya got to go, and disconnect the +. When ya leave, reconnect it and go.

 

[Imtjnotu 5,000+ posts]

i cant even drive the car batt is dead lulz for me

 

[bubbagumper6 5,000+ posts]

i will give this a shot....only ****** thing is i have a 70 pound batt and no way of getting to autozone.....and have work in an hour

What do you need to go to autozone for? You don't own a DMM?

 

[grandam888 10+ year member]

to charge his dead battery before work.

 

[Imtjnotu 5,000+ posts]

One method that I've found helpful in the past is disconnect the positive battery terminal and put a DMM on it to measure the current. Then start pulling fuses in the fuse box, when the current has a significant drop you've at least narrowed down your search. This doesn't work in all cases obviously but it's helped me. Also a ghetto fix for this if you do find a circuit that's drawing current with the car off is to break the fuse and wire in a relay to break that circuit when the car is off...

well its still happening...i pulled all the fuses and the dmm still read 12.8

 

[bubbagumper6 5,000+ posts]

well its still happening...i pulled all the fuses and the dmm still read 12.8

12.8 what? Amps? Volts? Magic fairy wings?

EDIT: I guess I should ask, what did you have the DMM set to and how did you have the probes connected?

 

[Imtjnotu 5,000+ posts]

12.8 what? Amps? Volts? Magic fairy wings?
EDIT: I guess I should ask, what did you have the DMM set to and how did you have the probes connected?

12 volts dc
i went and talked to the electrical guy at work who does the night time electrical on the busses...he said to disconnect the negative terminal and take the negative probe on the dmm on it

he then told me to connect positive probe on the dmm to the grounding point...if there is a short it will read voltage DC...it read 12.8...i then pulled fuses and no change in voltage at all

 

[bubbagumper6 5,000+ posts]

Dude I have no idea what he was trying to have you measure but that's not what I said to do. You need to measure the current, not the voltage. Disconnect one of your battery terminals and put the dmm in series (i.e. connect one probe to the actual battery's terminal and the other to the terminal you just pulled off). Your DMM should have a DC A mode, or something that says DC current. You'll probably have to change where the positive probe is plugged into your dmm. There's probably 2 ports that have a "10A" connecting them, those are the ports you need.

 

[Imtjnotu 5,000+ posts]

Dude I have no idea what he was trying to have you measure but that's not what I said to do. You need to measure the current, not the voltage. Disconnect one of your battery terminals and put the dmm in series (i.e. connect one probe to the actual battery's terminal and the other to the terminal you just pulled off). Your DMM should have a DC A mode, or something that says DC current. You'll probably have to change where the positive probe is plugged into your dmm. There's probably 2 ports that have a "10A" connecting them, those are the ports you need.

can u pm me ur number so i can text u?

 

[bubbagumper6 5,000+ posts]

Come on man it's not that hard. One probe on the battery post, the other on the terminal. Put your DMM on the option that says "10A" (see picture)

Then you see how you have 3 different ports? The bottom says COM, the middle says V (omega) mA, then the top says 10A. You usually have the black probe in the COM port and the red probe in the middle port (the with V, omega, and mA). But in this case we want to measure current so move the red probe to the port that says 10A.

This will allow you to measure how much current is being pulled from the battery. Ideally, with the car off nothing should be using power so the current should be 0 or very small. Since you have something that's draining your battery, yours will be something larger. This is where the fuse pulling comes into play. When your pulling fuses you watch for the current to drop.

Alternatively if you have a 12V lightbulb you could connect it in series (one pin to the battery post the other pin to your battery terminal) and it will only light if there's a draw. So it will probably light for you, then if you pull the fuse that the draw is going through the light will turn off.

 

[ampjunkielikeme 10+ year member]

how old is the battery? sounds like it may have dead cell ...