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chasing an electrical draw
#907302
01/17/11 03:24 PM
01/17/11 03:24 PM
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Joined: Jul 2006
Posts: 4,179
California
mickm
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OP
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Joined: Jul 2006
Posts: 4,179
California
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i have a smallish draw when the car is off that drains the battery after two weeks or so.
in starting to chase it, i disconnected the clock, which should be the only thing drawing current when the car is off. with a multimeter connected to the negative side of the battery, i was still drawing almost 12 volts, 11.8 or so.
i started popping fuses, and the only one that made a difference was the third from the right, (forget which one that is, have it written down in the garage), where the voltage dropped to ~8.9V. that was the only difference i could find by pulling fuses.
so my first question is even if there is nothing actually causing a draw, will there still be nominal voltage in the system? kind of like a battery that doesn't have enough charge to be useful still has a voltage?
the second question is if that is the case, now that i know which circuit it is on, do i just start following the circuit and disconnecting things, same idea as pulling the fuses?
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Re: chasing an electrical draw
[Re: 70SportSatellite]
#907304
01/17/11 03:48 PM
01/17/11 03:48 PM
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Joined: Jan 2008
Posts: 299
East Brunswick, NJ
finadk
enthusiast
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enthusiast
Joined: Jan 2008
Posts: 299
East Brunswick, NJ
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Use the test light method described. But i would not pull the fuse for the door light circuit that constantly on circuit is most likely the circuit with the problem. Is there an alarm or sound system amp in the car? Check the easy stuff first trunk light dome light, glove box light staying on?,
Scott
Scott
1956 Dodge Custom Royal Lancer (408 Stroker, 4 Wheel Disc Brakes, Rack & Pinion, 6 speed)
2002 Dodge Ram 1500 Quad Cab
1976 Corvette
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Re: chasing an electrical draw
[Re: 70SportSatellite]
#907305
01/17/11 03:50 PM
01/17/11 03:50 PM
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Joined: Oct 2010
Posts: 606
Montana
Yancy Derringer
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mopar
Joined: Oct 2010
Posts: 606
Montana
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You may be using the wrong technique, that is, "volts." As said above START with a test lamp. Depending on how big the test lamp is ---how much current it draws-- will determine the "draw" you can find with it.
That is, a test lamp using a huge bulb such as a stop or tail lamp will only "show color" on a fairly large "draw."
A tiny small bulb, or a 12V LED, will find smaller "draws."
After you have found the draw with your lamp, and it is now "dark"--------
THEN get out your meter, --BUT THIS DEPENDS--
You need to be sure to set your meter up for maximum AMPERAGE, NOT VOLTAGE.
On most meters, this should be 10 or 20 amps. BE CAREFUL, there are several pitfalls.
Some meters are fused in the amperage mode, some are not
DO NOT put your amperage selected meter TO GROUND. Current (ammeters) ALWAYS go in series, BUT---
If you screw up and cause a high current draw, like try and turn the key to "start" when the meter is hooked up---you will either destroy the meter, or blow it's fuse IF it is protected.
THEN after you have found the "draw" and the meter no longer shows anything on the heavy 10-20 amp scale, then set the meter down for more sensitive milliamp scales, but AGAIN be careful not to cause damage. On some meters (My Flukes) there are separate meter jacks which must be used for current scales.
THAT IS WHY you use the lamp test first---because a 12V lamp CAN NOT be damaged by your automotive system.
You started out right by removing the fuses. Don't discount anything in the car. I have found "draws" that were basically caused by corrosion, that is, a damaged harness under the doorsill that was full of corrosion. I also found a draw once that was caused by "junk" in the bottom of the cigarette lighter
The point? It's not always an object like the radio, or the "map light," etc. I can be something "in between."
ALSO be sure you unhook the alternator and check it for "draw." One or more diodes in the alternator may become what is called "leaky". These can draw anywhere from a tiny amount, to enough to cause problems.
HERE IS PART OF why you cannot check with a voltmeter "in series:"
EVEN DIRT on top of the battery will show "some voltage" on modern digital meters, or even sensitive analog type meters. This is because the combination of boiled acid, dirt, condensed humidity, etc, causes a conductive film, and the meter you are using does not draw very much current---in the millionth's of amps, or microamp range.
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Re: chasing an electrical draw
[Re: 70SportSatellite]
#907306
01/17/11 03:51 PM
01/17/11 03:51 PM
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Joined: Jul 2006
Posts: 4,179
California
mickm
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Quote:
Take a test light and hook it up in-line on the positive terminal of the battery. In other words, the clip to the battery and the probe touching the cable end. it should work the other way around but that's I how I normally do it.
The current draw in a normal situation should be so small it should not light the bulb. If the bulb lights then there is an excessive draw. If it does not light turn on the headlights. itr should now be very bright. If it doesn't, check your connections, it should.
Now, turn off everything and if the bulb glows start pulling fuses. When you find the fuse that turns the light out determine what it runs and start disconnecting those items.
Since you will have the door open be sure to depress the door jamb switch or pull the fuse for it, otherwise you will chase your tail.
sounds like a good plan. so the volt meter shows voltage, which isn't really what i'm looking for, i'm looking for current, which is exposed by there being enough current to light the light, correct?
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Re: chasing an electrical draw
[Re: finadk]
#907307
01/17/11 03:53 PM
01/17/11 03:53 PM
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Joined: Oct 2010
Posts: 606
Montana
Yancy Derringer
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mopar
Joined: Oct 2010
Posts: 606
Montana
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Quote:
Use the test light method described. But i would not pull the fuse for the door light circuit that constantly on circuit is most likely the circuit with the problem. Is there an alarm or sound system amp in the car? Check the easy stuff first trunk light dome light, glove box light staying on?,
Scott
ALSO in addition to above, DISCONNECT your sound system. This is because of two things. Many sound systems have circuits that are "always on." As with alternators, there are components that can be or become "leaky."
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Re: chasing an electrical draw
[Re: Yancy Derringer]
#907308
01/17/11 03:58 PM
01/17/11 03:58 PM
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Joined: Jul 2006
Posts: 4,179
California
mickm
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OP
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Quote:
You may be using the wrong technique, that is, "volts." As said above START with a test lamp. Depending on how big the test lamp is ---how much current it draws-- will determine the "draw" you can find with it.
That is, a test lamp using a huge bulb such as a stop or tail lamp will only "show color" on a fairly large "draw."
A tiny small bulb, or a 12V LED, will find smaller "draws."
yup, this makes sense.
Quote:
Some meters are fused in the amperage mode, some are not
DO NOT put your amperage selected meter TO GROUND. Current (ammeters) ALWAYS go in series, BUT---
my meter is fused. can you explain this, what do you mean "put your amperage selected meter TO GROUND"? so i want to
1) check that the current i'm chasing is within the range of what my meter will handle,
2) make sure i don't introduce more current than that, and
3) the meter needs to be hooked up in series, the same way the bulb was, correct?
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Re: chasing an electrical draw
[Re: finadk]
#907309
01/17/11 04:00 PM
01/17/11 04:00 PM
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Joined: Jul 2006
Posts: 4,179
California
mickm
OP
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Quote:
Use the test light method described. But i would not pull the fuse for the door light circuit that constantly on circuit is most likely the circuit with the problem. Is there an alarm or sound system amp in the car? Check the easy stuff first trunk light dome light, glove box light staying on?,
Scott
no, none of that in the car, but i haven't yet checked for the simple stuff. nothing is obvious anyway.
hmmm... i just remembered. it has been a while, but the cigarette lighter doesn't pop up on it's own when hot, and blows the fuse. i have used the lighter to power my LM-1, so i know it is good, but maybe as mentioned in another post there is some junk down in there.
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Re: chasing an electrical draw
[Re: mickm]
#907310
01/17/11 04:05 PM
01/17/11 04:05 PM
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Joined: Oct 2010
Posts: 606
Montana
Yancy Derringer
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Montana
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Quote:
my meter is fused. can you explain this, what do you mean "--EDIT DO NOT EDIT--put your amperage selected meter TO GROUND"? so i want to
1) check that the current i'm chasing is within the range of what my meter will handle,
2) make sure i don't introduce more current than that, and
3) the meter needs to be hooked up in series, the same way the bulb was, correct?
Right. Even fused meters CAN be easily destroyed. Some meters are fused for scales EXCEPT the "high amp" scales, still others are not fused at all in current scales.
So you have it right. I do not know what your knowledge/ experience is, just trying to be sure you don't blow up your meter. Even I have popped the rather expensive special fuse in my Fluke a "couple of times" here and there.
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Re: chasing an electrical draw
[Re: Andrewh]
#907312
01/17/11 04:12 PM
01/17/11 04:12 PM
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Joined: Jul 2006
Posts: 4,179
California
mickm
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Quote:
yes amps are what you are looking for not voltage.
Can't tell if you are trouble shooting an old car or new.
the 70 RR in my sig.
Quote:
Also there is an amperage shunt technique that allows you to put a resister inline with your meter so you don't blow it up and then multiply by 10 or something to get the actual amperage draw.
I have it in a motor magazine somewhere, but can't find it at the moment. Might be a good artical on line about it if you do a search.
for an old car a month before dead would be good. 2 weeks for an old car is bad.
for a new car, 2 weeks isn't so bad, but would be pushing it.
i have been disconnecting the battery, but would like to not have to do that every time. and would also like to not have to get up at 3am when it hits me and i can't remember if i disconnected it or not 
but even a month seems like a short amount of time. i hear plenty of people say their car sits for longer than that, and plenty of juice in the battery.
but yes, i do need to chase it down.
post that info from the article if you find it.
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Re: chasing an electrical draw
[Re: mickm]
#907313
01/17/11 04:23 PM
01/17/11 04:23 PM
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Joined: Jan 2003
Posts: 28,071
Irving, TX
feets
Senior Management
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Senior Management
Joined: Jan 2003
Posts: 28,071
Irving, TX
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Forget voltage. That's not what you want to look for. You need amperage.
When wiring trailer lights I've had 12 volts at the light but not enough current to blow a 3 amp fuse.
The test light will not glow unless there's sufficient amperage.
The battery can have a 12 volt draw for a month and be fine if the draw isn't pulling appreciable amperage.
The test light is also easier to mess with. Prop it up so you can see it from the driver's seat and start yanking fuses.
We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon
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Re: chasing an electrical draw
[Re: mickm]
#907314
01/17/11 04:28 PM
01/17/11 04:28 PM
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Joined: Aug 2005
Posts: 11,543
chicagoland,usa
buildanother
I Live Here
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I Live Here
Joined: Aug 2005
Posts: 11,543
chicagoland,usa
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The simple test light between batt post and batt term works great on old cars such as yours. Normally with doors shut, you can connect test light between post and cable terminal, the light should not light. Then if you open car door, the test light should light brightly. If you have a newer radio with memory, and a clock etc, it may light the light and then it should fade out, or almost out. That is not really a big enough draw to kill a good battery in only 2 months. 
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Re: chasing an electrical draw
[Re: Andrewh]
#907316
01/17/11 08:01 PM
01/17/11 08:01 PM
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Joined: Oct 2010
Posts: 606
Montana
Yancy Derringer
mopar
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mopar
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Montana
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Quote:
found it.
here is the jist.
wire a 1.0 ohm 10 watt resistor in series with the negitive cable.
so one end of the resistor on the battery, one end on the negitive cable.
Take a volt meter and measure VOLTAGE across the resistor.
According to the article, due to ohms law, the actual voltage measures will be equal to the amps being drawn through the resistor.
that way you don't risk your volt meter in amp mode.
I don't know what happens if the amps are higher than 12, since I didn't do the math. I don't know if it is possible to read a higher than 12 amp draw since you are limited by the battery's potential.
Ohm's basic law is I = E / R which means
AMPS = VOLTAGE divided by OHMS
So if the resistor (1 ohm) is put DIRECTLY across a 12V battery (as in a heavy short in the system, you get
AMPS = 12 / 1
So the 1 ohm resistor can draw a maximum current of 12 amps
However, WATTAGE is P = IE, which means
WATTS = AMPS X Volts
So your resistor is dissipating 12 amps X 12 volts or
144 WATTS!!! That little resistor won't last long if you were to have a heavy short in the harness
However, 10 watt resistors don't cost much, and are cheaper than damaging a meter!!
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Re: chasing an electrical draw
[Re: JonC]
#907318
01/18/11 12:13 AM
01/18/11 12:13 AM
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Joined: Jul 2006
Posts: 4,179
California
mickm
OP
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OP
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Quote:
Do you have an aftermarket radio that may draw current all the time?
stock am radio. and what a joy it is to listen to!
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Re: chasing an electrical draw
[Re: Andrewh]
#907319
01/18/11 12:16 AM
01/18/11 12:16 AM
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Joined: Jul 2006
Posts: 4,179
California
mickm
OP
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OP
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Posts: 4,179
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Quote:
found it.
here is the jist.
wire a 1.0 ohm 10 watt resistor in series with the negitive cable.
so one end of the resistor on the battery, one end on the negitive cable.
Take a volt meter and measure VOLTAGE across the resistor.
According to the article, due to ohms law, the actual voltage measures will be equal to the amps being drawn through the resistor.
that way you don't risk your volt meter in amp mode.
I don't know what happens if the amps are higher than 12, since I didn't do the math. I don't know if it is possible to read a higher than 12 amp draw since you are limited by the battery's potential.
nice, i'll do that if i need to. first thing is to look at the one circuit that dropped when i popped the fuse.
one question not answered yet though...
if there is no draw at all, will i still see voltage in the circuit? i had ~12v with the one fuse in, but only ~9v with the fuse out?
i know i need to be looking at amps, but i am curious about this.
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Re: chasing an electrical draw
[Re: mickm]
#907320
01/18/11 12:26 AM
01/18/11 12:26 AM
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Joined: Jun 2008
Posts: 1,822
Colorado
denfireguy
top fuel
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top fuel
Joined: Jun 2008
Posts: 1,822
Colorado
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Quote:
Quote:
found it.
here is the jist.
wire a 1.0 ohm 10 watt resistor in series with the negitive cable.
so one end of the resistor on the battery, one end on the negitive cable.
Take a volt meter and measure VOLTAGE across the resistor.
According to the article, due to ohms law, the actual voltage measures will be equal to the amps being drawn through the resistor.
that way you don't risk your volt meter in amp mode.
I don't know what happens if the amps are higher than 12, since I didn't do the math. I don't know if it is possible to read a higher than 12 amp draw since you are limited by the battery's potential.
nice, i'll do that if i need to. first thing is to look at the one circuit that dropped when i popped the fuse.
one question not answered yet though...
if there is no draw at all, will i still see voltage in the circuit? i had ~12v with the one fuse in, but only ~9v with the fuse out?
i know i need to be looking at amps, but i am curious about this.
Yes you will see voltage. Again, it is current flow that depletes a battery. Voltage is the potential to supply energy, current flow is the energy. It is like my kid, he has the potential to do work but until I actually see it, it is only potential and no work is done.
Craig
2014 Ram 1500 Laramie, 73 Cuda
Previous mopars: 62 Valiant, 65 Fury III, 68 Fury III, 72 Satellite, 74 Satellite, 89 Acclaim, 98 Caravan, 2003 Durango
Only previous Non-Mopar: Schwinn Tornado
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Re: chasing an electrical draw
[Re: denfireguy]
#907321
01/18/11 02:03 AM
01/18/11 02:03 AM
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Joined: Nov 2009
Posts: 2,177
ill
dennismopar73
top fuel
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top fuel
Joined: Nov 2009
Posts: 2,177
ill
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look at the alternator might have a diode out causing it to 'back up ' thru the system? , as on another post check you re radiator maybe it discharging hahahhaha l o l 
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