Welcome! Log In Create A New Profile Recent Messages

Advanced

Checking for dead (draw) shorts

Posted by rkj 
rkj
June 08, 2014 10:52PM
I'm no electrical genius but I think I'm ready for more of a refined way to test for draw shorts. I've always put a test light between the battery post and the cable and noticed how bright the light was to gauge how bad the draw is and move to pull fuses to run it down. I'd like refine this a little with a mutimeter.

Can anybody help me out here :eyes:

Thanks, Rick
June 09, 2014 01:17AM
You could install a multimeter between your battery power and the car (either the plus or the minus side), and set it to amps.
Then install one fuse after the other to read how much it draws. If you find a short, it can possible kill the fuse in your
multimeter.

If you don't want to risk the fuse, disconnect the battery, and measure each circuit (by installing fuses) with the
multimeter set to ohm. The draw on each circuit is 14 divided by your measurement.
rkj
June 09, 2014 11:39AM
Quote
Ove Kvam
You could install a multimeter between your battery power and the car (either the plus or the minus side), and set it to amps.
Then install one fuse after the other to read how much it draws. If you find a short, it can possible kill the fuse in your
multimeter.

If you don't want to risk the fuse, disconnect the battery, and measure each circuit (by installing fuses) with the
multimeter set to ohm. The draw on each circuit is 14 divided by your measurement.

Interesting Ove. Thank you.

On most cars I see today there is a digital draw normally, some have some sort of powered up device even when the car is off, and completely shut down. It would be nice if I had a way to measure this value, and maybe your amp reading will do just that. I'm off to a job right now and will try it.

Cheers, Rick :wavey:
rkj
June 09, 2014 07:43PM
So, this was an interesting day. With a fully charged battery (in a mint condition 1996 E320 cabrio Benz 30k) I started checking for why a brand new battery went dead in a few days... When I did my usual test light draw test I couldn't really tell any difference in the light intensity between the straight battery connection + to - and the cable and battery post. Both bright.

I then tried the DC-amp scale on 2M and got .039 between the battery post and the cable end. I switched to 20m scale and got 0.56

I don't know what these figures represent. Anybody?

I then put the scale back to DC 20 volt and measured between the post and cable and got 12.11- I don't really know if that has any real value either confused smiley but it might be it's my dead short, here again just guessing...

I then put the car through all my usual tests for charging and everything proved out well.

Thanks, Rick
June 09, 2014 10:24PM
I don't think I have seen a multimeter without autorange the last couple of decades, so I am not so familiar with your manual settings. You say you measure DC current, and have scales of 2M and 20M? As in MegaAmperes??? I highly doubt that a multimeter can take that much current, so I don't quite understand. And the readings seem to change a lot (39 and 56) between the ranges as well, which is not normal.

The voltage reading between the battery post and the circuit would only be valueable if you also measured between the battery poles, so you can see the difference. Then it would perhaps be possible to compute something.

If we assume a battery capacity of 100Ah, it would take a draw of something like 2A to empty it in 48 hours.
June 09, 2014 11:03PM
The scaling on the multimeter should be in Amps or Milliamperes. The scale change from two to 20 would also likely change the resistance of the circuit a little so if one had a current draw of a few milliamps, the resistance of the meter itself could change the current draw and the calibration accuracy. A current draw of 30 to 50 milliamps would not be a surprise. a half amp would be unusual. The car should be able to be parked for at least 3 weeks and still start.

There are some electronic switches and fan controls that can malfunction and draw the battery down overnight. See if you can tell if the AC fan is running at a low speed when the car is off. I have seen one fan control fail on an M5 E39 and when the mechanic got it out of the car it was too hot to touch.

Bob in Everett
June 11, 2014 06:09AM
Basic multimeters have a manual knob or keys to choose the range and units to measure, as well as several connections depending on what you want to do.
Basic electricity now: to measure voltage, connect in parallel to where you want to measure (example: the posts of the battery should read 12V give or take, with engine on, a bit above that).
Current is measured in series with the circuit, a way to do it is to remove the fuse and connect the multimeter set to the range of the current to be expected (or begin with the grater range and switch down). Make sure you have a good fuse in the multimeter, or use another fuse in line with the wire.
Ohm is the value of the Resistance and depend on many conditions, so not usefull here.

There are circuits always on stand-by, like the radio, keyless central locks, alarms, the clock, and so on.
Make sure all interior lights are off (glove compartment, trunk, etc), a small light bulb can drain a lot of energy over a long time.
In my Citroen BX, the cheap Chinese alarm and keyless door lock will draw the small battery in a couple weeks, if I don't disconnect the battery when the car will sit unused.
Old cars can sit without battery connected with no problem.
For the circuits that must be on stand by, up to 50mA can be accepted, hundreds of mA is too much, like Bob said.
For example, a 70Ah battery can give 70mA over 1000 hours, less than 1 1/2 month.

Good luck!



Edited 1 time(s). Last edit at 06/11/2014 06:11AM by Jose Pinto.
rkj
June 18, 2014 10:33PM
Quote
Jose Pinto
Basic multimeters have a manual knob or keys to choose the range and units to measure, as well as several connections depending on what you want to do.
Basic electricity now: to measure voltage, connect in parallel to where you want to measure (example: the posts of the battery should read 12V give or take, with engine on, a bit above that).
Current is measured in series with the circuit, a way to do it is to remove the fuse and connect the multimeter set to the range of the current to be expected (or begin with the grater range and switch down). Make sure you have a good fuse in the multimeter, or use another fuse in line with the wire.
Ohm is the value of the Resistance and depend on many conditions, so not usefull here.

There are circuits always on stand-by, like the radio, keyless central locks, alarms, the clock, and so on.
Make sure all interior lights are off (glove compartment, trunk, etc), a small light bulb can drain a lot of energy over a long time.
In my Citroen BX, the cheap Chinese alarm and keyless door lock will draw the small battery in a couple weeks, if I don't disconnect the battery when the car will sit unused.
Old cars can sit without battery connected with no problem.
For the circuits that must be on stand by, up to 50mA can be accepted, hundreds of mA is too much, like Bob said.
For example, a 70Ah battery can give 70mA over 1000 hours, less than 1 1/2 month.

Good luck!

Yes, I have a meter with a manual scale knob and three outlets for the leads (and a few other test sites ). The car does measure high 12s standing still and high 13s-14s running with everything running; ac wipers ect. I think you're saying to put the meter leads on both sides of the fuse that's causing the short (is that right?) and then get a reading in mill amps (?). Sorry I'm so short knowedge here...

I didn't see anything on. That's the first thing I check outside of pulling the cig lighter. I did hear a loud clicking when I connected the battery terminal up to the battery though, it sounded just like a relay hammering open or shut on the firewall somewhere.

Regardless, the test lamp keep bright when I put it between the battery and the cable so I sent the car down the street to be looked at by a shop that is familar with the car and has the wiring sheets. Now, the story gets wierd...

I called today and they told me there was a fuse burned out and that was causing the interior lights to not shut off.

Now, the thing is. I don't think I missed the interior lights not going off, I've been dealing with this car for many weeks now after it came back from the body shop (had a door replaced from a minor hit). The car has 30k and always is inside and it has never given any signs of any kind of electrical funny bussiness. I'll know more tomorrow when I pick the car up and after this weekends testing.

I think what you're trying to tell me (Jose) is I can put the meter on both sides of the fuse to see what the line is drawing but I can't do that between the battery and cable junction where I use my test light. Please correct me if I have that mixed up.

Cheers, and thanks..... Rick
June 19, 2014 12:52AM
When measuring current (Amperes), you need to open the circuit and measure in line, like you do at the battery.
While measuring how much current that runs through the battery, try to remove one fuse after the other, and putting it back.
Check how much the current drops. This will tell you how much current that goes through that particular fuse.

If you want to measure over a fuse with multimeter terminals on both sides, you need to set the multimeter to voltage.
This type of measurement will not really tell you much, unless you are searching for dead fuses.
June 19, 2014 06:11AM
Quote
Ove Kvam
When measuring current (Amperes), you need to open the circuit and measure in line, like you do at the battery.
While measuring how much current that runs through the battery, try to remove one fuse after the other, and putting it back.
Check how much the current drops. This will tell you how much current that goes through that particular fuse.

If you want to measure over a fuse with multimeter terminals on both sides, you need to set the multimeter to voltage.
This type of measurement will not really tell you much, unless you are searching for dead fuses.

Yeah, as Ove says.
You can pull out each fuse and conect the multimeter set to Amps, to check how much current flows to each circuit. If the multimeter is set to volts, it will read about 12V.
Or connect the multimeter in line with the battery, set to Ampere so the current flows through the meter, and pull each fuse at a time, so you can notice which circuit is consuming how much current.

About that door and interior light thing, there are switches at the door pillars that turn interior lights on and off. usually those connect to the ground when the door is open. You may go search the door switch and check if the wires are connected right, It may happen they are loose or making contact with the door shut and the interior light can drain a battery in hours. Anyway, a blown fuse will not cause interior lights to be on all time, there must be something else going on...Check the glove compartment and trunk light, those can go unnoticed...
The cigar lighter usually disconnects when the key is off, but the radio and other stuff may be permanently connected.

Good luck!



Edited 1 time(s). Last edit at 06/19/2014 06:15AM by Jose Pinto.
June 19, 2014 03:02PM
A fuse is a kind of resistance. Any current running through a resistance will cause a voltage drop. Put your multimeter in its most sensitive DC V setting. With open terminals, you'll see a fluctuating value. Connect both terminals directly, you should get a solid 0.000 V. Now, when searching a current draw, let the vehicle rest (modern cars need a couple of minutes, an E30 just a matter of seconds after the key is removed), make sure interior lights are off (in case you're opening doors to access the fuses). Now measure over your fuses, a 0.000V reading means no voltage drop, so no current draw. A fluctuating reading means an open circuit (aka broken fuse), any other reading means that there's a current running through this fuse.
This method is quicker than Amp measurements (no fuse removal) and much safer, as current is measured in a circuit, risking the multimeter (best case: its fuse) having to eat to much current, aka frying it.

Good luck!
June 19, 2014 03:21PM
Quote
Michiel 318iS
A fuse is a kind of resistance. Any current running through a resistance will cause a voltage drop. Put your multimeter in its most sensitive DC V setting. With open terminals, you'll see a fluctuating value. Connect both terminals directly, you should get a solid 0.000 V. Now, when searching a current draw, let the vehicle rest (modern cars need a couple of minutes, an E30 just a matter of seconds after the key is removed), make sure interior lights are off (in case you're opening doors to access the fuses). Now measure over your fuses, a 0.000V reading means no voltage drop, so no current draw. A fluctuating reading means an open circuit (aka broken fuse), any other reading means that there's a current running through this fuse.
This method is quicker than Amp measurements (no fuse removal) and much safer, as current is measured in a circuit, risking the multimeter (best case: its fuse) having to eat to much current, aka frying it.

Good luck!

If you come across a broken fuse, you can also risk a reading of 13 volts. Some of the cirucuits are always live, even with the ignition off. smiling smiley
June 19, 2014 03:36PM
True, thanks for the completion. Bottom line is, any low and stable reading means there's a current running through it.
Sorry, only registered users may post in this forum.

Click here to login

Online Users

Guests: 14
Record Number of Users: 3 on September 29, 2015
Record Number of Guests: 116 on November 11, 2017