12V shorted to ground

[NateDN10]

The situation - friends at our house, 2006 Chevy Cobalt. They start it up to leave, we're talking for a couple more minutes while it runs. They notice the gas gauge is at empty (tank was half full) - they've been having "weird electrical issues" - and smelled a burning smell. Turned the car off and it wouldn't start again.

I checked the battery with a voltmeter, it read 0.8V. A couple minutes later, 0.5V. Disconnected the battery (+ and gnd) and it went up to 3V. Checked resistance from the positive cable and negative cable - 0 ohms. Battery was very hot.

I've tried looking around in the engine bay for an obvious sign of a short, and in the fuse box. The fuses all look OK and I didn't see any obvious melted wires. Most the wires and wire bundles are sheathed in black plastic.

So before they resort to having it towed, any places to look for a short, or any other suggestions?

Thanks.

 

[Kool1]

I've actually seen a car with the battery connected backwards. The positive and negative posts were reversed. The car ran for a long time like that. The guy brought the car in to have a tachometer installed and when I connected the leads to the coil, a super huge spark from connecting wires to the coil told me there is a major short. I looked at the battery and saw the cables reversed. I asked the customer how long the cables were connected to the battery like that and he told me it probably happened a few months ago when he replaced the battery at Autozone.

 

[gathermewool]

Any electrical mods, stereo, lighting, etc?

 

[hansj3]

Charge the battery up and connect a dmm set to DC amps , positive to the positive post and negative to the positive terminal. Start withh the terminal on the post and the dmm hooked upthen lift off the terminal being sure not to break the dmm connection. After a few minutes read the amprage. It should read less than 50MA. If it reads less then 150 ma give it some more time to settle down. It can take up to 30 min.

If after a while it still reads high open the fusebox and pull the fuses and replace them one by one until you find one that cuts amps. Then research the fuses on the circuit.


Also don't rule out a short in the battery

 

[Rick in PA]

My opinion, you've got a major short somewhere. Be careful. A load that can drag a car battery down to under a volt is pretty major. If it was on a fused line, the fuse would have blown long ago. A car battery can supply as much current as a heavy fuse can carry for hours. Something major is wrong. Be careful if you reconnect the battery, sparks could cause the battery to explode by igniting hydrogen in the cells.

I'd look along the main cable to the starter and the starter itself.

 

[eljefino]

Yup, could also be the alternator with a shorted diode. A FLIR camera would be handy. It'd be hot over there, too.

I had a contour, which, among its other electrical disasters, had a sticky relay that continuously powered the ignition coils. Luckily (?) turning the key off also turned off the fuel pump.

I was walking by it and heard a random buzzing, putting my hand on various doodads narrowed it down.

 

[bubbatime]

Yeah this type of short is very dangerous. This is a dealer only repair in my opinion. Let them have the liability of fixing it.

 

[Dave Sherman]

Originally Posted By: hansj3
Charge the battery up and connect a dmm set to DC amps , positive to the positive post and negative to the positive terminal. Start withh the terminal on the post and the dmm hooked upthen lift off the terminal being sure not to break the dmm connection. After a few minutes read the amprage. It should read less than 50MA. If it reads less then 150 ma give it some more time to settle down. It can take up to 30 min.

If after a while it still reads high open the fusebox and pull the fuses and replace them one by one until you find one that cuts amps. Then research the fuses on the circuit.


Also don't rule out a short in the battery


That would not be a very good idea, if the battery is getting hot and going close to zero volts in a short time, this is tens or hundreds of amps. WAY beyond what your average DMM will measure. Since the high current range on many average DMMs isn't fused that would be a good way to fry your DMM, but would certainly blow any fuse if it is.

OP: While it is possible the battery has an internal short and killed itself, your resistance measurement sounds like there's a low resistance path to ground. Bad diode in the alternator comes to mind, although the alternator would probably be smoking if the battery was getting hot too. Might disconnect the alternator (DON'T start the engine with it disconnected!) and check again. Might look down by the starter solenoid and make sure the heavy lead to it isn't touching something.

 

[CT8]

Alternator? Bad battery?

 

[andyd]

Check the battery out and remove it, Clean the top of the battery and shine up the posts. Get it charged. Install it and try to start the engine. Look for smoke. Probably between the battery and the starter. I spent last Sunday afternoon fixing the harness that had fried the 12 awg wire going to the solenoid. I had replaced the starter and then damaged the 21 yr old Ford wiring and created a dead short. I had all the right stuff to repair it. Even some way better loom.

 

[clinebarger]

The Battery is in the trunk, Check the battery cable.

I would try a different battery with a 15amp fused jumper between the negative battery post & negative battery cable (with positive battery cable hooked up) If the 15amp fuse holds with the key-off, Try key-on, If it still holds....A shorted battery is a possibility.

 

[HerrStig]

Originally Posted By: Kool1
I've actually seen a car with the battery connected backwards. The positive and negative posts were reversed. The car ran for a long time like that. The guy brought the car in to have a tachometer installed and when I connected the leads to the coil, a super huge spark from connecting wires to the coil told me there is a major short. I looked at the battery and saw the cables reversed. I asked the customer how long the cables were connected to the battery like that and he told me it probably happened a few months ago when he replaced the battery at Autozone.
I call [censored]. That would make the starter run BACKWARDS and the Bendix would not engage. It would also blow up every solid state device in the car.

 

[Kruse]

Originally Posted By: HerrStig
Originally Posted By: Kool1
I've actually seen a car with the battery connected backwards. The positive and negative posts were reversed. The car ran for a long time like that. The guy brought the car in to have a tachometer installed and when I connected the leads to the coil, a super huge spark from connecting wires to the coil told me there is a major short. I looked at the battery and saw the cables reversed. I asked the customer how long the cables were connected to the battery like that and he told me it probably happened a few months ago when he replaced the battery at Autozone.
I call [censored]. That would make the starter run BACKWARDS and the Bendix would not engage. It would also blow up every solid state device in the car.


On most vehicles, the starter will turn the same direction despite it being negative ground or positive ground. A lot of older vehicles can be switched from positive ground (as they were wired from the factory) and put to negative ground and the starter will work either way. Your older starters will turn the same direction and it doesn't matter of the vehicle is negative ground or positive ground.
Now as far as the battery being hooked up wrong and it continues to work, it IS possible, and here's how:
You can take a battery and completely discharge it and then charge it back is the opposite polarity. So if the battery is completely dead, hook the (+) lead of the charger to the negative post of the battery and take the (-) lead of the charger and hook it to the positive post of the battery. The battery can charge up in the reverse polarity. I'm not saying it's exactly what did happen here, but it can happen.

 

[gathermewool]

To those who suggest checking the battery and hooking it back up: ARE YOU FREAKING KIDDING ME!?

The battery is toast. Install a new battery only after you've solved you problem.

Someone mentioned that the battery is in the trunk. If this is true, then I second that a very likely short is loss of insulation somewhere between the alternator and the battery.

 

[KrisZ]

Originally Posted By: gathermewool
To those who suggest checking the battery and hooking it back up: ARE YOU FREAKING KIDDING ME!?


+1, shows lack of understanding of the electrical systems.

There is no other way around it. If there is continuity between the positive and negative cables, you simply have to start with one cable, be it positive or negative and work your way down the line, checking for cable damage. Pay special attention to areas where the cables are touching metal parts, as that's where they usually rub through the insulation.
If the battery is in the trunk, I would check the hole that takes the cables outside of the cabin. Perhaps the grommet was damaged or installed improperly.

I recently went through something similar on my motorcycle and it can be frustrating.

 

[NateDN10]

I have a co-worker with a FLIR camera so we tried that, but the battery was too dead at that point to heat anything up.

The battery is in the trunk, and a cable runs to the fuse box. At that terminal another cable runs to the starter and alternator. When the cable to the starter and alternator was disconnected, the short at the battery terminals was gone, and with jumper cables to the fuse box, I was able to raise the windows and turn on the stereo.

The cable to the starter and alternator still reads 0 ohms to ground.

I volunteered to rip into it this weekend to figure out which it was an replace it, but they decided to have it towed and get the work done at a shop.

Thanks for the help all!

 

[asand1]

In case someone does not know, a starter has a one way drive called a Sprag or Bendix. A starter turning backward WILL NOT crank an engine over. And a DC permanent magnet motor will not turn the correct direction with polarity reversed, but a series wound motor would.

 

[mehullica]

I saw a mermaid riding a unicorn in this world of reverse polarity