How to locate a short in electrical system?

[Black_Blade]

My 02 has been sidelined for last week because something is killing the battery. Put my meter across the battery cables and sure enough, there is a direct short somewhere, now the big question of where to even begin trying to find out where this short is occurring! There must be a thousand miles of wire in it!

Also, a weird thing happened when I put my battery charger onto the battery to get it started a few days ago, before I had checked for the short, for I thought in the cold weather the battery just died on me. When I plugged the battery charger into the outlet to start the charging, one headlight flickers on, I never seen anything like that before, light switch was off keys not even in ignition...thats what made me think that there was something funky going on with the wiring, but several hours later the battery was charged enough to start the van. (the one headlight was no longer flickering, so at some point in the charging process it stopped doing that)

[aleekat]

Hook your meter back up and start pulling fuses. Once it drops, you have narrowed it down to a circuit that runs off that fuse.

[12Ounce]

.... also pull relays, that's what they are for ... sectionalizing the electrics

[Black_Blade]

Thanks guys, will start with that and see how it goes...just what you wanna be doing in the middle of January...lol.

[Black_Blade]

Well, as a follow up on my problem, I went through fuses and checking with ohmmeter. Finally got to the 25A (rear wiper motor, rear washer pump, rear wiper relay) in the box beside the battery to locate the one that had the issue.

I removed the fuse and put the battery back in, fired her up and let it run for a good spell to warm it all up. All seemed fine after a bit, so took daughter to work and then grabbed some take out to bring back home. The van had been running for a good 3/4 of an hour by now with no sign of trouble, so I turned it off to get the take-out food. Came back out and it barely turned over, but then caught and ran.
Now though, it was so low on power that all the lights in the cluster were flickering and fading out, it got worse if I accelerated. Limped it back home and checked the voltage across the battery with it running.

With all accesories off: 13.3 volts

lights and heater fan going: 11.5 volts (I did notice at stop lights before that the fan speed would reduce and then increase once stepped on gas when light went green)

I just replaced the alternator 2 yrs ago, but sure looks like its pooched again I guess and prolly ruined the battery as well for its been undercharging for quite some time now that I think about it. The fluctuation in heater fan speed has been going on since cold season started. Couldn't afford to fix alternator at the time...

[Black_Blade]

I have installed new alternator and battery. Also replaced a battery post clamp (neg), will be keeping an eye on it for corrosion build up. With engine running have 14.3 volts across the battery.

The resistance across battery cables, disconnected from battery is 5K, would anyone be able to verify if this is OK, or should there be even higher resistance?

[12Ounce]

That alternator voltage is much better ... that's for sure! Not sure about the open circuit resistance ... would think that is a bit low. Does your model have the special cabin fuse (#17 IIRC) that is pulled for long-term parking?

[aleekat]

Because battery cables are a low resistance circuit, checking for ohms is not reliable. You need to do a voltage drop test. Here's two links that may help:

http://www.aa1car.com/library/voltage_drop_testing.htm

http://info.rockauto.com/CarCareforC...Ch6SmplPgs.pdf

[Black_Blade]

Originally Posted by aleekat

Because battery cables are a low resistance circuit, checking for ohms is not reliable. You need to do a voltage drop test. Here's two links that may help:

http://www.aa1car.com/library/voltage_drop_testing.htm

http://info.rockauto.com/CarCareforC...Ch6SmplPgs.pdf

Will look into doing this, having some trouble locating a 1ohm 10 W resistor though. The Source has next to nothing anymore in the way of hobby electronics parts etc, don't know why they bother with the few pieces they still have...

[wiswind]

Very roughly......that 5K ohms on the 12V battery would be a draw of 2.4mA......or 0.0024 amps
Not a big draw.
Not something that I see killing a battery overnight.