How do you find a short in a autos electrical system?

Apr.25, 2009 in Autos

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I own a 1988 Mazda B2200 pickup. If it sits for two days or more the battery becomes dead with no juice to start it. I have installed a new battery and checked the alternator and it charges at 13.90 volts. I take the battery connections off and on and there is no spark showing a short. How do I find the short in the electrical system. would appreciate all the help I can get. Thanks in advance Doug

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5 Comments on “How do you find a short in a autos electrical system?”

bigfatandprego
April 26th, 2009 at 5:22 pm
take it to shucks they do a free test
oklatom
April 29th, 2009 at 12:09 am
Leave the negative terminal on, take off the positive. Put a test light between the battery and the terminal. Does it light? Good. Make sure everything is turned off. Find the fuse box. There are probably two of them, one under the hood with master fuses and one inside the cab. Start pulling fuses until the light goes out. When it does, you have found the circuit that is drawing the battery flat. Look on the cover for what it is…radio? Turn signals? Start tracing that circuit until you find the problem. Don’t forget to look and make sure a light isn’t staying on in the glove box or trunk…
carvinmark
April 29th, 2009 at 4:00 am
Oklatom is 100% right!!!!
John Paul
April 29th, 2009 at 9:41 pm
Doug this how I do it. First I Measure the drain Got a multimeter? or use a light bulb with wires soldered on between the neg.post on the battery and the battery clamp you took off brake light bulb is one amp side marker bulb is 1/2 amp. Lamp loop on to the battery bulb will light up. First place to check is a bad diode in the alternator remove the big wire at the alternator. Light went out there it is! Not ! put that wire back on and start removing the under hood fuses one at a time watching the lamp. then go to the under dash fuses. watch for the light to go out almost completely. Any extra amps radar detectors? unplug them too. when you find the circuit then you have to find what all it powers. Multimeter set to miliamps hooked up just like the bulb reading .025 amp or 25 miliamps is normal drain
br549
April 30th, 2009 at 7:10 am
Some good tips above, but first before anything else, wait for a moonless night. Go outside and let your eyes adjust to the darkness (about 5 minutes or so) then look all over the car for a light shining. It could be a panel light, glovebox, under dash light or such as that.

I have found stoplight switches shorted that would activate the brake lights normally, but would also dimly glow all the time. You could see it on a dark night!