Finding a short circuit

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93dsm

Member - Posts: 28

Joined: February 19, 2006

Location: United States

Posted: March 29, 2006 at 10:02 AM / IP Logged

Hi. Everytime I go to start my car, the battery is almost dead. Im pretty sure I have a short somewhere, and i know its not my dvd/radio/sound system because I have all that on a switch..and i cant accidently leave it on, because i have it set up so my alarm wont turn on if the switch is left on. I was wondering how you check a circuit for a short. I thought it was one lead of a dmm to the negative post of the battery, and the positive end to the circuit your testing (when its disconencted from the batter). And if theres cotinuity theres a short....is this correct?

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datspeed77

Member - Posts: 35

Joined: September 14, 2004

Location: United States

Posted: March 29, 2006 at 2:03 PM / IP Logged

You're actually not testing for a short circuit. You should be checking for a current draw while your car is off. Use an ammeter to find out if you have a current draw while the car is completely off. Also, I've had this happen before, I had a JVC radio with a fried diode. The car would draw current that way. The radio didn't stay on with the car off, but was just enough current to drain my battery. Hope that helps.

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Hornshockey

Silver - Posts: 520

Joined: January 31, 2005

Location: Texas, United States

Posted: March 29, 2006 at 2:27 PM / IP Logged

you'll need a multimeter with a DC amperage setting. Most have 10A and 300mA, use the 10A setting to start with.

Disconnect the neg battery cable and connect one probe from your meter to the post on the battery and one to the cable. There will probably be a little spark, if you do have a draw. The meter should read about .01-.02A draw when the car is off(be sure you close the doors or disable the domelight).

If the draw is higher than this it's time to start tracking it down. Start by disconnecting the main fuse on the power wire for your amplifiers(if applicable). Move on to the fuses for your alarm and stereo. Then move on to the vehicle's fuse box. Pull one fuse at a time and note any change in the amperage reading. When you pull a fuse and the current draw drops to an acceptable level, then you've found the component causing the draw. Then it's a matter of having that component repaired or replaced.

There's a page somewhere I remember reading that had this procedure on it, but I can't find it now.

Life moves pretty fast; if you don't stop and look around once in a while; you could miss it.

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Finding a short circuit

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