Electrical short draining battery/any advice?

Adidas · 08-06-2003, 08:22 AM

I have a 75 911 with a reocurring problem of an electrical short somewhere draining the battery. The latest new battery lasted 2 weeks before being drained. Any simple method for finding or tracing down electrical shorts causing this problem. What method do you use. Thanks Bill

echrisconnor · echrisconnor's Garage

There are a lot of ways of approaching this. You'll hear a bunch of them.

What I do is pull the ground off the battery and connect a volt meter between the (-) on the battery and the ground. I look for any sign of voltage reading on the meter. This means that something along the line is completing circuit and drawing power. Then I go through the fuse panel one at a time until I see the voltage drop to 0. Then I know I've found the circuit with the draw. Then chase down that circuit looking for where things are grounding out.

Your clock may give you a false reading, so pull that fuse first. It will usually appear as a pulsing bounce on your meter. Also, check things like your trunk light and interior light to make sure they aren't on when they're not supposed to.

c

Wil Ferch · 08-06-2003, 08:38 AM

Look at the two main leads off your battery. Remove the negative lead and connect a volt/amp meter ( or simply a small light) to the battery and to ground, and measure standby amp draw. One by one....remove the fuses and replace them. You'll then find that the amp draw goes down when you remove the ( one) fuse that has the problem in its circuit. Now...you trace the individual components in *that* circuit until you find the problem.

---Wil Ferch

targa80 · targa80's Garage

You can also connect an Amp meter in series with the battery + terminal and the battery + main cable. disconnecting any circuits that require a minimal current draw all the time clock, radio memory circuit. Any current draw after that will lead to your problem.

Early_S_Man · 08-06-2003, 09:10 AM

Bill,

You will need to use a DMM set to DC current or Amps ... on a 2 Amp or 200 mA range to use Wil's method. Two weeks to run a battery down undoubtably means a drain of less than 1.0 Amp!

Could mean radio, alarm system, or just one single lamp ... such as the trunk or glove box lamp!

Personally, I use an HP 428B clip-on DC Milliammeter, or a Fluke Y8100 DC/AC current probe connected to a DMM ... but, I realize those are pretty specialized. A DMM inline with the main battery cables will work just fine on this problem!

The work will be tedious, but it will find the problem!