Finding electrical shorts

Tiny
KEN53
  • MEMBER
  • 2003 FORD F-150
  • V8
  • 2WD
  • AUTOMATIC
  • 17,000 MILES
I have a short that is draining my battery how do I find it
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Tuesday, July 26th, 2011 AT 10:12 AM

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Tiny
CARADIODOC
  • EXPERT
It can be a little tricky with all the computers on the truck. Some of them can draw a high current for up to 20 minutes until they go to "sleep" mode. Anything you do to momentarily break the circuit starts that 20 minute wait period all over. That includes switching the range on your ammeter. If you start out on a higher scale you won't get the necessary accuracy. If you start out on a lower scale you can pop the meter's internal fuse. On top of that, some computers can lock up after the battery is reconnected but that doesn't seem to be a big Ford problem and that wasn't real common yet in 2003.

The secret is to disconnect one battery cable to insert your ammeter AND connect a jumper wire across the meter to in effect short it out. Some computers might "wake up" so let the meter and jumper sit there for at least 20 minutes, then remove the jumper wire. All current will go through the ammeter so it can be measured. If you need to switch to a lower scale, put the jumper wire back in place first, switch the meter, then remove the jumper again.

Unless the manufacturer specifies differently, the industry-accepted maximum is 35 milliamps to maintain the memory circuits in the radio and computers. At that rate a good battery will still start the engine after sitting for three weeks. Slightly more than that won't kill the battery overnight but you don't want to see 200 milliamps or more.

Typically when there is a drain, it's really a drain. You won't have to guess. You'll likely find an amp or more, especially if the battery goes dead overnight. Remove one fuse at a time to see if one makes the current drop to an acceptable level. That fuse's function will give you an idea of which component to look at. Don't forget that if you open the doors, the interior lights will turn on. You will also want to disconnect an under-hood light.

Before you go through all this work, it would make sense to be sure the generator is recharging the battery after you start the engine. Use a voltmeter to measure battery voltage. A fully charged good battery will measure close to 12.6 volts. If you charge it with a battery charger, then let it sit overnight and it holds a charge, suspect the generator, not a drain. If you find closer to 12.0 volts, the battery is good but discharged.

Remeasure the battery voltage when the engine is running. It must be between 13.75 and 14.75 volts. If it is low, again, suspect the generator.
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Tuesday, July 26th, 2011 AT 10:42 AM

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