Question: I have an electric leak on a 2007 GMC Sierra. The battery and alternator tested fine, but the battery goes dead every other time I let it sit overnight. Do you have any ideas why?
Answer: The "electric leak" is referred to as a parasitic draw, meaning excessive current is being drawn from the battery with the key off, doors closed, lights out, etc. A certain amount of current draw is normal. A general rule of thumb is anything less than 50 milliamps is OK. Even for a top-notch technician, it can be very time-consuming to diagnose a parasitic draw. It involves measuring current flow with an ammeter while all systems are correctly put to sleep. That's when certain systems (such as the alarm) stay active for a designated period of time before they power down and reduce the current drawn from the battery. Sometimes a vehicle may have to sit dormant for more than an hour before it's officially asleep. Once a draw above 50 milliamps is confirmed, and obvious stuff has been eliminated, systems need to be disabled (remove fuses/disconnect connectors) one at a time to find the problem circuit. There's a specified trail of systems to disable in the service manual. You can check some basics, though: Look for any interior lights that remain on -- glovebox, visor mirrors, etc. If you have aftermarket accessories installed, disconnect any and all to see if the problem stops. Then make an appointment with a GMC service department. Looks like there may be more than one software update related to the radio causing an intermittent excessive parasitic draw. If it does not apply, pay the diagnostic fee and have a technician find the problem It'll be worth it in this case.
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If you have a technical question regarding your pickup, SUV, or van, feel free to contact Alex, a master technician with the National Institute for Automotive Service Excellence. Send a letter to him in care of Truck Trend Garage, 831 S. Douglas Street, El Segundo, CA 90245, or e-mail us at firstname.lastname@example.org. Please include the VIN with your question. Due to the volume of questions received every month, we cannot guarantee that everyone's question will be personally answered or will appear in the magazine.
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