Theory Of Operation

What is current draw? 

Current draw, or parasitic draw is an electrical load that draws current from the battery when the ignition is turned off. Some are considered normal, some above normal. Normal draw comes from various electronic devices connected to battery positive at all times. Many vehicle modules draw a few mA (milliamps) as a means to "Keep Alive Memory". Whether it's the clock in the radio or the last known position of the memory mirrors, these tiny amounts of current typically will only add up to 20 or 30 mA. The vehicle can sit parked for days, even a few weeks without any problems of excessive battery drain that might prevent starting. As long as the vehicle is driven periodically in order for the alternator to recharge the battery there is no problem. A problem may occur, in situations such as new vehicles in dealer stock or long term airport parking situations.

Below are some examples of modules and components that require keep alive memory:

• Powertrain Control Module (PCM)
• Body Control Module (BCM)
• Memory Seat Module (MSM)
• Radio

What is excessive current draw? 

Excessive current draw can be determined by an ammeter reading in excess of 50 mA. A normal vehicle electrical system may draw approximately five to fifty milliamperes (0.005 to 0.050 ampere) depending on vehicle configuration.

How to locate the problem? 

Locating the problem involves a process of elimination. If the problem is not obvious, like the trunk light staying on, you will have to start troubleshooting:

• Start by removing one fuse at a time until you see the reading on the meter drop off (be careful to reinstall the fuses in their proper location).
• Once you've determined the high-draw circuit, there still may be a half-dozen loads, each individually drawing current from the battery.
• To zero in on that circuit or circuits, use the schematic diagram and disconnect each device on the circuit one-at-a-time and check the meter.
• When the milliamps reading drops off significantly, you've found the problem.

Perform the following test to determine where the cause of the draw is located.