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After visiting a couple shops, I learned that one of the biggest problems that Technicians see out in the field is Parasitic Drains. A customer comes in saying that they have to jump start their car after it sits for a couple of days. It will start and work fine during the workweek, but they let the car sit in the garage for the weekend and on Monday morning…. Low and behold, it won’t start. The likely cause, a Parasitic Draw. A Parasitic Draw occurs when there is an abnormal or continuous draw of power when the vehicle is off. This could be caused by a circuit or component left on or energized, like a sun visor or dome lamp, or a module staying “awake”. With today’s vehicles using more electronics and modules than ever before, more and more systems are intertwined through communication lines. This could mean that one system or module on a vehicle could cause an issue somewhere totally unrelated.
One of the many tests you can perform with our DMM is a Parasitic Draw Test. This will let you know whether you have an excessive draw in the vehicle. Before starting a Parasitic Draw Test, make sure that you have a fully charged battery. Remove the Key from the ignition, or make sure the keyless remote is out of the vehicle. Unplug any devices from the accessory outlets such as phone/tablet chargers, and make sure all doors are closed or latched.
To perform the test, disconnect the negative battery cable from the battery. You will need to connect your DMM in Series between the negative battery post and the negative battery cable. To do that, make sure that the black lead is in the “COM” Terminal of the DMM and the red lead is in the “mA” Terminal (Right Side) or the “A” Terminal (Left Side).
Connect a lead (red or black) to the negative battery post and the other lead to the negative battery cable (might need to you alligator clips). Next, set the DMM to mA. You should get a reading on the display. Let the vehicle go to “sleep” for about 30 minutes to 1 hour. A normal amp draw range would be 50 milliamps (mA) or lower, but can vary with different manufacturers.
In this case, we show .007 Amps (7 milliamps), which is good. There will be some draw occurring due to the memory settings of a vehicle (i.e. Clock, Radio Settings). If there is more than 50 milliamps (mA) showing on the display, most likely there is an excessive Parasitic Drain. Once you have verified that the vehicle has an excessive draw, the fun begins. It’s now time to locate the source of the drain… Stay tuned for PART 2 of DMM Usage: Parasitic Draw.