Troubleshooting Parasitic Electrical Drains
Albin Moore/Motor Age — How many times has a vehicle come to your bay with a new battery, new starter, new generator — and the vehicle owner complaining of the battery going dead if the vehicle is not driven every day or so? From time to time I read about problems like this, and see them in my shop quite often. Most times the vehicles have had many new parts installed, wiring harnesses cut apart and modules unhooked, all in an attempt to find where those elusive electrons are going.
The key to the diagnostic process is to get the problem to come to you instead of you chasing the problem. I would imagine that most of you folks own a cell phone and have lost it a time or two. When it comes time to find it, what path do you take? Do you start looking for it, turning the room upside down and searching, or do you grab another phone, call your cell phone number and then follow the sound of the ringing phone to its hiding place? So it goes with any diagnostic process; learn how the system works, do the appropriate testing, and use the testing results to find where and what the problem is.
The Challenge Has Changed.
Parasitic draw problems have changed a lot over the years. Back before electronic control modules were used on vehicles, there was nothing to draw current from the battery once the headlights, accessories and ignition switch were turned off.
Fast-forward to today, and the places on a vehicle that can cause battery drain are endless. Today’s vehicles have many electronic modules that draw current at all times, with some modules that can draw current for several hours even after the ignition switch has been turned off. With the advancement of electronics and the seemingly endless electrical systems, parasitic draw problems are getting to be more common all the time.
Back in the day, using a bulb or test light to complete the ground circuit of a battery was a great way to do the testing for a parasitic draw problem. If the bulb glowed, there was some current flow; when the bulb no longer glowed, the problem was fixed. This tried and proven test is no longer of value, since there is always some current flowing from the battery, powering internal “clocks” and the Keep Alive Memory (KAM) of the Engine and Transmission Control Modules (ECM and TCM).
So far, I have not yet seen a test light with a calibrated bulb (or a tech with a calibrated eye) that could tell the difference between a 2 milliamp (mA) draw and a 4 MA draw. Relying on this outdated method will only lead to frustration and continued comebacks from misdiagnosis.
Source: Motor Age
