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 20 milliamp (mA) draw and a 40 mA draw. Relying on this outdated method will only lead to frustration and continued comebacks from misdiagnosis.
Low-Tech Solution
While the potential causes of parasitic drain have become more complex, the tooling for detecting them does not need to be very high tech. It can be as simple as using a digital volt ohm meter (DVOM), a set of test leads, a high-current battery cable switch and a wiring diagram.
For some parasitic draw problems, having a recording amp meter can be very useful, especially in the case of a module that will wake up in the middle of the night and drain the energy from the battery. As a professional technician, you already should own a quality DVOM, and many high-end scan tools are equipped with graphing meters and/or digital storage oscilloscopes (DSOs).
Before You Start the Hunt
Before any testing for parasitic draw problems is performed, the battery should be tested to verify it is in good condition and that the charging system is working properly. Once these are verified to be in good working order, then move on to the parasitic draw problem.
Go to your service information and find out how much current draw the vehicle manufacturer allows. For most vehicles with a single battery, this current draw will be somewhere between 20 mA (0.02 amps) to 40 mA (0.04 amps). Once you know how much current is allowed, you will have a target for which to shoot.
Start the diagnostic process by putting a high current switch on the negative battery terminal and hooking the negative battery cable up tight. It is always a good practice to take the vehicle on a short test drive, and then operate all the accessories on the vehicles. This is done to bring the vehicle as close to its real-world working conditions as possible. The battery disconnect switch has been installed in the battery circuit so an amp meter can be hooked into the circuit without having to open the circuit and destroy all the evidence you were able to capture on the test drive.
Why not use a low-amp current probe around a battery cable, you ask? This will work in some cases, but if a low-amp probe is left on for an extended period of time, most current probes will drift a little. Then when you look and observe a reading of, lets say, 25 mA and the actual current flow is 35 mA, the information is not correct. This can mean the difference between fixing the problem the first time and having an angry customer stuck somewhere with a dead battery.
This is the reason why I prefer running the current through a meter, whether it be a graphing or digital device. These kinds of test meters always report with great accuracy.
With the meter hooked up and all doors closed, let the vehicle set until all the modules go to sleep. Depending on the vehicle, this might take 20 minutes or it might take six hours. If the vehicle is disturbed during this period, you get to start the process over again. In a case like this, you are at the mercy of the vehicle, and it doesn’t do any good to try and hurry the process.
If you must access the cabin, disable the driver’s door switch ahead of time so the door can be open without turning on lights or letting a module know the door is being opened.
Tracking it Down
Once all modules have gone to sleep, it is time to start finding where the current is flowing. I suggest printing out a picture of each fuse panel so you can easily identify the fuses. This also gives you a place to make notes as you work your way through the diagnostic process.
Before you grab a pair of pliers and start pulling fuses, though, consider what happens when you pull the fuse to the Powertrain Control Module (PCM) or the Body Control Module (BCM). Pulling the fuse will cut the current flow to those modules — and if you are lucky, you might find where the current draw is. But if you aren’t, when the fuse is inserted back into its cavity, you just woke up those modules and now you need to take a break while those modules go back to sleep.
A less-intrusive way to find the current flow is performing a voltage drop test across each fuse with your DVOM set to the milliamp voltage scale. I have found this procedure to be both accurate and quick.
Start by setting the DVOM to the lowest voltage setting the meter allows. You also should be using a pointed probe on each meter lead. If you take a look at the meter display, the DVOM will show voltage when the leads are not hooked to anything. Once the leads are hooked to each terminal of the fuse, the meter reading should show zero volts.
If there is any current flow through the fuse, a voltage will be reported on the meter screen. Why? The fuse is a thermal device that is designed to fail when the rated load is exceeded. It has a resistance, and this resistance varies with fuse design and current rating. As with any resistance in an electrical circuit, when current is flowing, a drop in voltage will be present — and measurable.
As with any diagnostic process, the technician should start with a system of gathering information and as the process proceeds, the information gathered will keep narrowing the problem down until the problem has been pinpointed. In this case, use the DVOM to test the voltage drop across each fuse and note this voltage on your fuse chart.
Once all fuses have been tested, use the fuse charts to identify the circuit each fuse feed. Identify the circuits that do not power up modules, and start removing those fuses one at a time, while watching the reading on your recording amp meter. The reason for using this approach is so none of the electronic modules are disturbed. If the current draw is not pinpointed, then start removing circuit breakers and the covered fuses you couldn’t test with your DVOM.
When the fuse powering the circuit with the current draw is removed, your amp meter will let you know. Once the circuit has been identified, you are on your way to doing the pinpoint testing for the problem.
A Real-Life Example
A 2007 Chevrolet Suburban K2500 truck came to the shop for a no-crank problem. The battery and starter had been replaced in an attempt to fix the problem. The testing process started by hooking a Tech2 scan tool and polling the modules on the vehicle. The scan tool found no communication with any of the modules, with the exceptions of the BCM and the AntiLock Braking System (ABS) modules. There is also a parasitic draw problem that will deplete the battery energy overnight.
The vehicle owner said the vehicle had been bought at auction. It was supposed to be a low-mileage vehicle in good running condition. The vehicle had been sitting for six months and the battery had gone dead. An attempt at jump-starting the engine had only caused all the windows to go down, then all systems went dead.
This is a classic case of jumper cables being hooked up backward, I thought, which would explain all the dead modules. The reason the battery is going dead is a 0.4 amp continuous draw from the battery.
With the electrical trauma this vehicle has suffered, I recommended replacing or fixing all the electronic modules before any time was spent trying to find the parasitic draw problem.
The reasoning behind fixing the module communication problems first is so the vehicle electronics are all working properly. Anytime electronic modules are involved, it is important they are in working order. Defective modules are common causes of parasitic draw problems.
With all modules working properly, I started testing the voltage drop across the fuses. I started in the engine compartment fuse box, where the only fuses I found flowing current were those powering up the PCM and BCM.
Moving to the interior fuse panel, the only fuse with any current flow was the fuse that powered the rear window wiper motor. Of course, this was the last fuse in the fuse panel.
Using a wiring diagram of the rear wiper motor, I found something interesting: In the wiring diagram of the rear wiper motor, there is a note saying “logic” or “logic module.” Anytime I see something like this, it is saying to me, “There is something inside that cannot be tested with an ohm meter.” In other words, all I can test is the inputs and the outputs.
After studying the wiring diagram, I found the rear wiper motor has an input to the BCM. The rear wiper motor has its own little computer module that is not on the Controller Area Network (CAN) bus, but is in control of the rear wiper and washer operation.
The current draw is internal to the rear wiper logic module. There are two ways of testing this problem: One way is to unplug the wiper motor plug, which will cause the current draw to go away; and the other is to use a low-current probe to check the current draw on circuit 2040 (a red wire with white tracer).
I prefer using the current probe, because the test in non-intrusive and will test the circuit with it intact and in its own working environment. You do not know whether unplugging the module will kill not only the module you are working on, but how it will affect other modules in the system.
With a new rear wiper motor installed, the current draw came down to .022 amp, which is normal for this vehicle. By using a systematic diagnostic process, jobs like this are both profitable and fun to work on.
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