VEHICLE AFFECTED: 2011 GMC Sierra www.VehicleServicePros.com MAY 2016 PROFESSIONAL TOOL & EQUIPMENT NEWS 51 VEHICLE ISSUE: Intermittent parasitic battery drain Tool Briefing ken wire. Other common open circuit causes could be due to blown fuses, faulty switches, faulty relays or poor connections. In some cases a shorted circuit can create a parasitic drain causing the battery to lose its charge. Common indicators of shorts to power or ground are, in most cases, very similar to those of an open circuit. The fun starts when you find an open circuit and need to determine if it is due to one of the common sources, or the root cause started with a shorted circuit or component. The issue we have is an intermittent parasitic drain on a 2011 GMC Sierra. The customer states they just purchased the vehicle, so they have no history of either prior issues or repairs. 1 OBTAIN INFORMATION As with any vehicle diagnosis the first thing that needs to be done is obtain information. Since the vehicle has an intermittent issue, the best place to start getting information is the customer. It is necessary to get as much information as possible from the customer in order to duplicate the issue and to recreate the circumstances that caused it in the first place. This step is even more critical when the issue is intermittent. The only information the customer Fig. 3. We use the Power Probe ECT2000 to assist with locating the short in the trailer running lamp circuit. This tool uses a transmitter (right) to send a signal through a circuit and a receiver that will capture the signal. The receiver (left) is used to follow the wiring and show the loss of signal just at the point of a short. The tool can also be used to find open circuits in a similar fashion. Using the tool, we located a short in the wire going to the left taillight of the trailer. was able to provide was that the battery was dead after the vehicle sat for a few days and it only seemed to happen every couple of weeks. Our experience with electrical issues dictate since this is an intermittent problem, we attempt to obtain more details from the customer by asking more pointed questions specific to how the vehicle was used just prior to the failure. Common questions to ask are: • Did you drive or use the vehicle differently prior to the failure? • Is it always the same driver using the vehicle? • Did you use it for a short or long trip prior to the issue? • Was it used at night? (We want to know if they had used the lights.) • Is it always parked in the same spot? (We once had a GM vehicle that had a short in the starter solenoid that only occurred when the vehicle was parked on a slope.) The answers we acquired did provide some possible clues. The truck is used to pull a trailer. Every time the trailer is used the issue does not happen, but when the trailer has been used for longer trips seems to be a common factor. The next stop for information was to go to Mitchell 1’s ProDemand to see if there were any related TSBs and to look at the wiring circuit for the vehicle, specifically the trailer wiring diagram. There were no TSBs. (We will be referencing the wiring diagram throughout the diagnosis and repair.) 2 TEST BATTERY AND CHARGING SYSTEM It is easy to overlook the obvious when you start diagnosing issues, especially intermittent problems. You may think that since we have narrowed the cause of the problem to be related to towing a trailer that basic tests like checking the battery and charging system are not necessary, but don’t ever forget the basics. It is possible that when the trailer is connected there is just enough additional load on the charging system or battery that voltage was not sufficient. We used our Snap-on MicroVAT battery tester and
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