Diagnosing a shorted electrical circuit

May 13, 2016
This diagnosis uncovers two issues: A short in a trailer circuit and battery drain caused by the brake control relay.

Vehicle affected: 2011 GMC Sierra

Vehicle issue: Intermittent parasitic battery drain

Tools Used:

  • Battery/Charging System Tester
  • Vehicle Information
  • DMM
  • ESI Fuse Buddy
  • Cal-Van Tools Amp Hound 2
  • Power Probe IV
  • Hopkins Tow Doctor
  • Power Probe ECT2000 Smart Circuit Tracer

Electrical issues can be some of the most challenging problems to diagnose on today’s vehicles. Not only are there many different causes of electrical faults, but their symptoms can vary as well. Throw in an intermittent issue and it complicates things even more.

 Common causes of electrical issues are:

  • High resistance in either power or ground circuits
  • Open circuits
  • Shorts to power or ground

 High resistance in a circuit can cause components to not work correctly such as a starter to crank the engine slower, or a sensor to report the engine temperature incorrectly. High resistance can also cause components to fail prematurely, like a fuel pump due to the pump needing to work harder to perform its job. In the worst cases, excessive resistance could cause wire insulation to melt due to heat being generated.

 Open circuits will typically cause a component to not work at all, such as a light not turning on because of a broken 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.

 Step 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 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.)

Step 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 Vat battery tester and confirmed both the battery and charging system were within specifications.

Step 3: Test for battery drain

There are numerous ways to test for current draw (amperage) at the battery. You could install a tester set to read amperage wired in series to either the positive or negative side of the battery between the cable and the battery. You can use a DMM, and it is a good idea to install a 7.5 amp fuse and holder between the meter and the battery. This will allow the 7.5 amp fuse to blow before the fuse in the meter, saving you time and money. Using a DMM works, but may not be the most efficient.

Other tools we have found very useful and easier are the ESI Fuse Buddy and the Horizon Tools Amp Hound. These tools can be installed in the same manner as a DMM, but provide a digital readout that allows amp readings of an individual circuit, right at the fuse.

In this case we used the Amp Hound, which allows you to read amperage on any circuit without needing to install the tester in series. This is beneficial because in the case of checking for an overall battery drain, disconnecting the battery to install a tool in series will lose memory and requires you to allow time for computer and radio memory circuits to recharge before reading residual current draw. Our tests determined there was no current drain at this time (See Fig. 1).

Step 4: Test all lighting, fuses and relays

 Since we are still looking for an unknown cause of an issue at this point it is not a bad idea to test all of the fuses to see if there are any issues such as either blown or missing fuses.  One of the easiest ways to test fuses is by using a Power Probe. We use the Power Probe IV which gives both a visual and audible signal when either ground or voltage is present. The Power Probe IV also has a built-in voltage drop test that indicates if there is excess resistance in a circuit. All of the fuses tested OK, except for fuse #25 in the main fuse box under the hood. This fuse supplies power to the running (tail) lights for a trailer.

We replaced the blown fuse for the trailer lights and since we do not currently have the customer’s trailer we installed the Hopkins Tow Doctor into the trailer connector. This tool allows you to check the vehicle circuitry from the driver seat by illuminating lights on the handheld unit indicating the correct light is turning on when the circuit is activated. Keep in mind this tool is great for testing the continuity of the circuit, but not if the circuit can sustain the load that may be needed to operate multiple lamps, or activate the brakes on a trailer. The tester showed the trailer/truck connection was correct and the fuse did not blow which indicates the issue with the blown fuse is most likely in the trailer, not the truck wiring.

We will also check all of the lighting on the truck and other electrical components to verify functionality. This step will assure there are no other issues and may assist in the diagnosis of the original concern, a parasitic battery drain. All other components worked without issue. We again used the Amp Hound to verify there were no parasitic drain that could have surfaced after energizing all of the components which showed no amp draw.

Since we needed to recreate the customer’s concern it was necessary to suspend all testing until we could have the trailer as well as the truck.

Step 5: Perform tests with trailer hooked to vehicle

We had the customer bring the trailer in immediately after driving it for a period of time to duplicate the conditions of the original concern. We repeated the amp draw test, this time we showed a 2-amp draw. Using the Amp Hound on the individual fuses in the underhood fuse box, we found the fuse to the trailer brake relay/controller (fuse 6) was drawing current. We also noted the trailer running lamp fuse was blown again. To confirm we were on the right path of our diagnosis, we disconnected the trailer connector and still had a 2-amp draw (See Fig. 2).

Step 6: Locate and repair the short

We have two separate issues; one is a short in the trailer running lamp circuit which is easiest, and may lead us to the other problem. The easiest tool we have found to locate a short is the Power Probe ECT2000. This tool uses a transmitter to send a signal through a circuit and a receiver that will capture the signal (See Fig. 3). The receiver is used to follow the wiring and show the loss of signal just at the point of a short (See Fig. 4). 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 (See Fig. 5). We repaired the wire that had been damaged from a screw rubbing the wire and had no further issues with the blown fuse (See Fig. 6). We still had the battery drain and needed to find its cause.

It seems unlikely that the trailer brake control module could have an issue that would cause a battery drain without having a trailer connected, but the GM system uses a relay that uses battery power to supply voltage to the trailer brake module. Without a trailer connected, there would be no reason for the system to draw current unless the relay itself was faulty, not allowing the relay to open when it’s operating signal was not present.

Using Mitchell 1’s ProDemand, we found the brake control relay and module were located above the spare tire under the truck. After we removed the spare tire, we were able to locate the relay and disconnect it. After disconnecting the relay, the Amp Hound was reconnected and confirmed the current draw was no longer present. With no current draw, we focused on why the relay was activated. Checking the wiring with the Power Probe, there was voltage present from the battery and a good ground. With the engine running, there was not a signal present at the relay control, so it appears that the fault is the relay itself having an internal issue causing it to stay energized once it had been engaged. We replaced the relay and reinstalled the spare tire. After verifying there was no current draw, we had the customer drive the vehicle and trailer to confirm the issue did not return.

Solving what turned out to be multiple electrical issues can be intimidating, but using the right tools can make it much easier and profitable.

About the Author

Barry Hoyland

Barry Hoyland has been in the independent aftermarket for more than 45 years as a technician, technician instructor, shop owner, and shop management consultant. He owned and operated a successful Southern California automotive repair center that offers complete auto care and specialized in emission and diagnostic services for over 28 years. Hoyland also owned a company that modified vehicles to perform as emergency response units and mobile command centers, incorporating high-end electronic components into today’s vehicles. Hoyland has experience with all size and types of vehicles including traditional gas, hybrid electric, alternative fuel, and heavy duty diesel trucks.

Hoyland has provided consulting services for many automotive shops, fleets, and government agencies in order to improve their operational efficiencies.

In addition, he has worked with many NHRA drag racing teams as a crew chief on supercharged alcohol and nitro-methane fueled cars and currently serves as a crew chief on a Top Alcohol Funny Car, a Nostalgia Funny Car, and a Nostalgia Alcohol Dragster

Hoyland holds certifications in ASE: A1, A6, A8, and L1, MACS 609, maintains a California Advanced Emission license, and a CDL with endorsements for double and triple trailers, tankers, and HazMat.

When he is not helping to run a shop in the Pacific Northwest, Hoyland travels across the U.S. as an instructor of technical and shop management courses, many of which he has developed. 

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