The next time you're crossing a parking lot or stuck in traffic, take a peak into the cars around you. You'll likely spot a variety of technological luxuries, from the now-standard cell phone to new gadgets like global positioning systems (GPSs), laptop computers, and even full-scale audio/video systems.
If you think the popularity of these devices is a passing fad, think again. According to the marketing research firm Frost & Sullivan, current sales for rear-seat entertainment systems is at $200 million and will likely double by the year 2007. The advances in automotive gadgetry make it easier to keep the kids entertained in the car, but they're creating the potential for new electrical problems, as well. I never thought much about power quality in cars until one of my neighbors approached me for help with the audio/video entertainment center installed in his SUV. Tackling this problem was my introduction to investigating power quality problems in automobiles.
As my neighbor Michael explained his problem, I began to envision a whole new dimension of power quality troubleshooting. Since installing a rear-seat entertainment system, Michael found his car's battery would die each night, and he'd need a “hot shot” the next morning to get going. The system installer told Michael he probably had a bad battery or charging system and recommended he get them checked at his dealer. The dealer said the battery system looked fine but thought the recent installation had created a short somewhere. He referred Michael back to the installer for a repair. Instead, Michael came to me.
A ghost load was obviously draining the battery when the engine was off. I knew it had to be a fairly large current draw to completely discharge the SUV's 400Ah battery overnight. The fact that there were no blown fuses and all the equipment appeared to be working correctly only made the situation more confusing. However, I thought troubleshooting the leakage current problem would be as simple as connecting a current meter to the load and beginning the process of circuit isolation by opening breakers until the current draw disappeared.
I don't use a DC clamp-on meter very often, so I didn't have one with me. I did have access to my digital mutlimeter (DMM), though, which can read up to 10A DC current. I hoped this would be enough to pinpoint the phantom load. We disconnected the positive battery cable from the battery and wired up my meter in series with the cable and battery. The meter measured a steady current draw of 5A. There was definitely a faulty circuit somewhere, but finding it was going to take some work.
In modern automobiles, certain loads draw current all the time. The alarm system, automotive clock, and seat memories all require power — even when the car isn't running. In some cases, the interior fan kicks on every once in a while to filter the air in the cabin. With all of these loads, it's no wonder automakers plan to increase the operating voltage from the 12VDC standard to a 24VDC-based system in the near future.
Michael and I began the testing process by pulling the fuses one-by-one to see if my meter uncovered a change in current. As soon as we pulled the accessory fuse, most of the current draw disappeared. Now we knew which circuit to focus on, but finding the short with all that concealed wire bundled together wasn't going to be easy. As I contemplated having to trace wires under the carpet and behind the panels of the doors, I remembered an old troubleshooting trick I learned several years ago while working on DC elevator control circuits. All I needed was a pocket compass.
With the fuse removed from the circuit, I had Michael use a jumper wire to tap the fuse terminals, which created an ON-OFF current flow through the conductors. Using a small key chain pocket compass, I followed the conductors from this circuit by watching the compass needle swing back and forth with the change in current flow. When current passes through a conductor, it creates a magnetic field, so I used the compass to detect the field.
After opening some covers and panels and removing tie wraps, we finally found the culprit. A mounting screw had penetrated some insulation on a couple of conductors that power up the passengers' heated seats. This explained why the current never became a fault current but was large enough to drain the battery. Once we removed the screw and covered the hole with some electrical tape, the system checked out fine.
When it comes to automotive power quality, the same basic rules of electricity still apply. Overvoltage and undervoltage conditions can wreak havoc on electronic systems and components. Most of the time, a faulty regulator or charging system is to blame. But as Michael's problem illustrates, sometimes the cause isn't so obvious.
Electrical Troubleshooting Tricks for Automobiles
Here are some “insider” troubleshooting tricks that should save you time and a few headaches while working on any vehicle.
Replacing a car battery can be frustrating because as soon as you remove the battery cable, everything wired into the car's electrical system loses power, erasing all of your settings. To avoid this problem next time, do the following:
Buy a 12V lantern battery at the hardware store. They go for about $5. Then connect a cigarette lighter plug cord to the lantern battery, making sure to observe the correct polarity. You can find these cords at most electronics stores for about $5.
Once you connect the cord to the lantern battery, plug the cigarette lighter plug into your cigarette lighter socket, which will parallel the lantern battery to the car's electrical system.
When you remove the car battery, the 12V lantern battery will feed the electrical system, keeping all of your electronic devices energized. Be careful with the car battery's positive terminal when connecting the new car battery — it is also energized.
Once the new car battery is in place, unplug the lantern battery.
The following technique is a must if you have a motor home or boat trailer. At some point in time, water will get into a turn signal or connector and start blowing fuses. Follow these instructions for troubleshooting the circuit without power.
Remove the defective fuse from the circuit, and wire up a 12V signal light in place of the fuse. The bulb will act as a resistive circuit element because it is wired in series with the circuit.
Start moving conductors around and tugging on bundles to see if the bulb flickers or dims. If this happens, you're close to the short, and the circuit resistance is changing.
Still can't find the fault? Have someone tap the connections for you, and use a compass to locate the conductor as mentioned in the article. If you move the compass along the wire and it moves back and forth, current is flowing in the circuit. When the needle stops moving, current is no longer flowing in that part of the circuit, so the short must be upstream. You'll find your short somewhere between these two points.