Analyzing electrical system issues

June 13, 2011

In this issue, we cover some of your questions related to electrical system testing. Obviously, it's an area where a lot has changed. Unfortunately, however, many technicians have failed to keep their knowledge and skills in step with those changes. Worse yet, there are still plenty of technicians who simply fear electrical diagnosis. Regardless of your knowledge and comfort level, follow along to bring both of them up a notch.

Q. We have an older AVR type tester with a carbon pile load control for testing batteries, starting and charging systems. Is there a quicker way to perform these test that doesn't require as much set-up time?

A. Testing a vehicle’s battery, starting and charging system can reveal small problems before they become more serious. Nowadays, the handheld, battery conductance and electrical system analyzer is becoming the most common choice for running these tests. Easy-to-use and requiring minimal training, the conductance tester can help streamline basic electrical tests. When performing battery tests, the tester will prompt you for information about the battery type, such as a standard flooded design, or one of the valve-regulated lead-acid types, such as AGM, spiral cell or gel cell. You may need to enter additional information such as the cold-cranking amperage or cranking amperage rating. These details can be found on the battery’s label. To follow up with the starting and charging system tests, just follow the instructions on the tester's menu step-by-step.

Q. After disconnecting a vehicle's battery, it ends up erasing memory settings like radio stations and things like memory power mirrors and seats. This irritates customers because they have to reprogram their settings back to the way they were. What can we do to prevent this?

A. One method of retaining memory involves using a power supply connected to the 12 electrical system through the lighter socket or power supply outlet. First, make sure the socket or outlet is powered with the ignition off. Then, insert the power supply adapter into the socket or outlet prior to disconnecting the battery. When you’re done servicing the battery, remove the power supply adapter.

Another method of retaining memory uses a power supply with a special adapter connected to the onboard diagnostic data link connector. Simply connect the adapter from the power supply to the data link connector and turn the power supply on. Once you’re finished with battery service, turn off the power supply and remove the adapter. If any system loses its electronic memory during service, consult service information for any reprogramming or initialization procedures needed to restore proper operation.

Q. When looking for a blown fuse, one school of thought prescribes visually examining a fuse to see if its conductor melted. Another school of thought recommends testing the fuse in the vehicle with a multimeter. Which is best?

Although a visual inspection usually works, it's sometimes difficult to tell a blown fuse using this method alone. This is especially true with some fuse types in certain locations. Generally, today’s vehicles use multiple fuse panel locations; one positioned in the vehicle interior and another mounted under the hood. There may be other fuse panel locations as well. To test a fuse with a multimeter, leave the suspected fuse in its holder for the affected circuit. With the meter’s ground lead connected to a reliable chassis ground, touch the probe of the positive lead to the terminals of the fuse. You may need to turn the ignition on for this check, or activate the circuit's switch, depending on the circuit. The meter should show voltage when probing each terminal. If the meter shows voltage at one terminal, but not the other, the fuse is blown and needs to be replaced. If you need to replace a fuse, make sure you use the correct type and amperage capacity for a given circuit. Don’t assume that the old fuse had the correct capacity. Also, never install a fuse with a larger capacity than what’s intended for the circuit, or electrical system damage may result. Use service information, the owner’s manual or the fuse block diagram to ensure the right choice.

Q. What should we use to eliminate guesswork when trying to identify faulty hall-effect sensors?

A. This type of sensor produces a digital square wave signal that alternates between off and on. By measuring the periods of on versus off, known as duty cycle and expressed as a percentage, you can get an idea of the sensor's health. You can test this type of sensor quite easily with a digital multimeter. Connect the meter's red lead to the sensor's signal wire and the meter's black lead to the sensor's ground wire. Leave the sensor connected to the vehicle harness or you won't get a reading. With the meter set to the duty cycle position, crank the engine and observe the display. A good sensor should show a duty cycle of 50 percent. A signal from a bad sensor will be less than 50 percent. If that's the case, replace the sensor.  

Q. We sometimes need to take circuit measurements in one spot on the vehicle while having to activate the circuit from another place. It's hard to be in two places at once, and short of using a helper is there any trick we can use for these types of situations?

A. A remote display digital multimeter might just be the ticket for you in these situations. This type of meter lets you connect the meter to a suspect circuit and then remove the display from the meter body so you can monitor it remotely. You can move the display up to 33 feet away from where the meter's connected. This feature can come in handy when checking circuits like brake lights, backup lights, and more.

Q. We recently had a car with a bad throttle position sensor (TPS) that we almost overlooked. Are there any tricks or tools that make diagnosis easier?

A. By checking the TPS output voltage with a digital multimeter, you can flag a faulty sensor and cure a drivability complaint. With the ignition switch off, set your meter to the DC volts position and connect the black test lead to a good ground. (If you have an auto-ranging meter, you may want to set it to a manual range so that an automatic range shift doesn't overshadow a sensor glitch.) Connect the red lead to the TPS signal return wire. Turn the ignition switch on and open the throttle slowly while observing the meter's display. Voltage should change smoothly as you sweep the throttle from idle to wide-open. A meter with a bar graph on the display can help catch a faulty sensor. If voltage changed erratically, replace the TPS. If the TPS voltage setting doesn't match the manufacturer's specs, adjust the TPS first, if possible. If the voltage reading still fails to meet specs after adjustment replace the TPS.

Q. We're relatively new to the world of electrical troubleshooting. Are there any “rules of the road” we can use to help ensure quality work with minimal comebacks?

A. First, always confirm the complaint. Never attempt to diagnose a problem without first confirming that the problem actually exists. If you can't duplicate it, you can't fix it. Next, understand the circuit. A few minutes spent understanding circuit operation can save hours of troubleshooting time. Know what, how and why the circuit operates first. Then, test the circuit. Use the equipment and procedures recommended by the vehicle manufacturer and never skip steps. Proper troubleshooting takes discipline, but saves time. After your diagnosis is complete, make the repair. Once you've concluded what the circuit fault is, fix it the right way. If the repairs require splicing or disassembly of connectors, follow the manufacturer's procedures to achieve a reliable, long-lasting repair. Finally, check your work. After repairs have been completed, make sure you try the circuit in all modes of operation. It's one thing to fix a headlight circuit that doesn't work, but if there are no high-beams, you're not finished.    

Q. We need a more exact way to determine whether a battery can sustain a load. In the past, too much guesswork has cost us comebacks. What can we do?

A. Often, a battery’s open circuit (unloaded) voltage is used solely as an indicator of battery health. This approach is a misdiagnosis in the making. First, look for the load test spec on the battery's label or in a shop manual. If neither of these is available, use half the battery's cold cranking amperage (CCA) or three times its amp-hour rating. Using a tester with a load device such as a carbon pile, connect the tester's load leads to their respective battery terminals and connect the tester's inductive amp pickup around one of the battery cables (It doesn't matter which cable). Zero the amps display or needle and set the volts switch to the proper position. Dial in the load with the load control to the specified value and hold it for 15 seconds. The voltage should stay above 9.6 with the battery temperature at 70 degrees F and above. Lower temperatures will show lower voltages. If the battery flunks this test, replace it.

Don't let electrical woes get you down. By combining the right approach with the right equipment, you're sure to get to the heart of the problem while ensuring customer satisfaction.

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