ElectricalSup_14

FleetMaintenance_NovemberDecember_2016

ELECTRICAL SYSTEMS DIAGNOSTICS Enter 9 at “e-inquiry” on VehicleServicePros.com Enter 10 at “e-inquiry” on VehicleServicePros.com 14 | ELECTRICAL TESTING SUPPLEMENT | DECEMBER 2016 Issue 2: Multiple failures of a part after initial replacement Another annoying situation occurs when a vehicle returns for the same failure, after an initial repair. Common examples range from a simple light bulb replacement to repetitive failures of an alternator or fuel pump. In many cases, a technician will install an incorrect part resulting in a repeat failure. For instance, we have seen an incorrect bulb installed in a taillight when a technician replaced the bulb with the same part number rather than looking up the application to see if the correct part was originally installed. In those cases a small mistake can perpetuate repetitive failures. The same thing can occur with any electrical part such as an alternator. You should not take any shortcuts during a “warranty repair,” whether it is looking up the part number or in the diagnosis. You should always start your diagnostic process from the beginning without assuming anything, especially relying on results of a test that was performed during the original repair. Charging systems on many newer vehicles use the PCM to set the proper charge rate by assessing demand on the system. Adjusting the charge rate by need maximizes battery life and improves fuel economy by reducing engine load. Because of this it may be necessary to use your scan tool or a battery reset tool, such as the BATT+ from CanDo International, to perform a relearn when replacing a battery or alternator. Failing to perform a relearn when replacing a battery or alternator may cause the system to overcharge and reduce battery or alternator life. When any part fails, especially higher amperage parts like fans, fuel pumps or alternators, you should check to make sure there is no excessive resistance in the circuit by performing a voltage drop test. A voltage drop will determine how much voltage is available to operate the component. If there is resistance in a circuit, it reduces available voltage and will manifest itself by having a dimmer bulb, slower starter or fuel pump, or less output from an alternator. In order to perform a voltage drop the circuit needs to be “on” with current fl owing. Reading voltage with a DMM and reading the voltage at any point will provide you with source voltage, but not necessarily give you an idea of how much resistance is in a circuit. Using a DMM to read resistance (ohms) is helpful for determining how much resistance may be in a component, but will not always provide you with dependable information on wiring or connections. Simply put, an ohmmeter sends a small amount of voltage through one end of a circuit and determines how much the voltage changes (drops) at the other end and calculates the results in ohms. With higher amp circuits and large gauge wires, a more reliable test is a voltage drop. One of the most effi cient ways to perform a voltage drop is to use a Power Probe III or Power Probe IV, as they have a built-in capacity to read how much voltage drop is occurring in a circuit. Another reliable way is to use your DMM. Let’s use a headlamp circuit as an example; start by turning the headlamps on. Next, set your DMM to a 20V scale and place the black lead of the meter at the positive battery terminal. Next, place your red lead at the positive terminal of the headlamp. The voltage reading should be as low as possible, and should be 0.2V or less. Repeat the process on the ground side by placing the black lead at the battery negative terminal and the red lead at the headlamp negative terminal. The result of this should also be less than 0.2V. By reading the voltage in this manner you are able to determine the amount of “drop” that is occurring through the circuit. For example, if your source voltage (battery) is 12.5V and you read 1.0V on the positive side of the circuit and 1.5V on the negative side, you actual voltage drop is 2.5V, lowering your source voltage to 11.0V. In order to determine where the actual resistance issue is occurring, work backwards from the headlamp connections to the battery and perform the voltage drop test along the way paying close attention to connections, splices or grounds. When your reading goes back under 0.2V you have located the area of high resistance. It is important to remember to perform this test with the circuit activated. Following complete diagnostic test procedures with every repair is necessary to perform quality service, but taking extra care on warranty issues can assure it will not be another comeback. CanDo battery reset tool Lang Tools CAT IV DMM Power Probe III Enter 14 at “e-inquiry” on VehicleServicePros.com


FleetMaintenance_NovemberDecember_2016
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