Auto Service Professional

JUN 2018

Magazine for the auto service professional

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14 A S P J u n e 2 018 Diagnosing the charging system on this ve- hicle is essentially done with the use of a scan tool. If there is a problem it will usually generate a diagnostic trouble code. However, if you want to perform an alternator output test you can do so by using a handheld electronic electrical sys- tem tester like the one in Figure 2. On newer vehicles, it's more difficult to per- form this test, as the battery charging tolerances are different due to the electrical loads placed on the vehicle are a lot less. Some of this is due to more LED lighting and other less-power-robbing components. To manually check alternator output using an electronic handheld tester, hook your two clamps on each post of the battery and your inductive clamp around the positive cable going to the al- ternator. Most testers will guide you through the test and point out if the unit has passed or failed. Remember, in order to obtain an accurate test, the battery must be able to operate at full ca- pacity. If for some reason you don't have access to a handheld tester, you can use your DVOM and an amp clamp suitable to handle the higher am- perage required. Some amp clamps like the one shown in Figure 8 have a built-in DVOM. Zero out the meter and place the amp clamp around the positive cable of the alternator. Set your meter reading to the amps scale. Start the engine and make sure all electrical load like head- lights, wipers, blower fan, etc., are turned off. Raise the rpms to about 2,000 and observe the status of the meter. Hold for a minute to let the system sta- bilize. Next, return the engine to idle. Now, turn on as many electrical loads as you can and repeat the steps that you performed when there wasn't any load on the system. Compare the two read- ings. You should see a slight increase depending on the state of charge of the battery. is is a crude test but it will give you an idea if the alternator is in fact working and how well it is working. Like I stated earlier, today's vehi- cle electrical management systems have notably more control over system charging than just the alternator itself. Figure 5: Clean surfaces thoroughly with battery cleaner before testing or charging. Figure 6: A battery current sensor acts like an amp clamp, connected to the battery negative terminal, sending a signal directly to the BCM. Figure 4: Here the battery cables were removed, and when we performed the exact same test on the same battery the results were different. This time it showed 969 CCA and a rated capacity of 102%. That's about a 25% difference and enough to skew the results! Also note corrosion deposits on terminals will add unwanted resistance during testing. C h a r g i n g S y s t e m s

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