Magazine for the auto service professional
Issue link: http://asp.epubxp.com/i/684411
16 | ASP June 2016 Technical doesn't automatically mean the battery is dis- charged though, because the same symptom could be caused by high-resistance in the starter circuit. Unfortunately, most people don't notice slow cranking speed because fuel injection and electronic controls will usually get the engine started after just a few compression strokes. As a result, their frst obvious clue to a battery problem is when the engine won't crank over at all, and that's when the car ends up in your bay. So, is it a bad battery or a bad connection somewhere in the starting system? With a DVOM that records min/max values, you can run a quick and reliable test that will point you in the right direction. But frst you need to charge the battery, and you should do it without disconnecting the battery cables so you don't erase any data that is hopefully still stored in the powertrain control module (PCM). When that's done, disconnect the charger and connect the meter to the battery terminals; not to the cable clamps and not to chassis ground, but directly to the battery posts. Now you're ready for the test. Read the open-circuit battery voltage. A fully charged battery will read 12.6 volts, and a battery with only 50% charge will read 12.3 volts. After charging a battery, the initial reading may be higher than the battery's true state-of-charge due to an afect called "surface charge" (see sidebar). Turning on the headlights for just a minute or two will give you a more accurate reading of the battery's true charge. Now turn the lights of, start the engine and note the charg- ing voltage. After about one minute at idle, note the voltage again, then turn the engine of and read the minimum and maximum voltages recorded by the meter. Te maximum number is charging system voltage. It will probably start high, and depend- ing on the vehicle and the condition of the battery, it should stabilize quickly at about 13.8 volts. If it's below 13.5, look for a bad battery connection or a charging system problem. If it's higher than 14.2 and stays high with the engine at idle, suspect a battery problem. Te minimum number will be battery voltage during peak starter draw. If it dropped below about 9.5 volts during cranking, it's time to test the battery itself. If the minimum is below 9 volts, that could explain a loss of data in the PCM, like learned idle speed or Just what is surface charge? Surface charge is when the chemical reaction that produces or stores electrical energy in the battery occurs only on the surface of the lead plates. This reaction runs to completion quickly and then begins occurring deeper and more slowly in the material of the plate. With the voltmeter set to capture min/max read- ings, watch the meter with the engine running. About one minute after start-up, our charging system voltage went from a peak of almost 14.2 down to the normal 13.8 volts. If the charging voltage stays high, test the battery. The minimum voltage occurs as the starter draws current from the battery. This reading is right in the middle of the normal range. A low reading indicates battery problems and a high reading indicates high resistance in the starter circuit.