Magazine for the auto service professional
Issue link: http://asp.epubxp.com/i/684411
18 | ASP June 2016 Technical long-term fuel, and you might also see strange problems with some of the other control units. If the minimum number is above about 10.8 volts, suspect high resistance somewhere in the starter circuit. Remember, these readings are taken from the battery posts, so start by checking voltage drop across the posts and the clamps. Even if everything looks clean, there could still be corrosion on the inside of the clamp that you can't see. Tis is not a defnitive test. It's just meant to point you in a diagnostic direction. But remember, a no-crank symptom doesn't auto- matically mean there's something wrong with the battery or starting system; it could be caused by mechanical problems. Parasitic draw Te most common cause of premature battery failure is what we call "parasitic draw," referring to something that con- tinues to draw current from the battery after the ignition switch is turned of. Some current draw after shut-down is normal. Te PCM and other control units are still turned on or "awake" after shut-down, and total current draw can be more than 500 milliamps for half an hour or longer. Even after all the control units turn of or "go to sleep," the PCM continues drawing current to maintain volatile memory of fault codes, freeze frame, learned idle speed, etc. Only a few manufacturers list the key-of current draw in their service information, so for those that don't, most techs consider 50 milliamps to be the maximum allowable parasitic draw 45 minutes after shut-down. You can test for parasitic draw with a DVOM that reads current, but be careful. First you have to get the meter connected in series between the battery and negative battery cable without breaking the connection to the car and erasing the volatile memory in the PCM. You also have to make sure the ignition switch is not turned on with the meter connected. Tat's because most meters are rated for 10 amps, and since the fuel pump runs when the ignition is frst turned on, it could blow the meter's fuse. If you have a low-amp probe that connects to your meter, none of this is a problem; just clamp the probe around the negative battery cable. With the Key On Engine Of (KOEO), you'll see the current draw of all the control units. With everything turned of and all the doors closed, you can watch current draw decrease in steps as each control unit times out and goes to sleep. If the vehicle has keyless entry, the control Next time you see a connection coated with green fuzz instead of this nice red grease, check the voltage drop across the battery post and clamp. This one is a perfect zero millivolts. Five minutes after shut-down, a "mystery" draw was still pulling 164 milliamps of current on our customer's Acura (the low-amp probe is set to 1 mV/10mA). And then after about 15 minutes, current draw suddenly dropped to 30 mA.