Magazine for the auto service professional
Issue link: http://asp.epubxp.com/i/684411
20 | ASP June 2016 Technical unit may stay awake longer with the key fob in the car. Move the key fob at least 20 feet away. If you're certain everything is of but current draw stays above the manufacturer's specifcation (or above 50 milliamps) for more than 45 minutes, it's time to check each circuit individually. Tere is one more thing to watch for when measuring parasitic draw. On many vehicles the PCM will wake up sometime after shut-down to pressure test the evaporative emissions control system (EVAP). It will draw enough current to operate a leak detection pump (LDP) and/or a few solenoid valves, but it shouldn't be enough to blow the fuse in a DVOM. Circuit testing You can check for current draw of indi- vidual circuits at the fuse box. Start with aftermarket equipment like a location tracking device, a fight recorder plugged into the data link connector (DLC), or any after- market or OEM equipment with a Bluetooth connection, like hands-free cell phone links. Next, look for aftermarket or OEM items located somewhere away from the driver's cockpit. For instance, we learned about a minivan with a back-seat DVD player that stayed on all night because someone tried to insert a second disc into the slot. Te player kept cycling the load/eject motor because no one removed the extra disc after it ejected, and next morning the engine would not crank. Instead of removing each fuse to fnd the circuit that's drawing current, it's easier to check the voltage drop across each fuse with a DVOM. Any time current fows through a circuit, there will be a diference in voltage Measure the voltage drop across each fuse to fnd the circuit that's drawing current. One tenth of a millivolt (0.0001 volts) across a 30-amp fuse indi- cates 50 milliamps of current. This reading shows the control unit for the interior lights is drawing just over 1 amp through a 10-amp fuse. How a battery works To make electricity, you need to make ions. An ion is any atom that has one less electron or one more electron than it normally would. This makes the atom somewhat unstable, and given the opportunity it will either accept or give up an electron to become stable again. Whether the ions are created by manipulat- ing magnetic felds with a generator or by a chemical reaction, electrons moving from one ionized atom to another produces the energy we call electricity. A lead/acid battery makes electricity with a chemical reaction, but frst we need to cre- ate some ions on the plates by charging the battery. When the battery is fat (discharged), the lead plates are actually lead sulfate, a com- pound of lead and sulfur. Charging the battery forces electrons to fow from one set of plates to the other, transforming one set into lead (with fewer electrons) and the other into lead oxide (charged with extra electrons). It also ionizes the electrolyte (sulfuric acid) so it can conduct electricity. The plates are electrically insulated from each other, so when they are connected together by connecting a circuit to the battery terminals, extra electrons fow from the positively charged plates (anode) through the circuit to the negatively charged plates (cathode). Eventually the atoms on the plates and in the acid are stable again and the chemical reaction ends.