On late model vehicles with computers and numerous electronic modules, the key-off drain on the battery can range from 20 to 50 milliamps, to as much as 300 to 400 milliamps. That is why the unpower test light will glow as long computers and modules memories are alive.
As a rule, the parasitic drain on most late model vehicles should be less than 50 milliamps one hour after the vehicle has been shut off and left undisturbed
Keep in mind, though, that opening a door, the trunk or turning anything on can wake up various modules and start the timer countdown all over again. So if you want to check the parasitic draw on the battery, leave the hood open.
Current drain can be measured with a digital multimeter (DMM) using the ammeter function.
1. Set the DMM to read milliamps (mA). If the meter is not auto-ranging, select the 100 or 1000 mA scale.
NOTE: the DMM should have a 10 amp fuse in it to protect the meter from possible damage if there is a large current drain on the battery.
2. Plug a battery backup into the cigarette lighter (or attach a backup battery or power supply to the battery cables) to protect KAM while disconnecting the battery cable
3. Disconnect one of the battery cables and connect the DMM test leads to the cable and battery post.
4. Unplug the battery backup and note the current reading on the DMM.
FINDING THE CAUSE OF THE CURRENT DRAIN
If the key-off current reading is above specifications (typically, more than 50 mA one hour after the vehicle has been shut off), the current drain is too high. It's time to start pulling fuses and relays to find the fault.
Pull the fuses and relays one at a time until the current reading drops
Avoid pulling the fuses for the PCM or other KAM-sensitive modules until you have checked all of the other circuits.
Once you've found the circuit that is causing the excessive current drain, check the relay, switch, module or other components in the circuit and replace as needed, or we can provide the wiring diagram for further help.
Saturday, November 21st, 2009 AT 9:03 AM