Thread: battery drain
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Old 12-15-2017, 09:30 AM   #12
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Join Date: Jan 2008
Posts: 719

1999 SL2
Default Re: battery drain

Originally Posted by fdryer View Post
1-The center of every cigarette lighter/power outlet (negative grounded vehicles) is positive(+). Ground is the side housing. Simply sticking a red probe into the center of the outlet (always HOT directly from a fused connection) with negative probe touching any nearby bare metal chassis should display battery voltage. If you want to make it easier, cutting off an unused power plug can be made into a plug in voltage adapter with your multimeter. Just be warned - the center is 12v(+) and the side housing is ground. An instant electrical short will occur, briefly before the fuse blows, if the multimeter probe contacts the center and side at the same time. An alternative, if you aren't comfortable with this arrangement is measuring one of the fuses in the console (access is needed). Examine your fuses for metal pins on top - this allows measuring for voltage. Some fuses are switched so some careful measuring with ignition off should reveal hot fuses like the power outlet.

2-If you finalize setup to measure battery/alternator voltages, simply measure with ignition off, after starting up while idling and after engine shutdown. Any trip will do and give you and idea of battery and alternator voltages. With a good battery, either fully charged or not, battery voltage should be at least 12 volts, better if around 12.5v. This is stand by battery voltage before being used for starting. Measure voltage after starting as the starter draws a lot of current and is always being recharged - at high idle the alternator, provided the drive belt is tight and not slipping/squealing, should display anywhere between 13v to 14.9v as the voltage regulator automatically determines what voltage is needed to supply all electrical needs of the car while recharging the battery. Batteries recharge after driving 15-30 minutes with voltage never dropping below 12 volts. It doesn't matter how long or short your trips are. You're assessing the entire battery, alternator and drive belt system by measuring standby voltage, cold engine idling, after a trip before engine shutdown and after engine shutdown. Record voltages for a few trips to see a pattern.

3-Main power; battery/battery cables and their connections/drive belt system/alternator. All should be examined for anything unusual.

4-All good batteries never show less than 12 volts. If you measure battery voltages lower than 12 volts, either the probes are incorrectly used, not making good electrical contact or the battery is failing from either premature failure, a drain in the electrical system or the alternator/drive belt isn't providing power to recharge the battery. Measuring voltages between 13.5v-14.7v indicates the drive belt and alternator are allowing the battery to be recharged for the next start.
Great info!
I don't quite get why I measure voltage after trip and shutdown. Voltage at idle tests alternator and voltage under load checks the belt, right? As I'm in a bit of a time crunch, I was considering just getting a warranty replacement after only checking my idle voltage.

I didn't quite finish charging last nite, but it was reading 13.xV. This morning it is 12.6V. This has been a usual pattern, which I assume was due to the MM measuring a surface charge , and is normal? Afaik the charger is 'smart' and does not overcharge.

1999 SL2 MANUAL ~76K


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