When the car is shut off, there should be a very small load on the battery to keep the clock running, the security system on etc. But the amount of current is really small. If the battery is dying overnight, something is loading it;
Charge the battery right up.
Have ignition off, doors and trunk closed, no interior /exterior lights on and leave the driver door window open.
Take a fairly large automotive bulb like an 1157 or even a headlamp.
Disconnect the negative terminal on the battery.
Connect one wire to the negative terminal of the battery from the light, and the second light terminal to the negative battery cable you diconnected; connecting the light in series. Don't put the negative battery terminal on the battery.
The bulb should not light if all your ignition system is off and you have no lights turned on.
Turn on the lights or blow the horn by poking through the open window without opening the door, the test light should light and the horn sound.
If the light lights without anything being on, then you have something constantly draining the battery.
Go to the fuse panel and remove and re-install each fuse one at a time from every circuit. You may have to close the interior light door switch by pushing it with something while you are getting to the fuses so the circuit is in the off state all the time. Everything has to remain off.
When the light goes off when you removed a fuse, that is the circuit which is loading the battery. Check the fuse description for the circuit affected.
My guess is a trunk light that stays on even when the trunk is shut. If it is a lighting system fuse causing the problem, remove the trunk bulb and replace the fuse. If the light goes off, the trunk switch is defective keeping the light on all the time.
You can also place a battery charger on the battery (connect the negative back on the battery) and go and check the car in the dark. Is there a light on like a floor lighting in the interior? Do you have a light on under the hood that can't be seen during the day?
Always make sure you have a good charged battery, and that the terminals are really clean before you start analyzing the problem.
Does the alternator charge the battery? Check by measuring the volatge across the battery engine off, no charger connected; should be 12V or close.
Start the car, the meter should read 14V or so, but more than without engine running. If not, the alternator/charging circuit may be the real problem.
Aug 09, 2009 |
1991 Cadillac DeVille