My 1996 Taurus will sometimes (of course not always) slowly drain down my battery. The FORD tech people replaced a relay switch associated with the fuel pump, but the problem has is now back. They can’t seem to find the problem, because it only does it once in awhile. Any suggestions.
Every time you turn off the car make sure nothing is operational that should not be. Radio, gauges, windows, and if it autolocks doors make sure all the doors are locked as it may be trying to lock a door lock that just won’t go occasionally. Also remove any trunk, hood, glove box bulbs. Also make sure brake lights are off.
How old is the battery, and has the alternator been tested? If the battery is old, replace it. If the alternator is not putting out enough volts and amps, replace that.
But if the battery is new and the alternator is strong, then you have an electrical short somewhere. The first thing to do is identify the circuit. The easy way to do this is to pull the fuses one at a time and read the battery and alternator output. At some point if a circuit has a short, you will pull the fuse for that circuit and see a jump in voltage. Unfortunately, many fuses cover multiple circuits. But it’s a start.
When you identify the potentially faulty circuits, you then need to see if any part of the wiring interacts with moving parts (such as car doors) or can be overheated. Especially concentrate on any accessory that seems iffy, that works sometimes but not always.
A simple place to start is to remove the bulb for the trunk compartment light, the glove compartment light, the underhood light and the light in the console (if your car has these features). If the battery doesn’t drain down, you have isolated the problem to one of the switches controlling one of these bulbs. If this doesn’t solve the problem, you aren’t out any money.
Also, check to be certain that the brake lights are off when you have parked the car.
Ever used a DVOM or ammeter? Install one between battery and cable. Pull fusesrelays till you see the mA or A current go away and that’s the bad circuit. Best and easiest start. I’ve seen a glovebox light staying on do this and get missed through hours of diag. Best tool in box DVOM. Slow drain is low current so meter in series, set to read mA, battery cable off and connected to one lead other lead on batt post. Make sure leads are in com and Amp. not Ohm not Volt. you can do this on positive or neg. Neg less sparks. If current is drawing at that time with key off everything shut and off it could be a bcm. Start pulling fuses then and see when current isn’t flowing that’s the circuit. If it’s labeled lights that’s a start. Make sure not to open doors and stuff or use accessories because that will draw current. and the DVOM will show it.Not hard. Electrical current is like water current. You have a pipe with a small leak and you need to isolate the other pipes until the leak stops. Thats all. If your problem in alt or battery you can figure that out too. DVOM on Volts across battery want around 12.6, crank engine and it changes to 14.4 or alternator voltage. May read positive or neg still voltage. Kinda a crash course. THe basics.
Ya know if your post did not replicate my post so closely I might not wonder. I try to make an effort to post new information or star posts I agree with rather than adding to the clutter by repeating what someone else said.