'97 328i Convertible-Electrical Problem

[jfreg]

I just bought my car about two weeks ago, and had to do a fair amount of work on it to get it through inspection. I installed new control arms, bushings, strut cartriges, brakes, rotors, and a rear window. While I was doing all of that out in the driveway, the battery ran down. I assumed I'd left a light on somewhere, but the battery continues to drain down if the car isn't driven for a day or so. The battery is good, and seems to take charge from a charger or from the alternator when the car is driven. The terminal connections are tight, and the indicator eye on the battery shows that it's good. The car's previous owner says the battery is only about 6 months old. I haven't been able to keep it charged long enough to accurately check the voltage.

I've check all of the various lights: in the glove box, the trunk, the floor, and the map lights by the rearview mirror. None of them are staying on when they're not supposed to. The previous owner says that he didn't do any electrical work or modifications to the car, except to plug in a BMW 6-disk changer in the trunk. I didn't mess with any electrical components when I did my repair work, with the exception of the brake sensor. I removed it and reinstalled it when I did the brakes. I don't see any obvious shorts at the battery, or with the main power cables under the hood. The only possible clue I have is that the power locks wouldn't work from the reomote when I first got the car, but they began working intermittenly later. I figured out that the remote won't work if the chrome hood switch on the passenger side of the engine compartment isn't engaged. However, the battery has drained when the switch was engaged, and when not engaged.

My first thought is to try to isolate the source of the power drain by pulling, one by one, the fuses for the systems that have power when the ignition is off; the alarm, door locks, lights, computer, cigarette lighter, etc, and see if the battery drains while a fuse is out. That will take a while, and be pretty brutal on my battery. Any advice or suggestions of a better approach will be appreciated.

Thanks,

John

[Jeron]

Pull all of the fuses, charge the battery then replace them one at a time while looking for the drain.

[jfreg]

I took a different approach and found the circuit with the short pretty quickly. I used an electrical tester to measure the current flowing from the battery with everything electrical turned off that can be turned off. I taped the trunk lid light switch in the off position while I accessed the battery, so the trunk lights wouldn't be on and affect my reading. I removed the negative battery cable, and measured the DC mAmps flowing between the cable and the negative battery teminal (positive tester lead on the cable, negative on the battery terminal). The flow was just under an amp, about 940 mAmps. I understand that the normal reading is about .25 amps, or 250 mAmps, so something was clearly pulling more power than is normal. Once the baseline current was established, I started pulling fuses one at a time, each time checking with the tester to see if the current flow between the cable and battery dropped. I found the culprit on the third fuse. When I pulled the "Glovebox" fuse, the current flow dropped to normal, about .2 amps on my tester. It was raining and dark while I was out doing this last night, so I haven't isolated the exact source of the short, but I've pulled that fuse in the mean time, so the battery doesn't keep draining while I wait to find the problem. The battery was still charged in the morning, so I'm positive I found the bad circuit. Now I just need to find the location of the short. I didn't see anything obvious at a glance. The fuse box indicates that the glove box light and the trunk lights are on that circuit. All of those lights work normally with the fuse in, with the exception of the built-in glove box flashlight. I tested current with the flashlight in and out of it's plug, and it made no difference in the current flow. Are wiring diagrams available online anywhere?

[Jeron]

It is common to tap the glove box light circuit to get power when adding devices to the car. I'd guess the previous owner removed something he had installed and left a short.

[Jeron]

Oh, and pulling the fuses one at a time as you did is the best way. I thought that wasn't an option based on your first post about it being such a large drain.