It looks like I found the source of the current leak, but the job is far from done. I pulled each fuse, one at a time, and when I got to fuse 27, a flow of only 40 mA was seen, so over a 100X reduction. It was 4+ amps after pulling each of the other fuses.

But my celebration was short lived when I found that fuse 27 protects not one, but three (3) circuits, as follows: Glove box light, windshield washer system, and passenger compartment/trunk lighting. None of these lights are on when the car is at rest. I suppose I could try disconnecting the windshield washer motor's connector - anybody know where it's located? Any other suggestions as to how to start hunting down the source of the huge current draw which apparently originates somewhere in one of these three circuits?

trader4: I read 4 amps (actually 4 volts dropped across a 1 ohm resister placed between the negative battery terminal and its cable) when I disconnect the negative battery cable from its terminal, and wait a good while (typically several hours). As stated above, this massive leak goes away only when I disconnect fuse 27, when it drops to what I understand to be pretty close to the normal residual draw that occurs when the car is in sleep mode.

OK, a couple of new data points, neither particularly encouraging. After I posted last night's post, a little voice inside my head questioned whether I had read the scale on my voltmeter correctly, and sure enough, I was off by 10X - with fuse 27 removed, I'm getting a 400 mA draw (0.4V dropped across the 1 ohm resistor), not 40 mA, which would be 0.04V. So, while pulling fuse 27 does give a 10X reduction in draw from the 4+ amps I get with the fuse in, it's still nowhere near the normal baseline level of 30-40 mA or so.

Second, I removed the connector to the windshield washer pump after putting fuse 27 back, and the current draw is back up to over 4 amps, so that's another (inexpensive) thing to scratch off the list.

I have to confess to not understanding very well what's to be expected as far as current draw when the car is in its various modes. Doing the testing the way that I am, with the negative battery cable disconnected now for nearly a week, and measuring the voltage drop across a 1 ohm resistor placed in series between the negative battery terminal and its cable, what should I expect the current draw to be? What does it actually take to put the car totally "to sleep"? I would think that having the battery cable disconnected for nearly a week should do the trick, but as I said, this is an area I'm not very knowledgeable. Thanks in advance the the continued advice.

You need to be in the car or else opening any of the doors will turn on one of the courtesy light and will wake the car up.

OK, I think I understand now. While I haven't ever re-connected my negative battery cable to its post at any time during my procedure, what I've been doing amounts to the same thing. I've been leaving the cable disconnected from its post, but then connecting a 1 ohm resistor in series between the cable and its post, and then measuring the voltage drop across the resistor. The problem has been that I had to disconnect the resistor after each test, because the current draw is so huge that it generates 16 watts (I-squared-R, where I = 4 amps, and R = 1 ohm), and the resistor is only rated at 10 watts, so if I had kept it connected long enough for everything to go to sleep, it would have burned up the resistor. This meant that I had to disconnect and re-connect the resistor every time I checked a fuse, so I inadvertently "woke up" the systems every time I tested a fuse.

So, here's what I just did:

- Removed the connectors to both of the rear hatch interior lights
- Pulled fuse 27
- Closed all the doors
- Connected the 1 ohm resistor in series between the negative battery terminal and the cable
- Measured the voltage drop across the resistor to be 0.4 volts, which equates to 400 mA
- Waited about 45 minutes
- Checked the voltage again - it had dropped to 0.038 volts, or right at the target 38 mA

So, I've established that my current drain is located somewhere in the circuits protected by fuse 27, and nowhere else (I've checked every single fuse in the vehicle, including the three that protect the blower, ruling out the FSR). Unfortunately, this one fuse, according to the label in the glove box, protects three separate circuits (glove box light, passenger compartment/trunk lights, and windshield washer system). I've ruled out the windshield washer pump by disconnecting its connector, so maybe it's one of the lights (other than the hatch lights, because I've already determined that I get the same 4+ amps flow with the hatch light connectors disconnected if I keep fuse 27 in place).

Thanks again for all the advice - I'm definitely learning a lot about how to troubleshoot this type of problem. Any tips on how to track down the actual source of the problem? I guess I'll start by disconnecting the glove box light.

I'm sure there are other things within the lighting system to check, but do all interior lights work properly when opening the respective doors? Perhaps you have a bad door switch that's causing a problem and can find it by its malfunctioning. Then i guess you need to look at how they tie into the unlock feature (I can't remember as my e53 has been gone for so long) as the lights come on with a remote unlock correct? you can do the same with glovebox switch and maybe check to see if the flashlight charger in the glovebox works properly too. Does yours have auto wipers? There might also be an issue with the windshield sensor if so....lot's to look at...electric gremlins are always the worst.