If you are locked out of your BMW with electric door locks and a flat battery, my experience might help you.

I own two E32 7-series cars, a 1992 735i and a 1989 750iL. The electric door locks are a nice feature. So is the location of the battery underneath the passenger side rear seat — good for weight distribution.

Problem is, when the battery is dead, I can’t get into my car any more. Right now, after months of being parked, each car has a flat battery.

Solution 1, courtesy of BMW, is to insert the key into either of the front doors, and turn the key slowly while lifting the outside door handle slowly and simultaneously. There is a mechanical link that barely works to unlock the door, this way. This makes the car time-consuming to steal, which is good. Presumably, you breaking into your own car will have more patience than a nervous car thief. In my experience, if you mess with this approach for half an hour to an hour, you can typically solve the problem, and get into your car. Yay!

Solution 2, courtesy of physics and simple wiring, is to energize the car’s electrical system externally. On 735i or 735iL cars with the M30 engine, this is not terribly hard though it still took me a couple of hours. I jacked up the car, and placed axle stands under the outer sections of the front sub-frame. Then, I placed a jack underneath the center of the front sub-frame, just in case. I angled it so it’s not in my way as I work on the driver’s side of the car, from below. With a 10 mm socket, I removed the 4 nuts and one bolt that held the plastic pan in place below the car. This enabled me to see and reach the starter from below. Then, I used a thin-nosed, sharp-toothed jumper cable.

I wrapped it in a pair of rubber gloves and used packaging tape so that only the very tip was exposed.

While lying underneath the car, I was able to discern the largest nut on the starter. This has at least one large wire leading to it. With the battery charged, this is normally positive. Working from below, I grabbed this nut with the teeth of the positive-side jumper cable clamp, and then attached the negative part of the jumper cable clamp to a place on the BMW with clean metal and a good ground connection. I happened to choose the power steering bracket. Then, I attached the positive clamp of the other side of the jumper cable set to the positive pole of a good battery. The last step would be to attach the negative clamp to the negative pole of the good battery, but … what if there was a place where my insulation was weak, so that attaching the battery would result in a massive short-circuit, possibly frying the good battery and/or making it explode? To be cautious, I didn’t clamp the negative clamp to the negative pole. I just brushed it over it, barely touching, and expecting the worst. I hoped to see some small sparks but not massive fireworks. Small sparks are what I saw. That meant the BMW was drawing some current, which is good and normal. (It might have been safer yet to connect the negative to the BMW body as a last step, now that I think about it). Encouraged, I attached the clamp, and then the door locks were energized, and I could unlock the car and get inside, yay! I removed the clamps from the good battery first, and then from the ground of the BMW, and then from the starter nut of the BMW. For the 750iL I’ll probably have to make another plan because the starter seems harder to reach. At least I have the problem solved for the 735i.

Solution 3, courtesy of physics and complex wiring, works on the assumption that you can unlock the trunk manually even if the battery is flat. With a healthy battery, the trunk courtesy light would come on, meaning there is a direct connection from the positive wire of the courtesy light to the positive pole of the battery, and from the negative wire of the courtesy light to the negative pole of the battery. By energizing these wires with a charger, it’s sometimes possible to trickle charge the flat battery inside the car. The trick is that the fuse in this circuit is sensitive enough that if you simply try to unlock the doors when the system is energized with a charger, you are likely to blow the fuse, and lose your opportunity to charge the battery with this circuit. The fuse is typically #33 and it’s under the driver side rear seat and thus not accessible with the door locked. I charged the battery for about 4 hours and then tried to unlock the doors with the charger contributing some of the charge. It was risky but it worked. When attaching the jumper cables to the courtesy light, remember that it’s the large central rectangular light just inside the trunk.

The red and white wire is the positive one. If you connect the polarity the wrong way around, you can fry the alternator diodes.

I connected the negative jumper cable to a ground connection instead of the negative of the courtesy light, so as to have some physical distance between the two jaws of the jumper cable set.

Once I was inside the car, I yanked up the rear seat to get to the battery. I undid the terminals, using 13 mm wrenches. Then, I could remove the battery to slow-charge it or replace it. I reminded myself that on these cars, the terminal cable color are NOT the traditional “red is positive, black is negative” but that “black is positive, brown is negative.”

I hope my article is helpful. And, if you ever need good used parts for your E32, please remember me. 🙂