I had an owner of a 2003 BMW 530i ask me to look at his car for an intermittent starting problem. At times while turning the key forward to start the vehicle the car would crank for approximately two or three seconds and then stop cranking without starting the vehicle. Other times it would "click" as if the battery were dead. After doing this two or three times it would finally start. When I tried to start the vehicle myself it did the cranking for approximately two seconds and then just stop cranking. After the fourth try it had started.
I began diagnosing by opening the battery compartment in the trunk to test the battery and check the battery cables. The positive side of the battery has two large cables connected to it and two small wires coming out of a black plastic box connected to the battery cable that comes from the battery all the way to the front of the vehicle. The other large cable (approximately a 2 gauge wire) connects to a metal connecter which is connected to the side of the black plastic box. This cable goes from there to a junction box less than two feet away from the battery. As soon as I touched the cable going to the junction box I nearly burned my finger due to intense heat. I then looked at the cable and noticed that the insulation was melted. The black plastic box also showed signs of melting.
I disconnected the battery cable that goes to the junction box and upon further inspection noticed that the battery cable had corrosion on the inside of the insulation close to where the connector to the black plastic box is. I also noticed that the battery was not secured which was allowing the battery to slide back and forth upon acceleration and braking. I was able to see that the battery moving was causing the burnt battery cable to rub against a plastic heat shield which had worn the insulation down. The plastic heat shield also showed signs of melting.
The cable that was getting hot which connects to the junction block has absolutely no fuse of any kind so the only thing that will stop this from burning and causing a fire would be if it melted the black box on the other cable enough to set off the small charge in the box which causes the main battery cable to break away from the battery terminal. The small black plastic box has two wires going into a yellow connector inside the box. This has a small charge in it which is supposed to go off when the air bag is deployed in order to disconnect the positive cable from the battery. The cable with the small plastic box connected to it is $450 and is a dealer only item.
This is the second BMW vehicle on which I have seen this cable which goes to the junction box with melted insulation. After fixing this on the vehicle I was working on this time; I decided to go on line to different mechanic's websites which I use and I found that this is a common problem on BMW vehicles. I have found it is common for the battery hold down to break or the metal tray which is connected to the floor of the trunk to break.This causes the battery to bounce around which causes the battery cable to rub against metal until the insulation begins to wear away which will cause a short to ground once the exposed wire touches the body of the vehicle.
This most recent BMW I worked on was completely missing any battery hold down so I used wood temporarily to stop the battery from moving around in the compartment until the owner can get to BMW and buy a proper battery hold down.
I suggest that anyone with a BMW with the battery in the trunk check their battery and cables on a regular basis to ensure this is not happening. I do not know exactly which years this is a problem on but I have seen this on a 2003 and a 2002 and have read about it happening on vehicles between the 2000 and 2004 model years. It may very well happen on other model years as well so it is a good idea to check this if you have a BMW with a trunk mounted battery regardless of year.
The owner of the 2003 530i also told me that he had taken it to a BMW dealer twice for the same issue with the starting problem and both times they stated that they were not able to find anything wrong. He said they stated that they hooked the vehicle computers up to their scanners and everything was fine. It is almost laughable the way that some mechanics rely solely on the computers to tell them what is wrong. There are still many problems on vehicles these days that can't be found with a scanner but rather with some common sense, time and effort.
This is a photograph of the original cable. You can see the insulation melted in the photo.
This photograph was taken after I changed the burned cable. This photo is a bit blurry but if you look good you can see the melting mark on the black box next to where the new black cable is. The purple is simply a corrosion inhibitor I put on the connectors. If you look closely at this photo you will see a tube connected to the battery at the bottom left of the photo. This is a vent tube which must be in place in order to vent the battery gases. The one in this vehicle was not connected when I first looked at it.
This is a photograph of the junction box in which the battery cable was connected to. The black cable coming from the bottom is the new cable I installed. The white panel is held on with clips and is easy to remove. Once it is off you will see one nut that holds the black (originally red) cable in place. Again; there is NO fuse protecting the cable that goes from the battery to this junction box.
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