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post #1 of 7 (permalink) Old 09-10-2008, 11:00 PM Thread Starter
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Battery Safety Terminal???

Recently I went to start my 1999 BMW 540i (after leaving it parked for just a few hours) and all electric power was gone, not even the smallest interior light would come on. Assuming a dead battery, I was about to try jump starting when I slammed the trunk lid shut and suddenly all electric power came back on like normal, but when I tried to start engine all power went off again. So I opened trunk and checked for loose battery terminals and both seemed to be tight, but as I wiggled the large red cable coming out of BST all power came back again! This time I was able to start engine and drive car home without a problem and the Check Engine light never came on.

In trying to determine cause of total power loss, my first thought was the battery safety terminal (BST) at battery end of +ve cable. From reading other posts I understand that this is an "explosive coupling " that "disconnects the main battery feed to the starter and alternator" but "does not disconnect power to the other systems". I assume that means once this coupling has "exploded" it cannot be reset and must be replaced. Is that true - does it really explode and break apart what is a very thick battery cable? Anyone know exactly how it works? Is it a kind of fusible link and does it mean that any 1999 540i would be totally disabled and have to be towed away to a BMW dealer after an accident?

Anyway there's no sign that my BST has exploded (I'd expect to see it burned out) and since in my case (a) all power was lost, not just power to starter and alternator (b) all power came back after either slamming trunk lid shut or jiggling the +ve cable, am I right to assume that in my case this explosive coupling has not irreversibly deployed and so is not at fault here? If so that leaves me wondering what to do if this happens again, as seems likely? I now have a multimeter in car, so I could check for voltage across the battery terminals (check for possible bad battery with loose internal +ve terminal - this battery was installed by Autozone last year). Could this be a problem with a short or a bad ground somewhere? (I'm not good with electrics) Any other thoughts/suggestions very welcome.

Now I wonder if this battery safety device could be triggered by a rear-end collision because coincidentally this car was hit in the rear bumper in a minor parking lot incident last year, when another vehicle backed into it. This was enough of a jolt to set off the alarm, but only caused superficial scratch damage to the bumper and certainly the air bags were not activated and there was no loss of electrical power at that time. Could being hit in the rear bumper trigger air bag deployment and/or BST?

More related questions:

1. Can someone advise what would happen if for any reason battery power is completely shut off while driving? Would you need to immediately put it in Neutral rather than coasting to a halt in Drive and would that damage the automatic transmission? (I've never had such a thing happen with any car and never wondered about it until now). This would seem to be very dangerous for something that's supposed to be a safety feature! Would it result in damage to the alternator, computer, voltage regulator or what? I read that you should never disconnect the battery while the engine is running,

2. Any special precautions needed when working around this BST? Any chance that it could explode just from jiggling the cable?

3. I read one post where someone cut out the BST and rejoined the cable directly to the battery terminal clamp - anyone think that's a viable option (short of having dealer replace the whole cable for $600) or would that cause some other problems, like with the wiring for the air bags? (don't want those going off!)
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post #2 of 7 (permalink) Old 01-24-2009, 01:27 AM
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Hey delfletcher, did you ever find out the answer to your question(s)?

I have a 1998 528i in which the driver's side air bags both deployed after hitting a curb. I continued to drive the car home, and once I had it inside the garage, I turned it off, and immediately attempted to start the engine again.... but alas, as I suspected, it the engine would not turn over.
It appears I am victim to this Battery Safety Terminal device!

My question is, how do I fix it? Can I replace it? Or like you mentioned, just rejoin the cable directly to the battery terminal?
Does anyone have pictures that would help?

I have replacement air bags and air bag control module on order from ebay, as well as a new control arm. The last piece of the puzzle is to figure out how to start the car again!
Any help would be greatly appreciated!

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post #3 of 7 (permalink) Old 05-08-2009, 02:41 PM
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Were you able to rejoin the cable directly to the battery terminal clamp?

Also anyone having problems with your BMW starting or if you have been

Tapping clamp on battery and car then starts read the following. There was a recall, apparenty BMW fixed the problem? When I called in with the recall number I was told my model 323i 1999 was fixed, lol.. It was??? Then why the F am I having the same problems listed below. These cars are crap

Vehicle description: Passenger vehicles. These vehicles are built with a side air bag system consisting of door-mounted thorax air bags (rear door air bags are optional), a head protection system (HPS) for front occupants, a central electronic sensor and diagnostic system, left and right satellite impact sensors, and associated wiring. This system is unduly sensitive to certain non-crash impacts, such as contacting large potholes or curbs at substantial speed.
This could cause the side air bag and HPS to deploy without an actual side crash. In addition, the battery safety terminal (BST) would also activate, disconnecting the starter cable from the battery. In this case, vehicle electrical system would continue to operate and the engine would continue to run, but after stopping, could not be restarted.
Dealers will reprogram the central computer control module that regulates the performance of all of the vehicle's occupant protection systems. For detailed information & supporting documents
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post #4 of 7 (permalink) Old 05-10-2010, 11:59 AM
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I have a 2001 BMW 540i Sport Wagon with the same problem. It appears that BMW did not fix the issue between 1999 and 2001. It seems like this is a persistent problem. BMW should probably wake up their engineers and ask them to redesign this fragile thing.

Unfortunately, the repair piece costs $300-$500. I found one on eBay for $290 from the UK (I'm writing from an airplane some 30k feet above the Rocky Mountains). IMPORTANT: What assurance do I have that it won't just break again?

Next step will be to tear mine apart and see if I can simply solder the connection together. Otherwise, I vote for buying an aftermarket battery connector. Both of these options obviously bypass BMW's safety. But having such a huge, show-stopping problem such as intermittent and _complete_ loss of power is a higher priority than the chance that a fender-bender may cause a short. If the car is totaled, it won't matter whether something shorted or not.

Anyone have an alternative? Here's an opportunity for an entrepreneur to make some money.


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post #5 of 7 (permalink) Old 05-21-2010, 09:57 PM
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When I goy my car last Dec 2009, The check light for the air bag was on when i was driving the car, the last owner said that when he changed the battery of the car it suddenly appeared after.. He said a simple fuse will address the issue.

I brought the car to European Motors here in Nevada and what a surprise THE Red Battery Terminal was hitting the wall of the car and totally got burned. WOW!

The said battery that was use to replace the old one was much bigger and the clamp of the battery was not holding it well and thus it was moving..

My luck....I had to replace the electrical harness (Battery Terminal Cable (Red One) ) and it cost me around 490 plus 200 for the labor.

They changed the clamps of the battery to make sure it was not moving at all..

I just want to make it clear that when it is the safety issue of the vehicle like this which entails explosives to activate the air bag or in general could make the car burn I would have to bring it to someone reputable.

Peace of Mine..

I would immediately bring it to a professional. Not necessary a dealer... but someone you can trust.
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post #6 of 7 (permalink) Old 11-10-2011, 11:17 PM
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post #7 of 7 (permalink) Old 11-11-2011, 02:23 PM
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Car should start... There is a rather large connection array above the battery, inspect this closely for dodgy conns, AND pull the fat fuses, inspect then replace. Also check the common earth/ground to body...

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Eso es todo!
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