Testing for bad Battery Safety Terminal?
 

Hey all - just picked up a "parts" 4.6is today. Turns out it may be a better car then I was expecting so I may trying and fix it up.

One thing - airbag light is on and previous owner says the problem is the BST.

Car was in a slight rear end collision - no airbags were deployed. He says the problem is the BST on the battery cable.

The car runs and drives fine, actually. Reading about the BST it typically says BST goes bad and the car won't start.

So just wanted to see what folks here had to say. Is it possible the BST is bad but the car still starts? Is there a check for this issue?

Thanks!

Testing for bad Battery Safety Terminal?
 

BST blown you can't start the car without jump start and won't charge the battery.

BST is a literal circuit breaker between battery and alternator/starter.

When you have BST symptoms ish you need to check the B+ terminal at the firewall. If it's loose you'll get symptoms like a loose battery terminal.

First off what codes does it show and second what scanner are you using?

There have been times where folks had a BST giving similar symptoms before but it's such a simple, cheap fix that I would have thought the original owner would have taken care of the "one" [emoji1787] thing that made his X5 unreliable.

So let's just see what current and pending codes show up before any money gets thrown for parts.

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So to be clear - this wasn't the only issue. The kid selling it just didn't want to fix it up, and I think he was pretty honest about everything wrong. It was also super cheap. However, the engine runs strong (new chains too) and shifts great but it needs a ton of little things. Fine with me, fun winter project. Worst thing I have a parts car for my much nicer 4.6

I'm using a Launch and will have to plug in for codes again.

I've read BST can allow the car to start but cause airbag errors hence my Q. Will post back with more.

I have heard a couple of the guys have BST issues like you described (not 100% same) but I have also found that with a good battery many e53's with similar issues had connection problems with the B+ or the ignition switch.

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Good to know. There is lots of cleaning and checking for me to do tonight, although I may be in luck. Looks like the car had the recall done which means I have the OEM splice already done. That means I just need to find a good used battery cable and I can just cut the end off and use that with the replacement connection (if, in fact, the BST is the problem). The plastic ends look pretty nasty so I may just swap it for cosmetic reasons.

Will keep you all posted with codes and as I learn more.

If the car starts BST not the problem. Other end is where to look for problem.


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Is it possible the BST has been "spliced" or by-passed and the SRS system cannot read the BST status or has a code for a BST "fired"?

Remember, the BST is part of the airbag (SRS) system and is possibly monitored like the other SRS components.

The BST can be coded out according to a discussion on bimmerworks but I have no idea if it will show up as fired or maybe unlisted in that case.

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That's a good point.

Here's what the TIS has to say on the MRS/MRS2 (SRS) system..

Multiple restraint system (MRS)

Function
The multiple restraint system MRS includes the functions of the central activation unit ZAE2 and is expanded by the functionality of the side airbags for the chest area (thorax) in the front.

The MRS2 system additionally monitors the ignition circuit of the two head airbags in the front and the two side airbags in the rear as well as the safety battery terminal.

Functionality as with ZAE2
After "Ignition ON", the system performs a self-test which lasts approx. 4 seconds. The airbag indicator light is illuminated during this time.

If the control unit detects a currently present or already stored fault during the self-test, the airbag indicator light will stay illuminated beyond the 4 seconds, otherwise it will go out. The system is operational after the self-test.

Sensors are housed in the central control unit which record and evaluate the deceleration values an accident.

It receives the information about the occupancy state of the front passenger seat from the electronics of the passenger seat occupancy detector.

It simultaneously monitors (for US vehicles) the seat belt buckles and thus receives the information whether the seat belts are fastened.

The control unit processes all this information and decides according to specific criteria whether and when to fire the driver/front passenger airbag. e. g. the front passenger airbag/belt tensioner will not be activated when the front passenger seat is not occupied.

The MRS control unit can process max. 3 accidents with activation of at least one ignition circuit. An internal fault is then set in the fault memory which makes the renewal of the control unit necessary.

Additional functions MRS/MRS2
The sensors installed in the multiple restraint system mainly detects decelerations in the longitudinal direction of the vehicle. In order to be able to more quickly detect and evaluate decelerations in the transverse direction, which typically occur during side collisions, there are an additional two externally-mounted sensors or so-called satellites. They are attached to the cross members below the driver and front passenger seats and report to the MRS control unit, whether an accident situation was detected which make the activation necessary of one or both side airbags for the chest area (MRS) and furthermore the head airbags and the side airbags and the side airbags in the rear (MRS2). The MRS or MRS2 control unit compares these messages with the measurement of the sensors installed in the control unit and then activates one or multiple side airbags if necessary.

Components of the system

MRS/MRS2 control unit
There are sensors installed in the MRS/MRS2 control unit which record and evaluate the accident decelerations.

The MRS resp. the MRS2 control unit decides which ignition circuit shall be activated independently from the information of the periphery and the installed sensors.

Airbag indicator light
The airbag indicator light lights up during the self-test which is performed after switching on the ignition and if a fault in the system is currently present or stored in the system.

If the control unit is disconnected from the wiring harness, the airbag indicator light lights up through a bridge in the connector of the wiring harness.

Passenger seat occupancy detector (SBE) (encodable)
A seat-occupancy mat installed under the seat surface of the front passenger seat changes its resistance depending on the load that is currently located on the seat. This load is evaluated by the seat occupancy electronics and sends the result "occupied"/"not occupied" to the MRS resp. MRS2 control unit via a data line.

The status "occupied" is detected when more than approx. 12 kg load is present on the front passenger seat.

Note
For safety reasons, the "occupied" status is still detected for approx. 2 minutes after removal of the load.

Satellites (sensors for side airbags)
The satellites (sensors for side airbags) are attached to the cross members below the driver and front passenger seats.

They are connected with the wiring harness via a 3-pin connector: Voltage supply and the data line to the MRS resp. MRS2 control unit.

The sensors for the side airbag detect accident decelerations in the transverse direction, which typically occur during side collisions, and notify the control unit that they are detecting an accident situation which makes the activation of one of the side airbags necessary.

The satellites are a mandatory part of the MRS/MRS2 and are not encodable.

If an open circuit of the data line between the control unit and one of its satellites occurs, a diagnosis can only be performed after approx. 30 seconds.

Warning!

The installation and removal as well as the disconnecting of the satellites may only be performed with the battery disconnected. This also applies to all work which could cause vibrations of the satellites.

Seat belt buckle contact (encodable)
Switches installed in the seat belt buckles signal to the control unit whether the seat belt is fastened.

Ignition circuits
- Driver's airbag (encodable)
- Front passenger airbag (encodable)
- Belt tensioner driver's side (encodable)
- Belt tensioner front passenger side (encodable)
- Side airbag chest area (thorax) left (encodable)
- Side airbag chest area (thorax) right (encodable)
Additionally possible ignition circuit for MRS2
- thorax side airbag rear left and right (encodable)
- ITS head airbag front right and left (encodable)
- Safety battery terminal

The MRS in the workshop
There is a test adapter for the 50-pin connector of the ZAE2 of the MRS and the MRS2.

Caution!
The installation and removal as well as the disconnecting of the MRS resp.MRS2 control unit and the satellites (sensors for side airbag) may only be performed with the battery disconnected!

After renewing the MRS resp. MRS2 control unit, this is to be encoded.

Seat occupancy detection
The status of the passenger seat occupancy detector (seat occupied or not occupied) can be read out via the diagnosis.

For safety reasons, the "occupied" status is still displayed for approx. 2 minutes after removal of the load.

Thanks all for the posts. Well - I did some digging today, including some internet "research" as well as junkyard diving. Here's a little mythbusting I can tell you after a day of playing around.

First, your BST can ignite and the car can still start. Mine is, in fact, this way. There is a video that shows how to repair a battery cable which also explains how they work.

https://www.youtube.com/watch?v=s-XO...nnel=BavTekInc.

The end of the (+) cable with the BST is a separate part from the main cable. The main cable is "pressed" into the end with the charge, and when the BST ignites is shoots the stud of the cable out so the car won't start.

Since the cable was made to be separated, all you have to do is press the two ends back together if the BST ignites. You can easily do this by hand, and the car will start although you will have the BST fault. SO - this explains how you can have the error code and still have a car that starts as it's trivial to repair the cable to make the car run.

This also means it's really, really easy to restore this cable without the cost of the BMW repair cable - you just need to find an unexploded terminal end from a parts car.

The TRICK is that the yellow SRS looking connector is molded INTO the metal battery cable end so you CANNOT remove the connector from the terminal end, you must remove it from the harness connector by the rear seat. This is also why it's unlikely that you can just get one from another car since that cable is a specific length.

I ended up purchasing just the cable end off ebay, as there is a guy who clearly hits the yards and correctly removes good BST's precisely so you can repair them. Pretty good deal for the money since he's done the work to find a good one.

https://www.ebay.com/itm/263752369902

I'm going to try that repair and report back if it clears the codes. (since I hit the junkyard today I didn't have time to pull codes, sorry)