Z4M died: need thoughts

[ncnmra]

I was driving back from a trip today and about 100 miles into the trip, at a country road stop sign I head a "pop" from the passengers seat. I thought it was a water bottle that I had rolling around under the seat. Seconds later the car stalled. Upon trying to restart I got nothing but a "click", it wouldn't crank at all. Dash lights appeared, but CEL and EML were on. I came across a helpful hitchhiker and in the process of moving the car off the road we figured we'd try to bump start it in 2nd and it worked!!!

I returned his favor by driving him 20 miles back in the direction I came from to a larger town. For the entire drive it was totally fine. We stopped in the town and I shut it down. Went to the bank to get some cash and tried to start. Same thing, dead. I bump started again and decided to risk the rest of the drive home because it was driving perfectly fine for the last 20 miles; no lights on the dash EXCEPT for the airbag light. Weird. Thought maybe the starter was bad.

I got about 25 miles down and all of a sudden the entire dash lit up. CEL, EML, ABS, seemingly everything and the car started losing power. I tried gearing down and getting more powering but it wouldn't Rev past about 2500 and it appeared that my tach and speedo were non operational. The car was stuttering and eventually I had to pull over. I tried cranking, but it was completely dead.

I had a booster pack with me but I'm not sure if it works. I disconnected the battery and put in the booster and car was completely dead. At one point after reconnecting the battery, I managed to get 1/2 a crank a few times but nothing more. I was able to put the top and windows up, but I could hear that the battery was strained. Eventually even my hazards stopped blinking and when I turned the key, only the ABS light came on, nothing else.

I'm writing this from my wife's car as she came to pick me up, and car will be towed to an Indy mechanic tonight.

Any thoughts? I'm thinking alternator....not sure if the "pop" I heard (definitely sounded like passengers seat; NOT engine bay) was just a huge coincidence? Is there a fuse there that could have blown if perhaps the alternator shorted?

The fact that it drive relatively normal for about 30 mins and then slowly died was perplexing, although I thought perhaps it was running thanks to the battery only which eventually couldn't take any more.

Any thoughts would be much appreciated.

[dc_wright]

Does sound like you lost the alternator and ran on the battery until it died. Pretty quick diagnostic to determine that when it gets to your indie's shop.

[Vanne]

Not sure why on earth these cars don't come with a freakin volt meter indicator.. Hope its something simple bud.
If i had to put money on something, id say DC is dead on.

[ncnmra]

I generally agree but there is one thing I don't understand. In the moment, I tried boosting the car at both the battery and also under the hood. With the battery disconnected, I couldn't even get the booster to power the electrics (completely dead).

I tried again this AM when I went to check that the car made it to the shop. Same thing, tryinf both a booster and a known good battery, I couldn't get any life out of the car (i.e.: door locks) with the negative of the battery in the trunk disconnected. However when I reconnected the trunk battery, I could operate the doors. What am I missing? Shouldn't I be able to boost the car with the battery completely removed? I thought if I apply a good battery to the positive post and the chassis, it should work, but I got NOTHING.

[yousefnjr]

^^havent tried myself, but prob incomplete circuit with the batt removed. You shouldn't ever need to disconnect the battery to jump start. Leave your batt hooked up, and connect your leads to the points under the hood

I assume you're using something like this?

[yousefnjr]

Should also add that I don't know what the pop was, but the low revs/ no power is your car going into limp mode. Should have some codes stored.

IIRC the fuse box is behind the glove box on the passenger side, I've never heard one make a loud pop before but it's kinda in the area you heard it from... worth a look

[XMetal]

What "booster" are you using?

A battery "booster" is exactly that - it helps boost the battery. It's only a "helper", if you will. If the battery is completely shot, the booster won't do jack. It won't be capable of starting the car without the battery being somewhat alive.

Also, having the correctly sized jumper cable will determined whether the car starts or not. A few years back, when the battery on my car was completely dead, a buddy tried helping me jump-start it with a normal 10-gauge jumper cable - the car just made a click...nothing. My neighbor brought over a heavy duty 2 gauge jumper cable and the car fired right up. Just to show you that with all the current needed to start the car, the tiny resistance difference in the cables is all it takes to start/not start it.

[inTgr8r]

Originally Posted by ncnmra

I generally agree but there is one thing I don't understand. In the moment, I tried boosting the car at both the battery and also under the hood. With the battery disconnected, I couldn't even get the booster to power the electrics (completely dead).

I tried again this AM when I went to check that the car made it to the shop. Same thing, tryinf both a booster and a known good battery, I couldn't get any life out of the car (i.e.: door locks) with the negative of the battery in the trunk disconnected. However when I reconnected the trunk battery, I could operate the doors. What am I missing? Shouldn't I be able to boost the car with the battery completely removed? I thought if I apply a good battery to the positive post and the chassis, it should work, but I got NOTHING.

Did you check the circuit interrupter for continuity at the battery?

[ncnmra]

The booster can start a car on its own. I tried it this morning on my Ford Explorer 3.5L. I disconnected the negative and then put the booster on the negative lead and the positive post. Started with no issues.

My concern is that plugging in my booster (or a battery on cables) did not give me ANY electrical with the car battery disconnected. With the battery connected, putting the booster on at the engine bay didn't help. It's like there is no connection from the engine bay positive post to the battery. I'm not sure how the positive post in the engine bay is wired.

The circuit interrupter is an interesting thought. Where is it located, how does it work and is it replaceable or resettable?

I noticed a "black box" on one of the positive leads, but I didn't bring a multimeter with me.

The "pop" could have been a big coincidence but who knows. I managed to open the fuse box but didn't bring a multimeter to diagnose.

[ncnmra]

I should add, when I was trying to boost at the battery, I couldn't get at the actual post because of the red plastic guard, but rather I clamped at the metal ring that is visible on the wire going to the LEFT from the battery. This ring appears to be connected to the wire, but potentially NOT. Also this ring is right before the "black box" before the battery terminal.

Assuming this black box is the circuit interrupter, this would answer a few questions. Also it is possible that the "pop" I heard was this, since it is basically in the passenger side, right behind the wall.

EDIT: I started searching, and it could be due to the "BST" cable.

http://www.bimmerscan.com/bmw-battery-safety-terminal/

If this is the case, it would explain a few things.

[ncnmra]

This post really makes me think that its the BST that went.

http://www.bimmerfest.com/forums/sho...d.php?t=461819

When the BST blows, it disconnects the alternator and starter. That's why I could bump start, but the car ran only off the battery. I'm pretty much 99% sure that's the issue, and explains why I could see the "metal ring" protruding from the battery wire...normally it would be inside the casing, but it got pushed out by the gas discharge tube.

[antennahead]

I wonder why it would just "blow" on it's own with no accident involved? Call me crazy, but I think the BST is typical BMW overkill.

[ncnmra]

All the symptoms make sense that it's the BST.... BUT why it would blow CONPLETELY unprovoked is beyond me and infuriating. I had literally gently stopped at a country intersection when I heard the "pop". No pothole, no impact, not even an aggressive turn or breaking.

I'm really tempted to bypass it all together. I see zero safety benefits if all it does is kill the starter and alternator. All other electrics continue to work.

[David70]

All of what you found makes sense. I also think like any other fuse, it blowing after many years is likely not to mean anything. If I were in your shoes I would try to replace this "fuse" with a cable without the fuse, I see it unlikely to make any difference, I don't believe most cars come with it and another problem area to cause your car to not run.

[ncnmra]

Originally Posted by David70

All of what you found makes sense. I also think like any other fuse, it blowing after many years is likely not to mean anything. If I were in your shoes I would try to replace this "fuse" with a cable without the fuse, I see it unlikely to make any difference, I don't believe most cars come with it and another problem area to cause your car to not run.

My gut feeling is to agree. BMW charges some astronomical fee for this "BST Cable", which IMHO is pointless. If it doesn't disconnect ALL of the electrics from the car then it is worthless.

To me, this safety feature actually caused a safety hazard. Fortunately it was a warm day and nothing happened, but a breakdown in the middle of NOWHERE due to a failure of a "safety" feature is the definition of irony.

It is at the mechanic's shop now and he will get back to me with options. I will do my own research as to any detriments of bypassing the whole thing. I know for one thing that you need to install a resistor to simulate the BST module otherwise the airbag light will remain lit.

EDIT: Here is a good cross section showing how the thing works. My mechanic will check if it actually "blew", but I'm thinking the "pop" I heard was exactly that. Again, no potholes, no collision, no impact, in fact the car wasn't even moving.

[ncnmra]

For everyone's reference (including my own), I think the replacement part is BMW # 61127841239, which supersedes #61127837136.

https://www.ecstuning.com/b-genuine-...s/61127841239/

[GuidoK]

Originally Posted by ncnmra

I see zero safety benefits if all it does is kill the starter and alternator. All other electrics continue to work.

All other electrics are protected by fuses, so if there's physical damage to another component which would cause a short, its fuse protected so the fuse blows.

HOWEVER: the alternator and starter are NOT!
So physical damage to those causing a short would start a fire or battery explosion. Thats the reason for this device, so its function makes perfect sense from an engineering pov.

[ncnmra]

Originally Posted by GuidoK

All other electrics are protected by fuses, so if there's physical damage to another component which would cause a short, its fuse protected so the fuse blows.

HOWEVER: the alternator and starter are NOT!
So physical damage to those causing a short would start a fire or battery explosion. Thats the reason for this device, so its function makes perfect sense from an engineering pov.

Point very well taken, and as an EE I'm embarrassed that it didn't cross my mind. Regardless, I'm pissed off that it went without provocation, and that a replacement is going to cost me a fortune. I agree that I won't screw around with trying to bypass it.

I have asked the mechanic to test the alternator. I am concerned that blowing this thing may be because of a failing other component. I did change the battery out last year.

Can anyone confirm: there is no "coding" required on these vehicles if the battery is changed? I'm pretty sure I researched that issue to death before switching out the battery, but I want to ensure that the battery isn't being overcharged or something which could have cause this thing to blow.

UPDATE: I have ordered PN 61127841239 from the local BMW dealership through the mechanic. $318 CAD+tax. Mechanic tells me that it calls for 2h of labor. Apparently you need to lift the car up and unbolt a bunch of frame bolts to get to the bloody nut that holds the positive terminal to the floor of the vehicle.

I want to have the codes scanned first in case there is something there that may indicate why the bloody thing fired in the first place.

UPDATE 2: I brought the car home because the parts need to come from Germany and will take over a week. I can confirm 100% that the BST (sometimes referred to as SBK - Safety Battery Terminal) has deployed. Manually reassembling it allowed the car to start like normal, with only the airbag light on as expected since the BST module is toast. I drove the car home on a booster.

[JCF]

Coding is not required for a new battery.

[ncnmra]

Update: I ordered the BST cable and had it replaced at an indy shop. It took them about 2h, because the battery cable must be installed from the bottom. Part took about a week to arrive. In that time I re-connected the BST, and "simulated" the sensor using a 0.25A fuse which has the same resistance (~2ohms), and cleared the Airbag light. My theory was to see if it had triggered on its own, or if the computer triggered it. Car drove fine for 200 kms, with no lights etc, so I'll chalk it up to a random failure of the BST.

New cable installed and all is well. Thought I'd post the update in case someone else has this issue in the future. I didn't want to simply "bypass" the BST for the reasons stated above.

[Kream]

Mine randomly popped last year while driving home as well. Road was a little bumping but no pot holes (135k miles). I bypassed the BST all together but the airbag light is on. What did you do to clear the airbag light?

[kav]

Mine went randomly too. I just got the airbag light. Suffered through the ridiculous dealer replacement charges and works fine.