[Thomachaan]

::The following was done for diagnostic purposes and as a temporary fix before buying a new positive battery cable. Defeating the Battery Safety Terminal leaves these cars vulnerable to shorting the battery, which could easily cause a fire. H8 batteries are big and tough, and could weld a screwdriver to its posts if you accidentally dropped one and shorted it::

Though it's been done before, I thought I'd share my experience in figuring out my Passenger Restraint Malfunction warning on my iDrive. ISTA+ shows code as:

93B2 - MRS Safety Battery Terminal

Before buying a new positive battery cable for my car, I needed to determine if the damage was deeper:

-Wiring
-MRS5 fault
-Passenger airbag module under right dash fault
-etc., I'm still learning the systems.

Based on the iDrive message, I was afraid the module itself was broken, or that my passenger OC3 matt sensor somehow fried, and that my airbags wouldn't deploy.

ISTA+ told me to measure resistance between the pins on the BST, specifically to ONLY use a BMW specified tool to measure. I think I understand why now, because even the small amount of current used by a multimeter in measuring resistance could damage the BST (by either igniting it or slowly burning it off, idk).

Well, while measuring mine with a regular multimeter, readings jumped between 0.2ohms and infinite (open circuit). I opened the plastic casing around my BST to check inside, that thing never fired off but something must have failed. Maybe when removing/installing my battery, maybe when my car hit a big electrical issue,

Following posts, I decided to jump the connector with a resistor between 1.5-2.5ohms. If it worked in clearing the code, I would have successfully diagnosed my BST as faulty. With the number of things I have to buy for this 3 series, the last thing I want is an extra working BST.

I don't have low value resistors around 2ohms (where'd all the Radio Shacks go??), but I do have a few 10, 12, and 22ohm resistors (electronics hobby projects). I decided to use as many of these as I could in a network to share the current load (since they're each rated around 1/4W). If the MRS module ever did try to fire off of the BST with resistors in place, I'd guess it'd send up to 12v for some amount of time, which would quickly burn a puny resistor to a bright red hot glow. The following resistor pack I made may seem over complicated, but isn't that how every system on an e90 goes? I did my DIY BMW style.

I soldered 2x22ohm resistors in parallel. Then I soldered a 10ohm to a 12ohm resistor in series (total 22ohms), made 8 sets of those. I individually wrapped each of these in electrical tape (to insulate), then soldered all of them in parallel (22 / 10 = 2.2ohms). Taped all that together, making a neat little resistor pack, and twisted the leads with pliers to a shape that would fit nicely into the BST connector.

After some bending and rebending of the leads I got the pack to make snug contact in the connector, tested by measuring resistance at the exposed part of the pins on the connector tip's underside.

I taped the resistor pack to the connector so it wouldn't vibrate out, then tucked the wire between the power distribution box and battery top brace. If it does fire off, it shouldn't cause a fire, maybe just get hot or pop. Car would still be able to start too, but there's the fire hazard.

After clearing the code one last time through ISTA+, the iDrive's passenger restraint warning went away for good. ISTA+ shows the MRS5 module as green in the module tree.

Pics cuz it happened:
#1 - The resistors used.
#2 - Vice to hold resistors in place for soldering.
#3 - Snipped excess leads, started individually taping series resistor sets.
#4 - Resistor pack next to a Honda key for size and V-TEC reference.
#5 - All taped up, chillen over the battery.