I have been thinking not less than a hundred times should or shouldn't I write up this post because the SRS Airbag light on the beautiful looking BMW is one of the trickiest issue to solve in the automotive industry. Well, at the end I have decided why not, if I can just share some of my experience with others and who knows, that might become one of the many thousand solutions on the internet that is just right for you.
Long story short, I been working on a customer's 04' 745li E66 with airbag light on in instrusment cluster and the 'very' informative message displaying on the iDrive screen:
In the first instinct, your workshop technician, and even the BMW technician, whom my customer first sent the car to, will say that: "oh, airbag unit is faulty!", "Your belt tensioner needs replacement", some will clear the codes and wait for it to re-occur, some will crawl everywhere under the seats looking for loose connections. The latter are standard pratices of the technician, nothings wrong with it. But jump straight into the conclusions that you need a replacement unit that will unfortunately cost you $1000 and it needs to be imported from Germany isnt't a very wise solution by the sound of it.
They are probably a lot of top auto technicians out there that can dig deeper to check resistance on the airbag modules, or advanced auto tech who can diagnose and code the system with fancy BMW Diagnostic Program. But the time and cost for you, as a customer will be way over the top to get this problem sorted out by professionals.
For me, I like to keep things simple - tackle the root of the cause. When did it happened? What was the last thing you did to trigger this message? Many people will go " I don't know. It just happened." Fair enough, sometime when you are about to go somewhere and started up your car and this thing came up staying on your dash, without any signal. Well I guess I am abit lucky as my customer recalled that the car was not in used for about 2 months with battery disconnected, when he planned to use his car he hooked up his car battery again but the battery was drained, so he put it on-charged. And after charging, he got the car started, but the Airbag light and iDrive message came up and won't go away. So he took it to BMW dealer, they scanned and cleared the DTC for him, but the fault came back again. He was told a new unit from Germany is needed. I am so suprised that they didn't even perform any testing or checking at all and opted for a trial and error process.
After some research, I was 200% confident that the culprit was the voltage supplpy to the airbag system. I don't want to go too technical on this matter as it would confuse many readers because they are multiple parts involved in just the power supply section like the battery, BST, B+, MRS, fuses to the airbag module etc.. So first thing first, check the battey. I used a DHC battery tester and it came up with "Good and recharge", battery voltage 12.54V. It looked fine to me the voltage so if I was just using a voltmeter to check the battery I would have thought that the battey is in good condition. Anyway, I pulled the battery out of the car and put it on my charger - it is very important to know if you are not familiar with batteries, that BMW stock battery is AGM battery. Choose the charging option carefully on your charger because you only want to charge the AGM battery with AGM charging option. If your battery is not BMW stock battery, you need to know how to differentiate what type of battery it is. Usually it will be just a standard maintenance free lead-acid battery instead.
I got 12.94V from the battery after it is fully charged with some surface charge on the battery of course. Cleaned the battery terminals with hot boiling water (you can add baking soda if you want) and inspected the battery before I put it back into the vehicle. Double checked all the cables on BST and the battery connectors to make sure they are all tight and secured. Started up the car, airbag light illuminated for the activation phase and went out, and no airbag message on the OB Data in iDrive. I didn't even have to scan and clear the codes this time.
The car has since been tested for another one and half day in the workshop on and off. Airbag light is not coming back and seems like a simple job like this fixed up the annoying error on the BMW. I hope this can provide a small assistant to all BMW owners when it comes to the airbag light issue.