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One of the most common annoyances associated with the late-model E36 BMWs is the somewhat troublesome air bag warning lamp. This warning lamp had dogged do-it-yourselfers for years. The lamp is like a little gremlin - there apparently is no specific reason why or how it is activated - any number of seemingly harmless actions can set it off. The airbag lamps on all three of my E36 BMWs have been on at one time or another. Sometimes the light will stay lit, and then go out after about 3 minutes of driving. Sometimes it will blink and then stay lit. Sometimes it will stay on continuously from when you start the car. Either way, each of these occurrences indicates that the airbag computer (also known as the SRS, or Supplemental Restraint System) has had a problem in the past.
The SRS warning lamp is often triggered for relatively minor reasons that do not require any maintenance on the system. Any number of harmless acts can trigger the SRS lamp:
- Driving the car through a large puddle, or being exposed to lots of rain
- Swapping steering wheels
- Removing the gauge cluster to replace light bulbs
- Aggressive and/or track driving
- Accidentally disconnecting an SRS component while hooking up electrical accessories, like an alarm or stereo
- Pulling a fuse for the SRS system while performing electrical troubleshooting.
- Bad seatbelt sensor in the belt receptacle (common failure)
You can't clear the lamp by disconnecting the battery - it will stay on. You can take the car to your local BMW dealer, but they will charge you anywhere from $80 to $300 just to diagnose the system and reset the lamp. However, there is a new tool available for the home mechanic that allows you to troubleshoot and diagnose air bag problems. The R5/SRS scan and reset tool from Peake Research (Figure 1) is very similar to the fuel injection code reader featured in our Pelican Technical Article on Reading Fuel Injection Fault Codes. The tool plugs into the same socket and allows you to diagnose the problem with your airbag, and also to reset a that annoying lamp. The instructions are very simple - just plug it in, read the codes, and then reset the lamp.
Most of the time, the airbag error code will be relatively minor, particularly if you know what caused the error. For example, the airbag light went on after I removed the gauges from my E36, and then accidentally turned the ignition on in order to move the steering wheel out of the way so they could be reinstalled.
Many people have told me that the airbags will not deploy when the airbag lamp is illuminated. This is an area of much controversy, as I have also been told by more than a few people that their airbags did deploy when the light was on. From this non-specific evidence, I would hazard to guess that the airbag system does indeed continue working when the light is on. However, the light is obviously there to warn of a potential fault in the system, so you should check it out immediately if it does happen to light up.
In addition, BMW factory documentation seems to indicated that the airbag system will still work if the airbag light is on, and there are only minor faults observed. I will quote from BMW factory documents:
72 01 93 (2137) - "The [airbag] Watchdog subsystem has the ability to disable the Output Stage in the event of certain faults."
72 01 85 (2042) - "If the SRS indicator light [lights up] the system is faulty; there is a risk of the system not operating in the event of an accident.
So it would seem, depending upon the type of fault recorded by the Watchdog system, the airbags may or may not deploy - it depends on how serious the fault it.
There are quite a few BMW Technical Bulletins available on the design and maintenance of the SRS system. Bulletin 72.01.85 details the SRS system, 71.01.93 discusses the later-model Central Activation Module, and 61.02.00 describes the procedures for installing various SRS sensor wire harness repair kits.
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