Gary Knox

I've had the airbag light staying on in my '94 for about 6 months for varying lengths of time. For example:

Haven't driven the car in several days, it would stay on for ~10 minutes. Stop at a store for 10 minutes, and it would stay on for ~5 minutes. Continue this stop/start pattern and 'maybe' eventually it would not stay on longer than the 2-3 seconds. This scenario would be repeated unless the car was driven every day, in which case it probably would not stay on more than a few seconds.

About 10 days ago, I felt my battery was weak (NAPA, 84 month with 3+ years left), even though it had a maintainer on it (car is stored for the winter). Checked voltage and it was about 11.9. So, I removed the maintainer and hooked up a charger. Charged at 6 amps for an hour, then at 2 amps for about 8 hours. When I removed the charger, the voltage had dropped to 12.1 within less than an hour.

Took it to NAPA, and they replaced it. Installed new one a couple of days ago, and put a charger on at 2 amps for 4-5 hours, then hooked up the maintainer. This morning, I disconnected the maintainer, and early afternoon I started the car for the first time since the new battery was installed. VOILA - the airbag light went off in 2-3 seconds. I think maybe it's a random event, so stop the engine. Thirty minutes later, I start it again, and the light goes off in 2-3 seconds. Once more later this afternoon and the same result.

So, I've concluded there is a capacitor in the circuit that must be fully charged with electrons for the airbag circuit to sense all is OK, and if the battery is less than full charge, that capacitor loses enough juice to trip the circuit - extended charging from the alternator brings it up to 'snuff'.

Look forward to hearing Alan's take, or that of any other Porsche 928 electronics guru who has an opinion. In the meantime, I'm a happy 'non-illuminated airbag light' camper!

SQLGuy

Low battery voltage will cause all sorts of odd behavior. I do wonder, though, if your use of a 993 airbag on the steering wheel makes any difference in this case.

Gary Knox

SQLGuy,

Well, perhaps it could, but I really doubt it. I've had that wheel/bag in place for about 4 years, and it was only in ~ the last 6 months that the airbag light stayed on and the 'notice' came on and stayed that way as I described.

My hypothesis is that the battery had gotten weak, but it still had sufficient voltage and current to start the car. That starting draw dropped the voltage enough that the Airbag system capacitor was low on electrons after startup. Then, the alternator was getting the battery voltage up to 12.6+, so it took a while to get the electrons into the capacitor.

Herman K

When I bought my car the PO had battled the airbag light on for 2 years it took me a while to solve the issue.
I did end up replacing the control unit.
I have an additional spare that you're welcome to use and see if it solves your problem.

Alan

Well the airbag unit does care about voltage. It indeed has a big capacitor that it charges so it is ready to fire the airbag igniters. It is designed to store enough energy that in an accident if the supply power is lost - it can continue to monitor the crash sensors and fire both airbags for a period of many seconds after the power loss. You can see why this would be important - the first part of an accident might sever the main electrical connections - say under the car - you'd still want the best possible chance that the airbags could still deploy when soon after you impact something solid.

If the supply voltage is too low the charge stored in the capacitor may be too low to meet this condition - and an airbag fault is the only way it can indicate this.

Alan

Tom in Austin

interesting about the capacitor and air bag remaining armed after ignition turned off. Advice I've always seen says to wait 30 mins after disconnecting the battery before touching the air bag system. Granted, there's a margin of safety in there, but suggests the capacitor can still fire the bags for more than just a few seconds.

Makes me think about the GM ignition-switch issue where the press reports say the failed switch "cut power to the airbags", resulting in death and injuries when the car crashed. Does the GM system have capacitors? If so, is it accurate to describe the effects as "cut power to the airbags"

Gary Knox

PM sent to Alan regarding the capacitor.

Tom, I've always used the 'suspenders and belt' approach of never doing anything relating to the airbag unless the battery has been disconnected for 24 hours!!!

Also, if you do ever disconnect the airbag, be sure you DO NOT turn the ignition key to the on position with the airbag disconnected. You'll not be able to get the airbag light 'off' after that without the use of the Bosch Hammer, a software program such as Theo's or similar.

davek9

Gary did you pull the "error code"?
Not sure if the Shark Tuner can do it, I think Andrew used the "Hammer" when we were looking at it.
Rob Budd's '91 had an Air Bag warning that would come on, cleaned connection under "A" pillars (not easy to do), no dice.
We would clear it with the Diag tool and w/ in mins it would come back, some times it might last a mile or two.
Finally Rob replaced the Air Bag control unit, problem solved for over two years now.

Dave

Edit: the code we got was the Airbag Controller, I'm thinking you seen the same on yours.

SQLGuy

Durametric can certainly reset and clear airbag controller errors. I would imagine it can read them as well.

siscogts

On my old bmw 635 abs light had the same behavior. Changed battery, to no avail.
Bmw sold me a new unit so I had the old one to open and look at, some condenser were not healthy anymore, changed them and tested the old unit. Abs light off,abs responding in the same way with old or new ecu.
Maybe something is ageing in Airbag ecu, but I will never deal with that system.