E30 SRS/Airbag Fault Diagnosis

[chomowitz]

I have a 1990 E30 325i sedan. The SRS light on the check panel flashes and then stays on steady. I went through the procedure given in R3V Limited Forums > E30 Technical Forums > E30 FAQ and DIY > How To: GET and RESET Airbag Codes/SRS Light (link... http://www.r3vlimited.com/board/showthread.php?t=173007 ) and got the code 1-4-3 as a permanent fault (Description "04 Driver's airbag supply line faulty" with a Remedy "Check supply line for open or break" ). I would like to follow up, but I am not sure what "supply line" specifically refers to and where I can check for an "open or break".

I have the 1990 BMW 325i Electrical Troubleshooting Manual, but if anyone has guidance on how (and where) to proceed, I would appreciate the information.

Also, I purchased the SIR3 tool to reset the SRS light and it worked properly, but after turning the light off momentarily, the light cam back on after the tool finished the reset. This is consistent with permanent fault indication from the procedure mentioned above.

Any information to help me execute tests will be appreciated.

[TobyB]

Execute the executives, and the people will follow...

As to more testing, the 'supply' is the +12v voltage to the airbag itself.
I'm guessing the SRS module supplies 12v all the time, and then grounds the other
side to fire the 'bag. If so, it's sensing that the bag is not passing voltage...
Although, that doesn't seem as failsafe as switching hot...
...if it does switch hot, then it's sensing that 'hot' is floating above ground,
while it's holding ground down. Either way, voltage is not passing through the
airbag firing circuit, meaning that it's pretty sure that the bag won't fire.
WHAT, you say, IT'S RUNNING VOLTAGE THROUGH THE FIRING CIRCUIT??? well,
yes, it is- it's not running any real CURRENT, though, and the current is what fires
the thing...

You can CAREFULLY meter it in the harness or at the connector. You run a small
but real chance of firing the bag if you do it incorrectly.
You can also remove the bag, then meter all of its connections safely.
It's most likely that the clockspring is shorting out, or that one end or the other
has come loose. I've never done much more than look at them, and replace the
entire thing with one from another car.

If all of this seems a bit scary, it should- the airbag is a little explosive device,
and while you'd have to get 12v at real amps across it to fire it, that's not impossible
to do by accident.

None of this is made any easier by the manufacturer's desire to avoid liability by
not providing any real repair info, so you really do have to figure it out yourself, with
the (very questionable) help of idiots like me who post on the internet.

t

[chomowitz]

Thanks for the information.

I will try your second approach (remove airbag and meter connections). I'm too nervous about the possibility of setting off the airbag. I'll report back after I meter the connections with the airbag removed.

[chomowitz]

UPDATE ... I removed the airbag and steering wheel from my 1990 BMW e30 325i sedan with no real problems. Now I am having a hard time metering the connections to see if there is an open. I tried checking continuity between C260 and the airbag connector (that connects on the rear of the airbag (NOTE ... Airbag has been removed, so just checking connections with std ohmmeter), but the results were confusing (readings unstable, but ohmmeter checks out ok). I am now trying to isolate the slip ring, but I cannot figure out how to remove it from the steering wheel.

I di remove the outter plastic/rubber cushion on the rear of the steering wheel (after removal from steering column) and then (after marking parts to allow proper alignment on replacement) I remove 3 2mm set screws that retaing the inner housing to the outer housing. I then rotated the inner housing so that I could extract it. I can see the slip ring assy, but I am not sure how to proceed to remove it.

I search for a procedure to replace a slip ring, but the only one I found was for an E36 ( link ... http://bmwnut.blogspot.com/2006/02/e...ism3-slip.html ), but the instructions are not correct for the E30 25i 1990.

Any and all suggestions will be appreciated ...