Beiträge von Trono

I think you can disregard my last post. I checked something I should've done some time ago. The restistance from the fuse in the power distribution box with only one fuse installed at a time and to the connector to the SRS ECU.

This is the result. (And here are the schematics again)

Fuse 54 (55 removed) -> SRS Pin 5 : <0.5 ohm
Fuse 55 (54 removed) -> SRS Pin 5 : inf resistance

Fuse 54 (55 removed) -> SRS Pin7 : 82 ohm
Fuse 55 (54 removed) -> SRS Pin7 : 7 ohm

No connection from any of the fuses to Pin 15 (through the occupancy mat (if I have one))


As far as I can read the schematics I guess I've found a short, but isn't it weird that it's only from fuse 54 to pin 7 and I'm not reading a connection from fuse 55 to pin5? Anyone have the brainpower to figure this one out?
Where the fault most likely located or best way to troubleshoot from here?

Thanks for the advice.

I just had time for a quick check under the seat. Found one loose connector and took a picture of it. Crappy mobile picture, hopefully clear enough.
Didn't have time to test anything, but does anyone reccognise this one? Could it be for the airbag? I noticed the color coding on the wires doesn't match though.


Edit: I had an half hour later this night to head back to the garage to do some testing. I meassured the contact and there's contact between pin 1 and 3, no contact from 1 or 3 to pin 2, which seems like it should according to the schematics I posted earlier (if this indeed is the airbag connector). Pin 1 to 3 goes through the mat, pin 2 is ground.
But I meassured that there's almost zero restistance between pin 1 and 3. I did not raise if I sat down in the seat either. Not sure what it should read here, but as far as I know the sensor should have some resistance that should rise when I sit down..?

Starting to think as you also mentioned TerryY that this can be that cause. Now that I feel I've tested almost everything else it wouldn't suprise me if the former owner got a faulty sensor mat and tried to "fix" it by disconnecting it and installing a ECU without the sensormat coding.

Appreciate all input here though. Should I just get one of those cheap occupancy mat overrides from ebay or is there anything else I should check? Anyone know for sure what readings I should get from the mat?

Argonaut: You remember the error code and did you get it fixed?

I recieved the slip ring today and fitted everything together, but still no go. Just to be sure first ran a diagnostics with just the slip ring, then with both the airbag and SRS ECU swapped as well. Still the same error.. *sighs*

Also ran the diagnostics with the charger left on and I've checked the battery connections and the wires I can see. They pretty much look as new.

Is there any good way to see what kinda voltage the control units get (and if it's stable)? I.e. connect the multimeter somewhere?

Another thing came to mind as well. At BMW the diagnostics reported that there was no passenger occupancy sensor installed. Isn't that a bit weird when I do have a passenger airbag? The mechanic thought this was unusual as well, but he meant it probably didn't matter since it don't give an error.

Since the control unit also reports as a newer date than the car (car build date is 09/93, SRS unit is 12/93), is it possible that it has been changed once and is not coded correctly? Dunno if that could result in the error I am getting.

I agree with you on the low voltage issue. That's why I though about running the diagnostics with a smart charger connected to the + connector under the hood to elminiate the possibility of a bad connection from the trunk. Haven't gotten around to try this yet though.

When I bought the car off a dealer they said the alternator had been replaced with a new one as the old one didn't work when they got the car. I guess that's the reason why the low voltage was stored in so many modules, but only way to be sure is to get all modules cleared and check again.

I checked both batteries with a simple charge tester and both checked out as OK with >12.5v charge.

Allthough it's worth to mention I had some troubles that I suspected to be because of a bad battery charge a few months ago. As it started to get colder outside the engine sometimes only cold-started on six cylinders and I even got the Trans FailSafe error once - but then I took the car out for a 3-4 hour drive and the problems haven't returned since.

Anyways, for now I'll wait for the new slip ring to arrive and just hope for the best. Starting to get tired of this problem now

The low voltage message came back again instantly after deleting the error message. I had been out driving for 2-3 hours before going to the BMW garage as well as checking my batteries before to see that they both had a good charge. The mechanic told me that if I had one just slightly bad power connection and the airbag is usually the first unit that starts to fail in the car.
We tried to clear all modules but the diagnostic unit crashed one time and got a communication error the next, so that's when he started suspecting a bad connection from the batteries.

The diagnostic unit they had didn't give any more instructions on the airbag though, but just in case I ordered a new slip ring last night as that's pretty much the only "movable" part I haven't tried to change yet.

And the saga continues..

I dropped by one of the local BMW garages today.
They charged about £30 for a full diagnostics (another one in town wanted close to ~£200(!)) and when I got there they let me use their garage and equipment for a couple of hours trying to figure this one out. Good service!

Anyway, the diagnostic gave the same error as I got with Carsoft, but with their equipment it also said Low Voltage as a detailed error. The samme error seemed to come again on several modules and the diagnostics eq got a communication error a couple of times. Many of the errors could probably relate back to when the old alternator stopped working, but communication errors can probably relate to low power.

Since I first was there they let me test another control module and a third airbag just to eliminate those completly. Still the same error.

Both my batteries should be fine and fully charged, so I guess I'm left with the extremly tiresome job of going over all connectors.

I'm thinking about the option of connecting a smart charger in front under the hood and run another test, but not sure if that's very healthy for the electronics. Allthough I guess this older electronics can handle more than the very fine electronics in new cars. Anyone with any experience on this?

Okay, that's good to know. I have access to a alot of E36 parts through friends. I'll see if I can find something suitable from there.

Hopefully I'll have time to drop by BMW for a full diagnostics next week too.

Both bulbs in the instrument cluster was fine. There was one removable and one soldered to the PCB. Just in case I also swapped the removable bulb with another one and checked continuity from pin 22 to 23 on the backside of the cluster.

Also checked the wires from pin 7 on the srs control unit to pin 22 on the cluster. All is fine.

Guess that leaves me the control unit again.

By the way, does anyone know if you can recode a used unit? As far as I know it's the same unit that's on the 7 (and 5?)-series. Atleast that would save me some money since I'm not even sure it'll work.

Aha, thanks Timm, that is some really interesting information! Heading out into the garage in a few minutes. If I figure anything out I'll let you know.

Zitat von Deep;70969

Thanks Reinhard and Trono, can I reset the light by disconnecting the batteries?, and if so, if it was just a temporary fault then the airbag light should stay off, am I correct in thinking this?

No, the fault is stored in the module's memory and needs to be cleared with proper diagnostiscs equipment. You can buy this off ebay (i.e. Carsoft), but as mentioned earlier in this thread it is not the most stable system with such old cars. The best solution is to get it checked at a garage. Only problem is that they might charge as much as buying the carsoft cables from ebay.
There are other reset tools too, but I have no experience with them.

This is starting to become a long thread, but I think I might just realised something here and hopefully this can help others with the same problem..
I found another page with schematics to E31 built from 09/93 along with this information on another forum.

Zitat

Fault code 2 >>>Airbag Ignition Circuit - Drivers Side - Defective
This fault is often stored during repair of other components (removal of steering wheel or instrument
cluster). Disconnect connector from generator module. Check wires from control unit pin 10 to contact
ring pin 1 and pin 11 to contact ring pin 2 for continuity. Measure resistance of squib (detonator) at
generator module (1.8 - 5.6 ohms). Replace if out of range. Check steering-wheel contact ring. Connect
generator module and measure resistance at control-unit plug between pin 10 and pin 11 (1.8 - 5.6
ohms). Replace steering-wheel contact ring. If the fault code recurs, replace control unit.

Alles anzeigen

Doesn't that leave the chance that the fault might as well be a short or broken circuit from the instrument cluster to the airbag control module? Does that sound reasonable?
If so can anyone point me in the right direction on how to check this? I've never even removed the instrument cluster before..

Those resistors that Reinhard talks about can be bought as a "plug and play" connector off ebay for about 10$.

I used one myself on my old E46 3-series and it worked like a charm! Took me 5 minutes top to install too.
The only disatvantage, if you can call it that, is that the passenger airbag will depoly in an accident even if there's no one in the passenger seat...

You will also have to clear the error stored in the airbag module after installing a resistor like that.

No, it doesn't matter, but as long as the airbag light is lit the system is deactivated as far as I know. So in other words, don't crash

A reset with a reset or diagnostics equipment might clear the fault stored, if not you have to do a diagnose with the proper equipment.

Okay, thanks for those links! I'll certainly read through them. Don't think I have a seat-occupancy detector either. As far as I know these are the correct schematics for my e31.

Just curious; Would an E38 750 steptronic transmission fit on on an e31 with the M70 engine?

Yeah, that thought struck me too..

BMW had the SRS2 system from 91-93 and the SRS3 is from 93-
Since my car is built in '93 I realised there was probably a chance it had the SRS2 system. With SRS2 error no 2 is "Front sensor triggered - multiple times"
The ignition circuit error message Carsoft gives me is on the SRS2 error code 44 and 45.

To check this I ran a diagnostic check without the airbag connected, but I did not get any new errors.

Source: http://www.scribd.com/doc/1703639/BMW-ERROR-CODES

Of course I guess that there's the chance that Carsoft even got the error code wrong.
Also when entering my VIN number into realoem.com the airbag module that came up is dated 9/93 - '96

So I feel pretty comfident I got SRS3 installed, but I'm not 100% sure. I've asked a few places before, but no one could really confirm what system I have installed.

No matter what I plan to go to the local BMW dealer and get a proper and full diagnostic of the car after this winter, but I was really hoping I could fix the few problems I know of on the car while it's in the garage.

Still no go..

I found the Airbag control module under the middle console and I did a resistance check on the wires, but they are fine.

So, to sum up.

- I've tried two different airbags. I know atleast one of them was functional in the old car.
- I've tried to use a 12v 10w bulb instead of the airbag which gave about 5 ohm resistance
- Checked, double and tripped checked the slip ring
- Checked the whole wire harness from the steering wheel to the airbag module.

Also made sure that Carsoft is working properly and I see that erasing the airbag memory clears the SRS lamp for about 5 seconds before the error is back. (The airbag also reports as OK if I do another diagnose before the light come back on again.

So I guess that leaves me with the module itself beeing faulty..?
And a new control module will have to be coded, right?

The only thing I've noticed is that according to the vin number my car is produced 9/93 (not 11/93 as I first thought), but carsoft reports 12/93 as the production date for the SRS unit.. Don't know if this is normal, it means the unit has already been relpaced once or if it's just carsoft though..

Zitat von reinhard;70723

Infinite resistance does not really prove anything other than that the two wires don´t touch

Haha, yeah. Sorry, really wasn't thinking clearly there. Too much work and too little sleep I guess

I'll do a proper reading later on.

Revtor: Thanks for you input. Good to have a bit more technical information on the subject than the other thread I read. I guess it can't hurt the system to try atleast, even though I've tried two different airbags already.

Well, it seems it wasn't the slip ring either ..

As the connection is shorted when you disconnect the cables I realised I could just hook everything back up except the airbag and measure from the airbag connector. As long as everything was connected like that there was infinite resistance, so if I'm not completely mistaken I guess that proves that the wiring is in order. As soon as I disconnected any part I got an open circuit

Going to check the passenger airbag asap and if not that helps I'm going to dig up the control unit.

By the way, I've head someone mentioning on another forum that you could try connecting a small light bulb, like the interior light bulbs in the car, instead of the airbag. That should give enough resistance that the control module won't report an error if everything else is in order. Anyone heard about this?

Hehe, I've taken all precautions and let the car stay disconnected for atleast an hour before even starting to work on the airbags. Nowadays she's parked for the winter anyway, so no need to have the batteries connected.

And yeah, hehe, tell me about Carsoft. It doesn't even recognise any modules except the airbag and zke modules. The rest "doesn't respond" ..

Anyways, I guess I have a few things to check out now. Hopefully I'll be able to fix this soon, been working on it for a while, but it's first now that the car is parked I've had time to go beyond the basics.

Build date is nov '93. First time registered is '95 sometime..

Anyways, the pictures are really helpful, I'll check when I get home

Edit: and agreed, that the ECU itself is broken sounds little likely, but who knows. The only thing that have had me wondered for a bit is that carsoft returns error 2 "ignition circuit driver side defective", but I also see that error 2 could be a defective controller unit in some pdf's. But I'm guessing/hoping that this is for the older SRS systems. As far as I know I should have the SRS3 system in mine with the dual airbag setup. I guess maybe Carsoft detects this itself and gives the correct error message too?