Most of todays vehicles have a diagnostic port and some embedded systems are constantly running an auto-diagnose check list. Such case is in engine management system (sensors, actuators, valves etc) light control module, tire pressure sensors and SRS airbag system. This self diagnose, that a system is making for itself, is to let the driver know in case any fail might appear. In case a fail is present in most cases a warning lamp will be lit on and it usually requires immediate attention. Two of this warning lamps constantly give chills to drivers as they suggest an expensive visit at the ´´stealership´´: ´´Check Engine´´ lamp and ´´SRS airbag´´ lamp. In this post I will talk about the SRS airbag lamp and how to get a false negative negative or false error free result upon automatic self diagnose.
DISCLAIMER!!! I strongly advise that any problems in the Airbag system should be immediately fixed by an expert and one should only do this in case of an immediate emergency!
Background: Immediately after the ignition is ON, the ECU (electronic command unit) will run a diagnostic test of the SRS airbag system, among other tests. The airbag lamp will be ON (indicating that the lamp is working) then after 3 seconds, if no errors are being detected, it will go OFF and stay that way. That was not the case here...
A friend of mine comes to me, desperate as his car failed the technical inspection and he needed the car the next day. This was the last technical inspection the car will ever have as it was too old and too many fails accumulated to be worth fixing. I guess when you get to the point that the fixing is more expensive than the car itself, you need a new car. The reason why the car failed the inspection was ´´SRS airbag lamp is lit on´´. This indicated that the car had a fail in the airbag system and the airbags will not deploy in case they´re needed.
So I decide to accept the challenge and try to fix the car in less than a day. First things first: I hook up my laptop to the car via the diagnostic cable and open up the diagnostic software. The fault history tells me that a cable in the door passenger was interrupted, however the fault was not currently present. I erase the error code from the history and sure enough, the lamp went off. But this was an intermittent problem so after I open and close the driver door a few times, after the ignition the lamp would be ON again. Normally, to properly fix this a new wire loom would be needed as I suspected one or more wires were interrupted in the drivers door. This wires go to the door airbag pyrotechnics and door crash impact sensors. Not only they were expensive to order, a few days would have to pass by until we would get them in the mail. This was not an option here.
An new and unconventional approach was needed here.
The ´´solution´´: If someone would diagnose the car, see the fault present, but yet the airbag lamp still goes OFF after 3 seconds would for sure ask himself if that is not Houdini´s car. The fault is there, but you can´t see it... The ´´solutions´´ is rather simple: you remove the airbag light and replace it with one that will obey your rules. Kind of like in politics. Here I needed to find a lamp on the dashboard that will go off after 3 seconds. They are a few of them: oil level, DDE (for diesel), battery, ASC (automatic stability control) and check engine lamp. However, after I would surgically remover the airbag LED, I would need a new one that takes it´s signal from one of these. The problem was that they were far away and is not a good idea to have wires passing over the circuit board, being pressed against it by the back dashboard cover. I would therefore need to take my signal from a near by lamp. And that was the seat belt warning lamp. Dealers can program the cars so it gives no warning if the seatbelt is not fasten. That is because they need to focus on making money and can´t afford to have the cars beeping around every time they move them across the saloon.
DISCLAIMER!!! I strongly advise that any problems in the Airbag system should be immediately fixed by an expert and one should only do this in case of an immediate emergency!
Background: Immediately after the ignition is ON, the ECU (electronic command unit) will run a diagnostic test of the SRS airbag system, among other tests. The airbag lamp will be ON (indicating that the lamp is working) then after 3 seconds, if no errors are being detected, it will go OFF and stay that way. That was not the case here...
A friend of mine comes to me, desperate as his car failed the technical inspection and he needed the car the next day. This was the last technical inspection the car will ever have as it was too old and too many fails accumulated to be worth fixing. I guess when you get to the point that the fixing is more expensive than the car itself, you need a new car. The reason why the car failed the inspection was ´´SRS airbag lamp is lit on´´. This indicated that the car had a fail in the airbag system and the airbags will not deploy in case they´re needed.
So I decide to accept the challenge and try to fix the car in less than a day. First things first: I hook up my laptop to the car via the diagnostic cable and open up the diagnostic software. The fault history tells me that a cable in the door passenger was interrupted, however the fault was not currently present. I erase the error code from the history and sure enough, the lamp went off. But this was an intermittent problem so after I open and close the driver door a few times, after the ignition the lamp would be ON again. Normally, to properly fix this a new wire loom would be needed as I suspected one or more wires were interrupted in the drivers door. This wires go to the door airbag pyrotechnics and door crash impact sensors. Not only they were expensive to order, a few days would have to pass by until we would get them in the mail. This was not an option here.
An new and unconventional approach was needed here.
The ´´solution´´: If someone would diagnose the car, see the fault present, but yet the airbag lamp still goes OFF after 3 seconds would for sure ask himself if that is not Houdini´s car. The fault is there, but you can´t see it... The ´´solutions´´ is rather simple: you remove the airbag light and replace it with one that will obey your rules. Kind of like in politics. Here I needed to find a lamp on the dashboard that will go off after 3 seconds. They are a few of them: oil level, DDE (for diesel), battery, ASC (automatic stability control) and check engine lamp. However, after I would surgically remover the airbag LED, I would need a new one that takes it´s signal from one of these. The problem was that they were far away and is not a good idea to have wires passing over the circuit board, being pressed against it by the back dashboard cover. I would therefore need to take my signal from a near by lamp. And that was the seat belt warning lamp. Dealers can program the cars so it gives no warning if the seatbelt is not fasten. That is because they need to focus on making money and can´t afford to have the cars beeping around every time they move them across the saloon.
I first program the car via the diagnostic OBD (On-line Board Diagnostics) port not to give any warning in case the seatbelt was not fastened. In my diagnostic software (BMW scanner 1.4) the option ´´seat belt warning´´ has to be unchecked. This will cause the lamp to come ON after ignition, (to indicate that is present and working) but then go OFF after a few seconds. Convenient... in this case.
The dashboard is removed and the back screws opened and spared.
The back cover is off, exposing the PCB. The PCB is held in place by some clips. Carefully pry them off so you don´t break them then remove the PCB out from the case.
After the back cover is off, you can remove the PCB and start removing the actuators. They will come off by pulling them out.
Remove the thin plastic info cover.
Next proceed to remover the white platic trim. It has no screws, just some clips in the back. Here you can also see the fake LED light.
Here the removed airbag LED was removed. At first I tried to have some wires routed from the one near by (also removed) and try to solder them to the airbag LED that would have no other connection with the PCB board. That proved to be incredible difficult as the LED is incredible small and no reliable soldering could be made.
In the end I just mounted a normal 12V LED and soldered it to the seatbealt LED. Unfortunately I had to cancel to seatbelt LED because there was no more room on that specific terminal to mount it. Maybe I could had mounted another regular one, on the same terminal, but at that time I did not had a red one. It doesn´t matter in this case, as the people from technical inspection are only looking at the airbag light to see it turning ON then OFF.
So everything was mounted back in the reverse order of removal, the dashboard was mounted back in the car and there were no problems. The SRS airbag lamp was working just fine, turning ON then OFF after 3 seconds. This, of course did not fixed the real problem so the car was dangerous to be driven since the airbags would not deploy in case of an accident. However, it passed the technical inspection, the owner did his things with it and two weeks later he took it to scrap yard as we agreed. I would had never done the job if I knew the car was supposed to be sold to someone else or the owner intends to keep it for himself. You should always have a clean conscience. However... it was a rather fun project to work on. I should call it ´´the Houdini airbag lamp´´.
So everything was mounted back in the reverse order of removal, the dashboard was mounted back in the car and there were no problems. The SRS airbag lamp was working just fine, turning ON then OFF after 3 seconds. This, of course did not fixed the real problem so the car was dangerous to be driven since the airbags would not deploy in case of an accident. However, it passed the technical inspection, the owner did his things with it and two weeks later he took it to scrap yard as we agreed. I would had never done the job if I knew the car was supposed to be sold to someone else or the owner intends to keep it for himself. You should always have a clean conscience. However... it was a rather fun project to work on. I should call it ´´the Houdini airbag lamp´´.