September 15, 2015 06:22AM
Hi all

About one year ago, my car started to act up. First the airbag light came on in the instrument cluster. The error on the airbag control module was "Undervoltage".
I did some easy checks on the battery and its terminals, tried a new battery - nothing. Went to several diagnostics, they all pointed to a faulty airbag control module, but none of them was 100% sure. Since the cost of the module is not small (about 450 EUR), I decided to read further.

Then the car became a xmas tree. Everything went nuts:
- the radio was turning off and on by itself
- the dome light was turning on from time to time
- the steering wheel audio controls stopped working
- the cruise control stopped working
- the horn stopped working ! Man, that's really USEFUL in this shitty town smiling smiley

I took apart the interior to find the airbag module. It did not look rusted or anything, the connectors were in very good shape. Checked other modules too, like the GM5 - everything looked good there, too.
I learned a lot about the wiring of this car, but I did not learn much about how to find the fault. I even found some nice photos on a forum (unfortunately I don't have the link sad smiley ) that explain how the stupid network runs in this car... I was expecting everything to be DIGITAL, but no, it's some sort of NOISE modulated over an analogue signal. So everything pointed, again, to a faulty module that was blocking the communication for other modules.

I disconnected the airbag control module and took it out of the car. First I thought one of the capacitors looked strange and tried to find a replacement, but only found cheap chinese crap. Then I looked for a replacement module, but I feared that if it came from a damaged car, it may explode my airbags, which wouldn't be so nice. Then winter came and I did not move the car any more for about 3 months. I did not research the problem at all during this time.

In spring, I decided to go to the official BMW dealer. I did not do that before, because I know them well and they are incompetent thieves. But, I wanted to have a written recommendation to buy a new airbag module, so I could return it in case this was not the real problem. Silly me...

I put the module back in the car before going to them, to have a complete car (although without interior smiling smiley ). Interestingly, the car had no more errors, other than the airbag error. All other modules were working well now, even the horn.

So I went to the dealer, told them the story, they charged me for one hour of diagnosis and said they would call. They called after 30 min and said they found more errors and they suspect a problem with the wiring somewhere, and they would like to keep my car for 2-3 days and look for it. Weird, no extra costs, I said ok. Again, silly me. I know for fact that they steal parts from customer cars, or try to charge them for made up problems, in order to buy parts for their friends.

After 3 days they called and said there is no more error, I can come get the car. What ?? I went, payed the 1 hour bill, asked if they found anything, they said no, everything works perfectly. What ?!
I went to the car, started it and... the entire dash was lit up. I had errors on airbag, DSC, steering, gearbox. Went back inside and asked the guy... he said, very naturally "oh, yes, I FORGOT to tell you that you need a new steering wheel angle sensor, yours burned when we connected the car to the diagnostic computer. It's a well known fact that various modules burn when diagnosed, you have to buy a new one, the cost is 500 EUR + works, please sign this order right now". WHAT ?!?!?! WHAT ?!?!?

I went back to another service, connected the diagnose computer, indeed there was a new error on the steering wheel angle sensor. They deleted the error and guess what, everything works, again. They said they suspect the other guys disconnected the sensor in order to trigger the error, and the other errors were associated.

I went back to the BMW dealer and I talked to the complaint department and to the service manager. They did not give a shit about me, they probably take their share in everything they steal from customers. They even said my car is OLD so it's normal.

So, back to square one. After resetting the steering wheel angle sensor, the car ran fine for about a month. No errors (including airbags), everything was working well.

After one month, airbag light on the dashboard, again. This time I noticed something funny - if I switch on the ignition but do NOT start the car, then switch it off, then on again, the airbag light would go off. I was able to do that one more month. I was more and more convinced the problem was coming from the airbag control unit, namely the capacitor that looked a bit funny.

Well, I was wrong. The xmas tree started happening again, all modules went nuts again. Plus, I had a new light, for the gearbox. I did not find the time or the mood to take out the airbag control unit again for a long time, so I just drove like that. I could feel that the car was behaving a bit different already, gas consumption went higher and the power was not the same.

One day I was ready to order the new airbag control unit. I unplugged the old one, thinking the car will go back to normal. It didn't, even after I left it without battery for 4 days. Not thinking straight, I ordered the unit anyway smiling smiley

More time has passes and my friend (that has a diagnostic computer and promised to code my new unit) was always busy. so I thought fuck it, let's go to a service. I went to the same one that diagnosed the problem the first time, the guy seemed quite smart and experienced. I went in just to code the new unit, he connected the interface and... surprise - half of the car was not visible any more smiling smiley I had errors eveywhere, including the VANOS, the exhaust, gearbox, GM5, lights etc.

At this point, I was ready to set the damn thing on fire. The guy was very confident that he will find the problem and fix it, for a cost. I accepted the cost and let him work, knowing that he may take EVERYTHING apart, including the soft top (which may lead to more expenses... and more things not working correctly after...).

Amazingly, next day he called and said everything works ! He did not take any pictures, so I cannot be 100% sure of the story he sold me, but he said that the cause was... rusted and dirty contacts at the ignition switch, the light module and the GM5. I'm pretty sure the GM5 contacts were perfect, since I inspected them myself several times, but I cannot be sure about the others. Seems plausible, though. He said that the airbag module WAS defective, too, but maybe he just did not want me to feel even more sorry about the money smiling smiley

The car runs perfectly now, the power is back, consumption is back to "normal", the feeling is amazing again.

tl;dr: whenever you have weird electrical issues, check all connectors in the car for dirt/rust, before spending ridiculous amounts of money on parts.

A physics truck just turned over outside. There's physics everywhere!
September 15, 2015 10:07PM
Wow, what a story. I have heard of ignition switch causing problems with other BMW models.

Bob in Everett
September 16, 2015 05:36AM
What a story!

Corroded terminals, mositure and bad wiring can cause all sorts of problems.
OTOH, electronic modules built to automotive specs are musch more durable than people think.

Obviously, at the shop they will never blame the 0,25€ conector, but the 500€ module instead.
September 28, 2015 04:12PM
During diagnosis, I often ask to check connectors or to use 'flying' leads, unplugging the original ones. More than once mechanics don't believe it at first (it sure isn't a wiring problem), to see the issue is being fixed doing so.
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