Technical Main beams gone, Mot next week!

[GeorgiaDoors89]

Hi guys. I have a 2011 automatic fiat 500 pop. Months ago the main beam and tail lights went, auto electrician couldn't find any fault and said it had to be the bcm. Rang a fiat specialist and he didn't sound hopeful. A couple of weeks later, lights are all good again without being looked at. But it's happened again, just the main beams this time and occasionally they turn on when I test it but not reliably. I have a full service and mot next Tuesday and I'm bricking it. Really hoping there's someone on here that might know the right path to go down. I've got everything crossed that they will decide to work properly before the mot but it's not looking good. Any advice would be much appreciated!

 

[John202020]

When you flick to main beam does the tell tale come on, on the dash?

 

[GeorgiaDoors89]

Thank you for responding. Yes the blue light shows on the dash. It's registering that I'm turning them on but isn't passing the message along the line!

 

[John202020]

Check/replace(regardless) fuse 14 under bonnet have someone flash the main beam put a finger on relay 2 under the bonnet can you feel it click?
You can swap fuse 85 and 14 if you don't have a spare

Unrelated do your day time running lights work?

 

[GeorgiaDoors89]

Brill thank you.

Yes they do. Dipped headlights also all good.

 

[Ralf S.]

I suspect the auto electrician is correct and it is the BCM.

The BCM works out what you want to do (based on your inputs) and activates whatever module it needs to. The tell-tale blue light on high beam is on a separate loop than the lights themselves, so if the tell-tale comes on, all it means is that there's no problem with the tell-tale circuit or the lights stalk. High beam is also separate from low beams, so if low beams works and high beams doesn't, then that's less good than if neither worked.

I would take all the connector plugs out of the BCM and give them a good brush with some contact cleaner or WD40 and a toothbrush, just in case there's a vaguely loose connection. The BCM will be behind the fuse box and all the connector plugs on it are different enough that you'll be able to re-assemble it, but if you want to take them one at a time, clean the plugs and then replace them, that would work too. You just want to make sure it's all clean and it's all tight.

If that doesn't help then you need to test the "out" pin signal when you activate the high beam. On the Stilo from memory it's pin 15 but you need to find a wiring diagram for the 500's BCM .. it may or may not be the same pin.

You can't just swap the BCM for another identical one, since the BCM also holds the immobiliser and ECU recognition codes. Your high beams might work but the car won't start. Most ECU repair places ought to be able to track down the dodgy PCB circuit and repair it. Failing that, you can fit a new BCM but transfer all your chips from the old BCM to the new one, if you know someone handy with PCB repairs.


Ralf S.

 

[John202020]

If the tell tale lights then there must be a signal from the stalk? It's not that hard to tap into that wire and run it to a separate relay(fuse etc) and feed the lights directly by passing the cars system, it would work for the rest of the cars life? Far cheaper than a new BCM

 

[Gina500]

(Where has the „yeah that“ emoji gone ??)

 

[GeorgiaDoors89]

I suspect the auto electrician is correct and it is the BCM.

The BCM works out what you want to do (based on your inputs) and activates whatever module it needs to. The tell-tale blue light on high beam is on a separate loop than the lights themselves, so if the tell-tale comes on, all it means is that there's no problem with the tell-tale circuit or the lights stalk. High beam is also separate from low beams, so if low beams works and high beams doesn't, then that's less good than if neither worked.

I would take all the connector plugs out of the BCM and give them a good brush with some contact cleaner or WD40 and a toothbrush, just in case there's a vaguely loose connection. The BCM will be behind the fuse box and all the connector plugs on it are different enough that you'll be able to re-assemble it, but if you want to take them one at a time, clean the plugs and then replace them, that would work too. You just want to make sure it's all clean and it's all tight.

If that doesn't help then you need to test the "out" pin signal when you activate the high beam. On the Stilo from memory it's pin 15 but you need to find a wiring diagram for the 500's BCM .. it may or may not be the same pin.

You can't just swap the BCM for another identical one, since the BCM also holds the immobiliser and ECU recognition codes. Your high beams might work but the car won't start. Most ECU repair places ought to be able to track down the dodgy PCB circuit and repair it. Failing that, you can fit a new BCM but transfer all your chips from the old BCM to the new one, if you know someone handy with PCB repairs.


Ralf S.

Thank you Ralf I really appreciate your detailed response. It's been very frustrating taking it to mechanics who won't even look at the bcm to see if its something simple. I'm going to study some videos, take a deep breath and have a poke around. Thank you again

 

[Ralf S.]

The BCM is basically a printed circuit board with a load of chips on it, inside a plastic box. The tracks are absolutely miniscule so unless you have very good eyesight, it's difficult to see where there is any break. I took a photo of my PCB (zoomed in) so I could then zoom in again on different areas, looking for a break.

Look for any area that seems to have a brown scorch mark, since PCBs tend to go where the solder hasn't laid down at the correct thickness and therefore causes resistance, heat and then melting, which breaks the circuit. If you're "lucky" the break will be in plain sight, not under a chip, so you'll know that there is a problem with the PCB and that it's not something else.

On mine, I had what looked like a break just under the edge of one of the chips... so I couldn't be sure that would be the place to fix (even if I had a soldering iron the thickness of a human hair...) but in the end it didn't matter .. an ECU repair place just swapped the chips from mine onto a second-hand BCM and it's been fine ever since.

If you can't see an obvious break anywhere then the pro's can just test the circuits on the PCB to prove that the fault is on the PCB. That will cost a bit... but it will save you looking for other problems if the BCM has the fault. Obviously, once/if they find the fault, they might be able to just repair it there and then.

You could try wiring the lights (or at least a relay to operate them) via another circuit, like the tell-tale light.. but the system is CANBUS i.e. voodoo and I don't know how that will react to having an extra draw on that wire. It might be quicker to get it done this way/interesting to see how it works.. but it might cause other issues. instead.


Ralf S.

 

[GeorgiaDoors89]

The BCM is basically a printed circuit board with a load of chips on it, inside a plastic box. The tracks are absolutely miniscule so unless you have very good eyesight, it's difficult to see where there is any break. I took a photo of my PCB (zoomed in) so I could then zoom in again on different areas, looking for a break.

Look for any area that seems to have a brown scorch mark, since PCBs tend to go where the solder hasn't laid down at the correct thickness and therefore causes resistance, heat and then melting, which breaks the circuit. If you're "lucky" the break will be in plain sight, not under a chip, so you'll know that there is a problem with the PCB and that it's not something else.

On mine, I had what looked like a break just under the edge of one of the chips... so I couldn't be sure that would be the place to fix (even if I had a soldering iron the thickness of a human hair...) but in the end it didn't matter .. an ECU repair place just swapped the chips from mine onto a second-hand BCM and it's been fine ever since.

If you can't see an obvious break anywhere then the pro's can just test the circuits on the PCB to prove that the fault is on the PCB. That will cost a bit... but it will save you looking for other problems if the BCM has the fault. Obviously, once/if they find the fault, they might be able to just repair it there and then.

You could try wiring the lights (or at least a relay to operate them) via another circuit, like the tell-tale light.. but the system is CANBUS i.e. voodoo and I don't know how that will react to having an extra draw on that wire. It might be quicker to get it done this way/interesting to see how it works.. but it might cause other issues. instead.


Ralf S.

You're a star Ralf. Your time and expertise is so so appreciated