bcm damage

[therev]

hi everyone can some explain how damage is cause to the bcm buy fitting hid's or led headlight bilbs or even led number plate bulbs running around with the bulb out warning light on and explain what the bcm actually does thanks kevin 

[Ju1ian1001]

The BCM is the Body Control Module, It control everything thats electronic or electrical fitted to vehicle; it's nerve centre in simple terms, fitting Hid's or LED headlamp bulbs can overloadthe circuit therefore shorting and burning out the BCM. it uses what we call canbus  to supply power to different circuits (nerve signals if u like) and 1 wire can power more the one circuit, everything sends a code so if an LED bulb is fitted the system can't sense this so puts that light on, on the dash.

[therev]

so will i cause damage as i am running led number plate lights at the min and the dash light is on 

[Ju1ian1001]

39 minutes ago, therev said:

so will i cause damage as i am running led number plate lights at the min and the dash light is on 

No won't cause damage, if the light on the dash gets annoying you can have it coded out by some one with VCDS, there may be some one local to u with this. 

 

It's just the high power/output LED headlamp bulbs and HID's the can cause damage as our cars were designed to use them, Low power/output  won't damage anything.

 

VCDS is Vag Com Diagnostic Software

Edited 19 March, 2017 by Ju1ian1001

[therev]

ok thanks so how will led headlight bulbs cause damage as i thought they were low power 

[Ju1ian1001]

7 minutes ago, therev said:

ok thanks so how will led headlight bulbs cause damage as i thought they were low power 

From my understanding they draw higher current to effect the same brightness is halogen bulbs, but have not worked on an LED system,  being and MOT tester i have not actually worked on LED headlights as they haven't filtered through as yet, some one with more electronics experience may correct me though.

 

You always learn new things everyday.

[Breezy_Pete]

Someone should open up a damaged BCM from one of these and see what device actually feeds the headlights power. That might give some clues.

HIDs may present a high inductive load at switch-on/off as the 12V is presumably upped to something considerably higher by a transformer in the ballast? Some types of device not designed for inductive loads may not like this. BCMs designed for HID lights may have a different drive device. See John Popelish's rather good post (#5) here about resistive and inductive loads, for a better description than I can manage.

Here's the most relevant chunk:

 

"The practical upshot of this characteristic of an inductive load is
that the switch turning it on does not have to handle a sudden current
rise, since the current ramps up smoothly, after the switch is closed,
making it easier on the switch than if it was switching on
instantaneous full current into a resistor or a big inrush current
into a cold bulb filament.

Turning that current off, is another matter. The only way to get an
infinite rate of change from steady state (or any other) value to zero
in zero time, requires an infinite reverse voltage to be applied ot
the inductor for zero time. This, of course is very difficult to
arrange. But if you just open a set of contacts or turn off a
transistor in series with the inductance, the inductance generates a
very large voltage to ramp the current back down very fast. There is
no other way the current can change quickly. This high generated
voltage can be very hard on both mechanical contacts and transistors.
So practical switches have to make provisions to limit this voltage by
allowing a path for the decaying current, or handle the arching the
voltage produces."

 

 

LEDs would load the circuit differently again.

 

Edited 20 March, 2017 by Wino

[hrvrs]

OK, lots of misunderstandings going with these threads so I'll lay out what I know and let someone with more knowledge than me correct any mistakes.  I'm not an auto-electrician but I've fitted HID and LED bulbs/kits to around ten of my own vehicles without issue...

 

HID ballasts draw a similar current to whatever a rated halogen of the same wattage would, but only once the arc is established and settled - for the first few seconds the ballast will draw a higher current in order to strike the arc and stabilise it.

 

An HID arc requires many thousands of volts to function, the ballast is what takes the 12v auto supply and converts it to this greatly higher voltage

 

35W HID draws less current than a 55W halogen bulb, 55W HID will draw about the same - simple electronics people...  P= I x V

 

LEDs simply draw less current to produce the same light output (halogens waste energy as heat) - so they will confuse the car's testing circuitry by not drawing the expected current and it will think a bulb has failed...

 

Other than bulb failure warnings I don't foresee any way an LED bulb could damage the BCM, the relationship is very simple, all 12v, apply voltage, draw current, produce light...

 

HIDs on the other hand, yes, I can see the unexpectedly high current draw at startup could confuse the BCM and even overload it, possibly before the fuse has time to heat up and blow to protect the circuitry.  I would fit HIDs to an Octavia via a separate harness and a relay

 

Hope that clears it up a little bit

Edited 20 March, 2017 by hrvrs

[Breezy_Pete]

It may be the switching off of HIDS that causes the problems, as per the reference I quoted above, rather than the switching on. Incandescent/filament bulbs have very low resistance when cold (measure one with a multimeter if you doubt this) so big inrush currents are 'old hat'.

[therev]

ok thanks to all for the posts well as i said my mate has had my led bulbs in now for 4 days with no issues as he has maxidot and the only issue showing is rear number plate light so i will be fitting then in my fl tomorrow night i will keep yous informed but i must say seeing them in his they are way better than a halogen bulb thanks again kev

[paddypaws]

Relays are are an inductive load and if they are not protected by a flyback diode pose an equally dangerous hazard to a mosfet or any other semi conductor for that matter. As soon as the current is switched off the relays coil will produce a huge back EMF voltage. This is how ingnition coils work. Some MKII BCMs are perfectly happy with HIDs. Mine have been in for nearly 3 years with no problems. However to fully be sure a diode in series and one in reverse bias orientation in parrallel with the load will protect the mosfet.

A couple of articles that will help with the detail: 

https://progeny.co.uk/back-emf-suppression/

http://electronics.stackexchange.com/questions/110574/how-to-choose-a-flyback-diode-for-a-relay

 

Edited 30 March, 2017 by paddypaws
addition of links

[Breezy_Pete]

@paddypaws "Equally dangerous hazard"?

Would need to be a very large relay coil to have similar stored energy to an HID ballast, no?

Edited 30 March, 2017 by Wino

[paddypaws]

9 hours ago, Wino said:

@paddypaws "Equally dangerous hazard"?

Would need to be a very large relay coil to have similar stored energy to an HID ballast, no?

No, actually a relay could be more hazardous, a relay coil will provide a huge voltage back spike, it's a theoretically infinite (it's not) but as the voltage is proportional to the speed of the collapse of he field in the coil (which is instant) the potential is huge. Its high voltage that kills semi conductors, static discharge will kill a semi conductor quick sticks. HID ballasts are an unknown as they are all different designs, but some will have better filtering than others.

This page below is a good simple explanation. This stuff is very basic electronic principles, you always protect a driver transistor for a relay with a diode:-

 

Quote from linked page '' When the transistor switches “OFF”, the current flowing through the relay coil decreases and the magnetic field collapses. However the stored energy within the magnetic field has to go some where and a reverse voltage is developed across the coil as it tries to maintain the current in the relay coil. This action produces a high voltage spike across the relays coil that can damage the switching NPN transistor if allowed to build up''

http://www.electronics-tutorials.ws/blog/relay-switch-circuit.html

Edited 31 March, 2017 by paddypaws
add link

[Breezy_Pete]

"could be"...

"unknown"....

[langers2k]

The pulse comes from the coil on the input side of the relay rather than the switched output side.

 

Plenty of automotive relays already have protection built in, you can see the diode between 86 and 85:

[paddypaws]

1 hour ago, mikeholroyd said:

I really can't see any issue with a relay in this situation,  as you're only using the original halogen circuit to SWITCH the relay and not power it.  If there are any back spikes from the relay they will be going through the new harnesses to the battery,  not the BCM. 

 

Mike 

It won't Mike, the relays coil is the problem, which is directly connected to the Mosfet in  the BCM any spike will kill it. The diode has to be there whether built in or external.

Edited 31 March, 2017 by paddypaws

[paddypaws]

59 minutes ago, Wino said:

"could be"...

"unknown"....

Obviously. It depends on the relay, the diode and the HID kit.

[Breezy_Pete]

Fair enough mate. Just seemed a bit blunt to say "No" followed by ifs and buts and then technical explanations that have already been offered further up the page.

[paddypaws]

Just now, Wino said:

Fair enough mate. Just seemed a bit blunt to say "No" followed by ifs and buts and then technical explanations that have already been offered further up the page.

I didn't mean to be blunt but its a very well documented fact that a relay coil will always damage a semi conductor driving it if it doesn't have a proper shunt diode, either external or internally.  

Unfortunately some of the other technical explanations further up the top of the page are wrong, but I'm not getting in to that.

 

[Mike97]

I remember someone posting on here that a resistor melted off a cheap led bulb and burnt his bcm out so maybe best to get good quality led replacements.

 

Mike 

[Mike97]

2 hours ago, mikeholroyd said:

We're not talking about LED bulbs,  cheap or otherwise.  HID gas discharge bulbs are totally different. 

Op also asked about led number plate bulbs and led headlamp bulbs causing damage to bcm and i am sure the post i read on here was about a cheap led number plate bulb burning out his bcm.

 

Edited 31 March, 2017 by Mike97

[Breezy_Pete]

23 hours ago, paddypaws said:

I didn't mean to be blunt but its a very well documented fact that a relay coil will always damage a semi conductor driving it if it doesn't have a proper shunt diode, either external or internally.  

Unfortunately some of the other technical explanations further up the top of the page are wrong, but I'm not getting in to that.

 

 

Sorry I was a bit of a tit about this yesterday, fella.

Your info deserved respect and appreciation, not the ill-thought-out doubting that I gave it initially.

 

[paddypaws]

42 minutes ago, Wino said:

 

Sorry I was a bit of a tit about this yesterday, fella.

Your info deserved respect and appreciation, not the ill-thought-out doubting that I gave it initially.

 

Hey, no problem, I was a bit blunt when I re-read it. That's the trouble with forums sometimes, If things are written quickly the intonation is lost and can seem rude.

[therev]

well guys i have been running my led's h7 bulbs now for nearly a week in my octavia fl vrs tdi these are the ones http://www.ebay.co.uk/itm/172433922804?_trksid=p2060353.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT there is no flicking what so ever on powering up they come straight on and go straight off so fingers crossed these are ultra white when on 

[FatblokeVRS]

On 02/04/2017 at 11:32, therev said:

well guys i have been running my led's h7 bulbs now for nearly a week in my octavia fl vrs tdi these are the ones http://www.ebay.co.uk/itm/172433922804?_trksid=p2060353.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT there is no flicking what so ever on powering up they come straight on and go straight off so fingers crossed these are ultra white when on 

 

I got a set of similar led bulbs from a mate to try. Are you still running them ok?