Hi can anyone tell me how the can bus system works? For instance if I change a number plate bulb to led why does it show a light error? What is the system measuring that changes in lights show faults? Is it voltage etc? Can I add something to the wiring to give a correct reading back to the can bus system?
Cheers
Craig
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Can bus system
Re: Can bus system
[quote="Zakbelle"]
Hi can anyone tell me how the can bus system works? For instance if I change a number plate bulb to led why does it show a light error? What is the system measuring that changes in lights show faults? Is it voltage etc? Can I add something to the wiring to give a correct reading back to the can bus system?
Cheers
Craig
Sent from my iPhone using Tapatalk
[/quote]
It fires a low voltage across each bulb on start up, it reads the resistance across each bulb too see if its still intact.
When you fit an LED bulb it doesn't have the same resistance as a conventional filament bulb, so it thinks its blown.
Also when you have LE d bulbs fitted you can see them flash about 10 times after start up as the low voltage test is enough to light up the LED where it wouldn't light up a conventional bulb.
Hope that helps
Hi can anyone tell me how the can bus system works? For instance if I change a number plate bulb to led why does it show a light error? What is the system measuring that changes in lights show faults? Is it voltage etc? Can I add something to the wiring to give a correct reading back to the can bus system?
Cheers
Craig
Sent from my iPhone using Tapatalk
[/quote]
It fires a low voltage across each bulb on start up, it reads the resistance across each bulb too see if its still intact.
When you fit an LED bulb it doesn't have the same resistance as a conventional filament bulb, so it thinks its blown.
Also when you have LE d bulbs fitted you can see them flash about 10 times after start up as the low voltage test is enough to light up the LED where it wouldn't light up a conventional bulb.
Hope that helps
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BMW E83 X3. 3.0d M Sport, 2008 in black ( now gone )
BMW E93 330i Convertible Msport 2007. ( Now sold )
VW Golf 2.0 TDI 140 GT 2010 ( now sold )
Now Sold 55 plate, X5, 3.0 D Sport. Silv
Re: Can bus system
As above....
t's not the CANBUS causing the issue. That is only a communications network between modules.
The system for checking the lights sends a very short pulse on the lighting circuits which a bulb will not react to due to the resistance of the filament and the time taken to 'brighten'. The LCM sees that as a working bulb so no error goes over the CANBUS to the instrument binnacle (a CANBUS connected computer in its own right).
LEDs however need a tiny amount of voltage to switch on and this makes them look like a short circuit to the lighting module which promptly flags a fault as it thinks there is a short. Putting a reactive component (capacitor) in circuit across the wires (+ve to -ve) rather than in series brings the resistance up so the LCM is fooled into thinking the bulb is OK. BUT if you get the resistance too high then the LCM thinks the bulb has blown (open circuit), so you get another error!
Capacitors do not allow DC to pass hence not being in series, but they do allow a 'pulse' through so you put them in parallel to the lights power circuit, so between live and return.
Those that have done it will give you the figures. Somewhere between 20V to 35V handling with perhaps 4,700mF? Needs to be above 15V to cope with the alternator output voltage plus some headroom.
Richard
t's not the CANBUS causing the issue. That is only a communications network between modules.
The system for checking the lights sends a very short pulse on the lighting circuits which a bulb will not react to due to the resistance of the filament and the time taken to 'brighten'. The LCM sees that as a working bulb so no error goes over the CANBUS to the instrument binnacle (a CANBUS connected computer in its own right).
LEDs however need a tiny amount of voltage to switch on and this makes them look like a short circuit to the lighting module which promptly flags a fault as it thinks there is a short. Putting a reactive component (capacitor) in circuit across the wires (+ve to -ve) rather than in series brings the resistance up so the LCM is fooled into thinking the bulb is OK. BUT if you get the resistance too high then the LCM thinks the bulb has blown (open circuit), so you get another error!
Capacitors do not allow DC to pass hence not being in series, but they do allow a 'pulse' through so you put them in parallel to the lights power circuit, so between live and return.
Those that have done it will give you the figures. Somewhere between 20V to 35V handling with perhaps 4,700mF? Needs to be above 15V to cope with the alternator output voltage plus some headroom.
Richard
Last edited by X5Sport on Sun Jun 15, 2014 8:43 pm, edited 1 time in total.
Never anthropomorphise computers. They hate that.Re: Can bus system
Hmm okay, this is all to do with a reverse camera which has an led light which should replace one of the existing bulbs on the number plate, could I possibly leave the light connected and wire the led in or would that also show up as a fault??
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