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Exactly how does the "Bulb-Out" sensor work?

Fred930

Scirocco MK II Track Car
Location
Wisconsin
There are many threads discussing how to fool VW's bulb out sensors (by using a parallel load resistor) when switching to LED replacement lights. An active thread right now is for MK V LED Fog lights, where the kit builder includes an 8 ohm, 50 watt power resistor - which seems like a pretty conventional method.

But does anyone really know how VW's bulb sensing circuit works?

I could imagine each sensed bulb's power wire could be monitored by a hall effect current sensor ; or they could monitor the voltage across a low ohmic value sense resistor in series with each bulb. But either one seems kinda expensive - so maybe they have some other way of doing it?

A normal incandescent bulb exhibits a large positive temperature coefficient, meaning the resistance of an operating (hot) filament is much higher (maybe 15x) versus when it's cold. So a simple parallel load resistor (when installing LED replacements) doesn't very accurately model the incandescent filament's behavior.

Also, I've read VW's do a bulb check when starting the car that might actually be different than the check made when each individual light is turned on (but they must use the same sensing method - right??). That difference was reported to have caused the car to fail the power up (starting) test, while passing the test made when the LED light (with parallel load resistor) was actually switched on. So maybe the start-up test is just a very short current pulse into the cold filament?

Anyway - does anyone actually know how those bulb-out sensing circuits really work?
 

bowerhour4334

Ready to race!
Location
SoCal
Car(s)
MKV GTI
This is just a guess, but since the lights are controlled by a module and not just a switch, I'm assuming the module knows when it's an open circuit.

I would imagine you could fool it in a number of ways but why wouldn't you just replace the bulb?

Sent from my SM-G930V using Tapatalk
 

Fred930

Scirocco MK II Track Car
Location
Wisconsin
Thanks for the reply - no one else has chimed in. It's actually a pretty detailed question, generated from another thread where an LED Fog Light supplier was having difficulty trying to "fool" VW's bulb-out function. The car apparently makes a system wide test on many lamps during start-up, and a somewhat different test when the circuit is actually switched on, making it possible to pass one test and fail the other.

Since LED's consume so little power, the car "sees" them as burned out bulbs and sets a fault code. So people installing LED replacement lamps also have to install a parallel power resistor, which continually wastes power in a simple attempt to pass the bulb testing process.

I was hoping someone had detailed knowledge of how VW implemented the (pretty sensitive) system tests, which might lead to a better way to avoid the error codes.

Contributors to the Fog Light thread have experimented with different parallel resistor values -most recently having good luck with a 5.6 ohm, 50 watt unit. But other LED replacements might need different resistor values, depending on the power rating of the original bulb.

I suppose the easiest solution is to simply avoid "upgrading" to LED's!!
 

zrickety

The Fixer
Location
Unknown
Car(s)
VW GTI
I think you can turn off the cold diagnostics for all the lights, that would stop the error. You need vagcom. Resistors are another way but then you are just generating wasted heat.
 

GIACUser

Master Wallet Mechanic
Location
USA
Car(s)
MK 6 GolfR
I think you can turn off the cold diagnostics for all the lights, that would stop the error. You need vagcom. Resistors are another way but then you are just generating wasted heat.

After dealing with this recently (LED fogs - look great by the way).

1. There are 2 types of bulb checks going on that work differently.
2. Cold - happens at startup - can be disabled by vagcom.
3. Warm - continuous while running - no ability to disable.
4. 5.6 ohm power resistor turned out to be the perfect solution that would take care of Cold and Warm diags and prevent false bulb out.

Credit to GTIslow for defining the 5.6ohm solution to the issue.
http://www.vleds.com/vlr-il.html

These came with a 9006 splitter cable, I swapped one of the ends to a H11 turning it into a splitter/adapter cable:


Bottomline there was no way with vagcom to option off ALL bulb checking. Cold yes, Warm no.

My LED setup came with an 8 ohm power resistor. It turned out to be of a value that would take care of the warm diag problem but not the cold diag problem. So, I could turn off the cold diag with vagcom and the resistor took care of the warm diags.

Way more info than anyone wanted probably.
 
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zrickety

The Fixer
Location
Unknown
Car(s)
VW GTI
Good info, most detailed resistor number I've seen.
 
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