How does check panel know bulb is out?

[blocke]

Working on a project for school..trying to wrap my head around this.
How does the check panel in our cars know a bulb is out? I am assuming its based off an open/closed circuit or a break in continuity, but can anyone explain?

Take it a step further..newer cars can tell you exactly what bulb is out. How is that accomplished?

Thanks for any help!

[Quadrajet]

As I read about it a couple years ago. When there is no measured resistance in the ground, the warning light turns on. And apparently it only takes a millisecond. As periodically my check panel would indicate a bad taillight.

Of course when I checked they would be fine. Finally pulled the bulbs one day and had a detached filament end, that would detach away then reconnect randomly, I assumed thru vibration.

I'm sure I'm simplifying the explanation. You may want to do a search, or PM Davesmed.

[Gregs///M]

Download the ETM (electronics troubleshooting manual) on this site and view the schematics for the check control panel. It should become all clear and the diagrams will make it more clear than anyone can explain.

[DaveSmed]

Here's the inside of the check control relay in the trunk (this one has been modified slightly).



What is going on with that circuit board is each circuit for the individual monitored lamps (each wire to the separate bulbs) passes through this circuit board where it is routed through the copper wire coils. It then goes right back out to the respective light.

Sticking out of those coils, you will notice silver axial leads. Those are magnetic reed switches.




What happens is when you step on the brakes, the brake light switch closes and allows current to flow through the switch, to the circuit board, through the coil, and finally to the bulb. This current flow also induces a small magnetic field around the wire, which is amplified by the many turns of those coils.

This magnetic field from the current to the brake light bulb in turn acts on the reed switch, closing it. That reed switch closure is what check control is looking for. Check control is connected to the brake light circuit directly after the switch, so it "knows" when to expect the reed switches to close. If they do not, then it sets an error.


The specified minimum current is set via the size and number of turns in the coils. The difference in size relate to the different current draw of a brake light vs. a license plate light for example.

[kmarei]

I picked up a spare check control relay
Will see if I can cut half of the length of the coil for the license plate bulbs
So I can install LEDs and not lose the bulb warning if one of them went bad
I will measure the current draw with the standard festoon bulb
And see if I can shorten the coil to fit the current draw of an led vs a standard bulb

Any clue on how to find out how many loops of the coil I need for a specific current draw?
The standard bulb is 12 volts, and is 5 watts
Which means about 400mA
LED uses a lot less, so I'd need to change the coil to work with say 50mA etc

[AndrewBird]

I'd try cutting it down based on the percentage of current drop. I.E. - If the LED uses 40mA, that is 1/10 the amperage, so you should keep 1/10 of the coil. I'm not sure if it would work out quite this way, but it would be a good place to start.

[Todd Black 88]

Here's the inside of the check control relay in the trunk (this one has been modified slightly).



What is going on with that circuit board is each circuit for the individual monitored lamps (each wire to the separate bulbs) passes through this circuit board where it is routed through the copper wire coils. It then goes right back out to the respective light.

Sticking out of those coils, you will notice silver axial leads. Those are magnetic reed switches.




What happens is when you step on the brakes, the brake light switch closes and allows current to flow through the switch, to the circuit board, through the coil, and finally to the bulb. This current flow also induces a small magnetic field around the wire, which is amplified by the many turns of those coils.

This magnetic field from the current to the brake light bulb in turn acts on the reed switch, closing it. That reed switch closure is what check control is looking for. Check control is connected to the brake light circuit directly after the switch, so it "knows" when to expect the reed switches to close. If they do not, then it sets an error.


The specified minimum current is set via the size and number of turns in the coils. The difference in size relate to the different current draw of a brake light vs. a license plate light for example.

Great post.
One question I have pertains specifically to my m3.

If I start the car with the brake pedal depressed, my rear lights bulb on my check panel will NOT go out, even if I release and reapply the brake.

If I start my car with my foot off the pedal the light, the light comes on but if I now press the brake pedal the light DOES go out.

WTF is wrong with my car? Brake switch or as I suspect, the check lights relay you posted?

Tia
Todd


Thread jack over.:)

[dwvw]

Wow, using the info in this thread I fixed my brake light check panel problem today. broken solder connection at the third brake light check relay!! Thanks guys.

[kmarei]

Figured out how to bypass monitoring for specific lights
I wanted to bypass the bulb check for the license plate lights because I put LEDs
You need to bridge the 2 tabs right underneath the B in the picture

Here is a guide to bypass the various lights
You need to refer to the plastic cover for the check panel for the names of the terminals
To bypass a light, you need to hook up ground or terminal 31 to it
K(E) is for the low beam
58KL is for the tail lights (excluding brake light)
KKL is for the license plate light
54KL is for the 2 rear brake lights (excluding the 3rd brake light)
To bypass the you'll need to do that in the small circuit board attached to the 3 rd brwke light

[treff-7]

There are 2 things you can do if you want to go LED and get rid of the warning light:

1. You can bypass the reed switch by soldering a wire between the 2 ends of it (just like the red wire on the bottom of the picture in DaveSmed's post. If you look closely it actually bypasses 2 of them. One for each side of the car). This way there will be NO warning light even if the light is out.

2. You can modify the coils to close the reed switch even with the reduced current. In this case you need to INCREASE the number of turns in the coil. The current decreased so did the strength of the magnetic field in the coil. In order to increase the magnetic force you need to increase the number of turns. If I remember correctly with ten times less amount of current you need 10 times more turns to have the same magnetic force but I might be wrong. Somebody smarter than me will correct this. You can use thinner wire because the current is lower.

[treff-7]

Great post.
One question I have pertains specifically to my m3.

If I start the car with the brake pedal depressed, my rear lights bulb on my check panel will NOT go out, even if I release and reapply the brake.

If I start my car with my foot off the pedal the light, the light comes on but if I now press the brake pedal the light DOES go out.

WTF is wrong with my car? Brake switch or as I suspect, the check lights relay you posted?

Tia
Todd


Thread jack over.:)

Todd,

I think your problem is either your battery or pure connection or ground somewhere in the circuit. When you start the car due to the high power draw of the starter the voltage drops too low to create enough magnetic field in those coils to close the reed switch so the module will think that the light is out. After the car is started and you depress the pedal again it will be fine but the module has already registered a bulb being out and the warning light stays on. Of course if you depress the pedal for the first time after the car was started you don't have this issue because the starter is not drawing all that power anymore and the alternator is charging.

[downforce22]

bump, anybody done this recently? I'd like to turn off the christmas tree i've got on my cars check panel when I turn on the lights and this might be just the ticket.