E46 Headache

My E36 cars (I've had two of them) displayed BRAKE LIGHT CIRCUIT ERROR on the OBC. The cause of this trouble is a switch on the brake pedal.

If your error is BRAKE LIGHT OUT, then your problem may be different. I do not know if the E36 and E46 cars share the same messages for the same problem.

In my case, there was an error that would pop up from time to time, and turning the ignition off then on again would clear the message for various time periods and applications of the brake pedal, but the message would eventually return AND the brake lights worked properly all the while. Basically, it was a false report.

I removed the switch and cleaned it, and made a slight adjustment (bend) to the internal contacts to change the timing, but I suggest that most people should simply buy a new switch and install it. My theory is that the switch is binding internally, and the circuit is expecting to see a result based upon a different set of contacts internal to the same switch.

Contact Set A does something (as a result of you pressing the brake pedal) and contact Set B binds for a brief moment, and the expected electrical result passes through these contacts. Since the contacts are bound (or bent) then the expected signal does not exist at the moment it is expected, and an error is reported, but the bind has since gone away so the brake lights actually work properly. This malfunction gives an E36 a BRAKE LIGHT CIRCUIT ERROR message.

Jeff, Thanks for the response. I'm all too familiar with the common E36 woes. Most of my previous cars were E36s. I checked the brake switch, contacts, etc before bringing the car to the local BMW Stealer... err dealer. :-)

Since my original post, I've done more internet research and found that I probably need to break out the digital volt meter to do some resistance checks on the bulbs while they are in the assembly. I must remeber to have the assembly unplugged from the chassis's electrical system. :-)

I'm headed out to the garage to go do that now. I'll update you when I know more!

-Karl

Ummm, the Miata is one of the most bulletproof -- and fun -- cars in existence.

...and this piece of junk you bought is superior to a Miata? 8;)

-- Larry (2002 E46 325i and a '95 Miata R...)

I have heard that cheapo bulbs can play havoc with the BMW sensors!

I've seen it reported some 'no brand' aftermarket bulbs can give this result. Was it a BMW main dealer and did they use genuine spares? The clue seems to be if the base is plain brass rather than 'silver' plated. Bulbs which have the EC mark on them should be ok.

Have you looked at the bulb contacts for corrosion?

Fun, when the top is down. Claustrophobic, when the top is up.

"Top"? What is this "top" of which you speak?

-- Larry (6'3", 200lbs; '95 R, Hard Core, 5-pt. belts; _no_ top; no claustrophobia...)

Geez, I don't suppose you've ever driven a Lotus Europa, then...

-- Larry (ex-'73(?) JPS Europa...)

btw, the reason I mentioned this: the bulb sensor uses a low current signal to check the bulb, while the signal for lighting up the bulb has much more current and voltage.

Some minor corrosion on the contacts will interfere with the low-current sensor signal, while the much higher "light the bulb" current will just blast through the corrosion.

Dunno how you arrive at this conclusion. A given resistance has a greater effect at higher current.

When I swapped the old bulb high level brake lamp on my old 740i I used the LED type - Nice but the bulb check would flash the LEDs once every 10 secs so I had to make a smal timer circuit that would allow the LED brake light to work but not flash. The Bulb test circuit is quite powerful but is only microseconds in time so the incandescent filament doesn't have time to get hot but the LEDs don't have to get hot to output light.

Your problem is probably corrosion, cheapo bulbs or a burnt track on the light unit itself which is NOT unusual.

What about the third brake light?

That's what I am talking about

That is true, the voltage drop across a given resistance is greater for higher current. But only if the resistance does not change with the amount of current. A thin corrosion layer may give way when faced with the higher current level of the "light it up" signal.

I just mentioned it as a possibility, as I have seen a similar effect in other electrical devices.

I have had to replace 1 assembly, I was fed up of getting false warnings on the dash, the bulbs were always ok. it is a problem - the dealer acknowledged they sell a lot of assemblies. Sad but true.