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Brake LT Circuit - See Owner's Manual

Discussion in 'E34 (1989-1995)' started by Stephen King, Oct 8, 2008.

    Stephen King guest

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    I have a 1995 E34 540i. "Brake LT Circuit" - "See Owner's Manual" This warning message appeared on my dash. I asked my BMW mechanic to check it out. He told me the K3 Brake Circuit Relay needed to be replaced. He replaced the relay for which our Dealer charged me $32. The warning message stayed off for about a month and then returned. I got another relay from the dealer at no charge. The faulty relay was replaced a week ago and now the warning message is back. Has anyone ever had a similar experience? What did you do to correct the Problem?
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    mooseheadm5

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    You need a new brake light switch. This is the cure 99.9% of the time. There are two sets of contacts there and the computer recognizes that one set is intermittently or continuously bad.

    RTrent guest

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    Make sure you buy the BMW part not an aftermarket part.

    I tried two Febi switches and they both gave the brake circuit error.

    Also make sure all the bulbs are osram nickel plated or you'll get an error too.

    The E34 CCM is very finicky

    Aloha

    pldlnr guest

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    I have owned 4 different E34s including two 1992 525i, 1992 535i, and a 1992 525it. Every single car had the same problem. That message would come up about once every two weeks. I have never fixed it. It is kind of a running joke in the family now.
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    mooseheadm5

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    It is not that much of a joke, there. It can mean that your brake lights are not lighting up when you step on the pedal. That could mean a wreck where you are at fault and you and your family can be seriously injured. I would take it a little more seriously if I were you.

    pldlnr guest

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    My brake lights have never stopped working because of the brake light circuit issue. I thoroughly investigated this situation the first few times it came up. The switch is not providing proper feedback to the Check control system. It has not caused my brake lights to stop working.
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    mooseheadm5

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    Yeah, I thought that the first time I encountered this problem as well. So I sat int he customer's car and stepped on the pedal about 50 times and was about to give up. I had a mirror behind me and then one more press of hte pedal and lo and behold, the brake light circuit warning is illuminated and no brake lights in the mirror. I have been able to verify this failure mode on about 10 different cars, and have since replaced about 30 or more brake light switches in E34s and E36s, and a few E32s. One of the times when you step on the pedal, your brake lights may not light up. I hope there is nobody tailgating you when that happens.

    pldlnr guest

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    Okay, I get your point. I will have to look into it. I have never noticed that they have stopped working and I have always paid attention, although I am sure it is impossible that I have noticed every single time the message has come up.

    I have done quite a bit of reading on this issue and have seen many people who have felt like I did, that it is not a big deal. It is the fact that the switch wears and becomes dirty and the resistence in the switch becomes slightly higher creating a feedback issue to the check control. I understand that the raised resistence could also be an indicator of issues with the wiring, sockets, and bulbs.

    But, if you are telling me that it is possible that the lights could stop working, than I agree that is is more serious than I had thought. I appreciate your advice.
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    mooseheadm5

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    Actually, there are two sets of contacts in that switch, so the bulb resistance has nothing to do with it. It checks both sets of contacts and when it sees that the monitor contacts are closed but that the brake lights are not lit (which it monitors through the Active Check Control,) it sets that warning. I have only had one instance where replacing the switch did not fix that problem, but that car was a nightmare 750iL anyhow.

    xlr8tin guest

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    So, where is this switch located that needs to be replaced?
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    mooseheadm5

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    Brake light switch located on a bracket atteched to the pedal box. You have to remove the knee bolster.

    xlr8tin guest

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    On the right side of my leg?
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    mooseheadm5

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    It is actuated by the brake pedal. You should be able to find it pretty easily if you can remove whatever is in the way of you seeing the brake pedal arm.

    xlr8tin guest

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    Are you talking about the cover with the speaker behind it...or something different?
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    mooseheadm5

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    I am talking about the panel under the steering column, directly in front of your knees.

    xlr8tin guest

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    Gotcha thanks...I'll take a look when I get the chance

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