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yet more rear brake check fun

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    yet more rear brake check fun

    my rear check relay controls and checks all the lights properly except there's no power going out of pin 54KL to the third brake light. my understanding from the attached circuit diagram is that when 54K and 54L light up, which they do, it completes the circuit to 54KL when then goes to the third brake light, but that's not happening. I can live with two brake lights, but any thoughts as to why that might happen? I'd luv to get rid of this annoying check light. thanks!

    54KL at the check control unit needs to be pulled low in order to not trigger a brake check, it should have continuity to ground when you step on the brakes. When all 3 brakes work, their respective relays close.

    54KL is not a power source, I'm pretty sure it's just a digital input pin, which detects when it is pulled logic low (ground).

    If I'm reading right, you say your 3rd brake light doesn't work? That's your problem, it needs to work in order to trigger the relay tied to 54KL and the subsequent reed relays in the rear brake check relays module.

    I have a wagon which doesn't have a 3rd light, so I'm not familiar with it. Is there a relay inside of the 3rd brake light? You could short across it's switch contacts if you didn't want a 3rd brake light, but personally I would fix your 3rd light, it's an important safety feature. E30's need all the help they can get in the safety department.
    Last edited by earthwormjim; 01-19-2020, 10:49 AM.


      I would advise not to troubleshoot wiring unless you have a specific reason why the wiring may be comprised. The active check control uses a many transistors which fail over time and will cause false alarms. Only after replacing your check panel control system with one that is brand new or refurbished can you truly be informed if the system is working properly or not. Using another used one will only have the same problems.

      This same issue occurs within the cluster SI board. This is why people report that "smacking the dash" will make their gauges work again, this is because the 20+ transistors on the SI board, which are responsible for powering and controlling the gauges, fail to work after a few decades. Once replaced, gauges working robustly and accurately again. Same deal with the check control panel, when we refurbish these, we replace all the transistors and it functions as it should again. I have seen this occur on most E30's.

      Hope that helps!
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        very interesting, thanks for all the detail. my car came in parts and I'm slowly piecing it back together. my car should have a 3rd brake light and I will add it. Upon further inspection I can't find continuity between 54KL and the check panel at all. I need to dig deeper and try to find that green/black wire and see what happened to it. alternatively I could just delete or jump this check light as it's not even that useful. thanks!