Check Panel Issue - brake light warning

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jacobthegoat
Posts: 60
Joined: Mar 20, 2020 12:59 PM
Location: Nashville, TN

Check Panel Issue - brake light warning

Post by jacobthegoat »

e28 - 1988 528e manual

Issue - I've erased all the check panel indicators EXCEPT the warning light telling me my brake lights aren't working.
I've chased this issue to no avail.

I've replaced the bulbs, bulb housing, brake light sensor behind the brake pedal, swapped 3rd brake light to a new (known working) unit and new bulb. I've also recently replaced all my fuses. The little LED haunts me and I cannot figure out why.. Am I missing something?

The warning light has been on since I bought the car 4 years ago and I've fixed all the issues except the brake light warning. I am aware the indicator comes on until you push in the pedal, but should go away once you depress the pedal. I just feel like i'm missing something. Any ideas?
RonW
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Joined: Feb 12, 2006 12:00 PM
Location: Long Island, NY

Re: Check Panel Issue - brake light warning

Post by RonW »

Assuming your brake lights actually work, My guess is there's an issue with the check relays. The check relay is a very low impedance coil of wire and a reed switch. When current goes through the coil (and through the bulb), it closes the reed switch. This grounds the signal back to the check control, and tells it the bulb is working.

Everything following is to be found by examining the ETM for your car.

The reed switches for all three brake lights are wired in series, so that if any bulb is out, the brake LED in the check control will turn on.

The check relay for the 3rd brake light is part of that assembly. The green & white wire is the signal back to the check control. When the brake lights are not on, that wire should not be grounded. I expect it should be at +5V, but I don't know that for a fact. Check it with a voltmeter, relative to ground. If it's at 0V when the brake lights are off, then your problem is either at the check control or at the intermediate connection at pin 9 in C206, which is on the firewall above the driver's kick panel.

The green & black wire from the 3rd brake light reed switch goes to the reed switches for the other brake lights, and the 3rd brake light reed switch connects the green & white wire to the green & black wire when the 3rd brake light is on.

The check relay box for rear lights is a separate unit from the rear light clusters. The reed switches for the main brake lights are, as I said, wired in series. They connect the green & black wire from the 3rd brake light to the brown wire, which is ground.

When the brake lights are on, the green & black wire should be at 0V. If it's not, the problem is within the check relay box.
Ju@n
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Re: Check Panel Issue - brake light warning

Post by Ju@n »

Along with what Ron mentioned, you could try buying another used check panel, I had one that randomly told me that brake lights or tail lights were out (and they weren't), checked every thing, even vdrop but it turned out to be a cold solder joint in the ground pin on the check panel itself.
cek
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Re: Check Panel Issue - brake light warning

Post by cek »

If all else fails in diagnosing this, just leave it as is, as I have on Maytag.

Seriously, the only thing left to try for me is to bypass the reed switches completely.
Mike W.
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Location: California Whine Country

Re: Check Panel Issue - brake light warning

Post by Mike W. »

RonW as always out out some excellent info. I will however throw out a little more.

The check control relay in back can fail, I've seen them bad, and cause your problem. Also, the taillights (assy) are prone to bad grounds, especially on the brake light circuit. They rely on a folded steel bracket making good contact and after 20 or 30 years it doesn't always. It's possible you have a less than perfect ground that is good enough to work the lights (at least most of the time) when you push on the pedal, but on one side or the other is marginal enough to set off the check control light. This usually rears it's ugly head troubleshooting cruise control issues, but it could apply here too. Actually that's a corollary question, does the CC work with no issues?
jacobthegoat
Posts: 60
Joined: Mar 20, 2020 12:59 PM
Location: Nashville, TN

Re: Check Panel Issue - brake light warning

Post by jacobthegoat »

Thanks for the info guys.

Ron, i'm breaking out the multimeter today and will come back with my results. Thank you for the info.

Mike, my brake lights are working as they should, and my first inclination was to check the grounds. They are making good contact. My CC is not functioning, the stalk is broken on the column when I purchased it. Occasionally, maybe 5-6 times total, the brake light warning would function properly, and after a couple miles and a few red lights, the warning light would come back on. It has been permanently on for about 8-12 months now.

I will keep my eyes out for a check control panel, and might pull it out today for an inspection, as I have not done this yet.

Thank you all for your info!

JW
cek
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Re: Check Panel Issue - brake light warning

Post by cek »

jacobthegoat wrote: Dec 20, 2020 3:45 PM Thanks for the info guys.

Ron, i'm breaking out the multimeter today and will come back with my results. Thank you for the info.

Mike, my brake lights are working as they should, and my first inclination was to check the grounds. They are making good contact. My CC is not functioning, the stalk is broken on the column when I purchased it. Occasionally, maybe 5-6 times total, the brake light warning would function properly, and after a couple miles and a few red lights, the warning light would come back on. It has been permanently on for about 8-12 months now.

I will keep my eyes out for a check control panel and might pull it out today for an inspection, as I have not done this yet.

Thank you all for your info!

JW
FWIW, I think I just fixed Maytag (again). I pulled the center brake light and put a jumper across the reed switch. I tested it and it fixed the issue.

So I pulled the jumper off and reconnected everything. Tested again and the issue was still fixed.

(So, in other words, nothing changed but it was fixed).

So I reflowed the solder on the little reed switch board that's part of that assembly. It now seems really fixed.

I'm skeptical but happy. YMMV.
jacobthegoat
Posts: 60
Joined: Mar 20, 2020 12:59 PM
Location: Nashville, TN

Re: Check Panel Issue - brake light warning

Post by jacobthegoat »

Mike W. wrote: Dec 18, 2020 9:52 PM

The check control relay in back can fail, I've seen them bad, and cause your problem. Also, the taillights (assy) are prone to bad grounds, especially on the brake light circuit. They rely on a folded steel bracket making good contact and after 20 or 30 years it doesn't always.
After chasing this issue for 3 years, cleaning every ground and checking all the relays, I removed the entire rear tail light assembly, removed the sockets and gave the metal grounds a good cleaning and viola, the check light is gone.

Thanks for the info; another problem solved. On to the next one!

JW
jacobthegoat
Posts: 60
Joined: Mar 20, 2020 12:59 PM
Location: Nashville, TN

Re: Check Panel Issue - brake light warning

Post by jacobthegoat »

So the problem reared it’s ugly head again.
Took the 3rd tail light out and reflowed the solder for the reed and the ground. Issue gone.

Thanks for all the input. I hope this helps somewhere in the future.
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