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Wednesday, October 01, 2014

Doug's Domain

Doug Vetter, ATP/CFI

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April 12, 2004

Brake light switch replacement

(Image: Closeup of brake pedal switch) The car has been running well the last month, so there hasn't been much to report. However, the other day the OBC reported "Brake Circuit Failure". The warning appeared as I was pulling into a parking space, and it didn't say anything about a Brake *Light* Failure (a warning I'd seen before), so I instinctively figured it had something to do with the brake pedal switch. No big deal, really.

However, the real question at this point was whether I had any brake lights for the drive home. Being alone at the time, I managed to wedge my ice scraper / brush (about 3 feet long) between the brake pedal and the front seat to discover that I DID, in fact, have brake lights. The OBC kept flashing the annoying warning (because, according to the manual, any failure having to do with the brake system is considered a "priority 1" failure, or something serious enough to warrant incessant nagging). I turned the car off to go into the store, and when I came out and restarted it, the warning was gone. Evidently, this was an intermittent fault. The fault occurred once more that weekend, so I decided to take care of it first thing Monday morning.

I needed only repeat the warning message to my mechanic for him to confirm that the problem was, indeed, due to a faulty brake light switch. He offered that the switch had been updated several years ago due to a high failure rate and that I was "pretty lucky" to have received more than double the expected service life out of it. Apparently they had a nasty habit of failing just out of warranty (like everything else on a BMW). :-)

When I inquired as to how the computer actually determined that the switch had gone bad, he said that the switch is actually two switches that are actuated in tandem, and that if the computer doesn't see both switches open and close simultaneously, it flags the error. Obviously, that would be the case if one of the two switches failed entirely, but it obviously caught an intermittent condition as well. Methinks the OBC was worth the money, as without it, I'd have never known about the problem until the other switch failed, leaving me without brake lights.

When I asked about whether this stored any kind of fault code, he said "no". Turning the key off and then back on again is sufficient to clear the warning, unless, of course, it's a permanent failure, at which point the condition will be flagged again the instant you depress the brake pedal.

The switch took a couple minutes to replace and I was quickly on my way. And, yes, this is something I could have done myself, but I was in a rush -- as usual -- and couldn't take the time to screw with it. A little pricey given the time it took to do, but I've received enough freebies from my mechanic lately that I considered it a wash.

Total parts $25, Total Labor, $36, Total $65. Mileage: 94244.