RWD - Bulb sensor repair -'Haha! Made you Look'

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Bulb sensor repair -'Haha! Made you Look'

Thanks, Art. If you can remember that old childhood taunt, it sure worked for me as your suggestion made me go back and have another look at my bulb sensor even though I'd gone over it a number of times with a fine toothed comb (kept getting caught up in the coils).

You surmised a broken circuit between terminal 54 (from the brake switch) and the sense coils (which then go to the brake lights). That was my first thought also, but it had checked out okay. My tracing had it narrowed down to 54 majorly leaking out (as in shorting) into the dash bulb light circuit (31-K), hence power from the brake light switch taking the easy path to ground rather than illuminating the brake lights. Why it wasn't blowing a fuse was a bit beyond me at the time, but it seems it was going through enough circuitry and wiring to put a small load on it.

Here now, thanks to your encouragement, is the conclusion of my tale.

Double checking everything in case you were right, I also re-checked the short I'd previously seen into 31-K, but it was no longer there. What?? With alligator clips on assorted pins, all of a sudden the meter beeped while I was handling the bulb sensor. Twisting the boards a little was all it took to make the short happen and go away. OMG, like maybe a bad solder bridge I'd missed that was barely shorting under the wrong tension, like when being inserted in the relay tray socket. Well, 30 minutes later with my trusty meter, dental pick, plastic tweezers and the brightest light and best magnification I could muster, I finally isolated it as an intermittent bridge between the reed switch and the inside of the brake light inductance coil. Trying to tweak and centre (yes, that's how we spell it up here) the wire leads for the reed switch only provided limited benefit, so I decided to remove it, straighten it and put an insulating sleeve on the leads to guarantee it would stay away from the inside of the coil. I almost had it all back together when the ceramic sleeve holding the teensy reed switch inside decided to crumble -end of that thought, so heck, why not just leave the reed switch out permanently, next best thing to a bypass. Then, during subsequent handling and testing, another reed switch shorted out, obviously the ceramic on these reed switches is now crispy fragile after all these years. So, high time to declare this bulb sensor relay toast and re-wire it as a bridge. As a bit of a purist, I'll probably put in a new bulb sensor at some point as they are mildly useful especially as I'm not the principal driver, but for now I'll use the bypass.

BTW, Can you remember these were introduced in the 1974 140s and first known as a Bulb Integrity Sensor, whereas they are now known variously as Bulb Failure Warning Sensor (later 240 manuals), Bulb Failure Sensor (740 manual), Bulb Malfunction Indicator (940 manual), and in catalogues as Bulb Failure Relay or just Bulb Sensor (most aftermarket catalogues) and even Brake Light Relay (in one catalogue) -makes them a little trickier to find when google searching for prices.

Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now


New 1 Bulb sensor repair -Art? Anyone else?
posted by  Dave Stevens  on Sat Apr 6 22:27 CST 2019 >

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