Hi Dave,

I changed the topic heading from repair to delete, just in case there is some lonesome soul out there who arrives here wishing to repair a bulb failure warning sensor rather than bypass it. I knew this thread was heading for the delete category as soon as I read your original posts, with references to reed switches, Dave Barton, transistors, diodes, capacitors, and by gosh, an ESR meter. None of those things have any involvement whatsoever in lighting your dark brake lamps!

The theory of the sensor is to take the smallest samples possible of the current to the individual lamps to provide an alert when a path fails. That tiny sample is just enough to magnetically move the iron reed in the glass tube, yet because of the exponent in the relation between voltage and power, that reduction in lamp brightness is tiny-squared. What goes on with the stuff that turns on the warning indicator I've explained somewhat in that link Kitty posted Deleting Bulb Failure Circuits (TB)

You won't notice the difference between a bypassed unit and having the sentry in place, doing its job for lighting integrity, safety, and avoiding increasingly dangerous traffic stops -- unless you could perform the bypass at night while your headlamps are illuminating a reflective sign down a dark highway, switching it in and out observing the increase in light output.

The trouble with these 86 and later sensors might be said to have its root cause in the high-mounted brake light mandate. Hella was building them with reliable point-to-point wiring previously, then with the added complexity stuffed into the same form factor put the new circuitry on three tiers. The interconnection had to pass the high lamp current through wave-soldered pins whose joints eventually fractured like most every other high current solder connection in the 80's-era "electronic" relays.

That broken solder connection in the path from pin 54 is what kept your brake lights from lighting. It was difficult to build, and hard to get in there to repair. It is even hard to photograph. What's easy is to ruin one trying.

It would only be luck that you could diagnose this open circuit with the continuity beeper on a multimeter. Like you noticed, a fractured solder joint is mechanically sensitive. Diagnosis is done in circuit, under load, not by "testing" components; you have battery voltage on pin 54 and none on pin 54R, 54L, or 54S. I understand this is pretty easy poking a test light or meter probe in the back of the socket in a 240, but maybe a challenge doing it in the ashtray of a 7/9.

While I like to have mine working, and by "mine" I mean those in the cars my family drives, I do have one unit which was ruined converted to a "test" relay.

Delete the Bulb Sensor:







--
Art Benstein near Baltimore

"Sometimes I think the greatest talent of all is perseverance. But only sometimes." -Mitch Albom