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Bulb Out Indicator Question 700 1990

uid 18083's pic
posted by  lucid on  Mon Mar 29 11:42 CST 2004
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last visit: Fri Oct 24 12:31 CST 2014
[RELATED]

How the system checks the three bulbs, I don't know but it does.

This bit of Volvo trivia had me stumped too until I autopsied a BF Sensor. The basic principle is that.an equal current thru two opposing coils (having the same number of turns) will have no magnetic effect on the reed relay inside the coils.

The 3-Light Brake Light test circuit is similar, except that the Left bulb is tested by one coil (say 20 turns), opposed by two coils end-to-end, 10 turns for the Right bulb and 10 turns for the High Stop bulb.

As long as all three bulbs work, the 20/20 coils cancel out. However, a failure of any one of the three bulbs will upset the 20/20 "magnetic balance" and the sensor relay will light the BF Light.
--
Bruce Young
'93 940-NA (current) — 240s (one V8) — 140s — 122s — since '63.






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