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Here are the basics of the Bulb Failure Sensing operation —
The Bulb Failure Sensor tests for equal current on each side of these three pairs of lights (this includes 3 bulb brake lights):
Note that there will be NO warning light if BOTH bulbs in a pair are out.
• Tail lights
• Brake lights (including rear window light)
• Low headlight beams
You can isolate the source of the BFS warning light to one of these groups with the car running—or with Key ON and small red D+ Alternator wire disconnected—as follows:
1 – Step on Brake pedal. If no Warning light,
2 – Turn Parking (tail) lights on. If still no Warning light,
3 – Turn on Headlight Low beams. This should be the cause of the Warning light.
If Warning light is ON when all these lights test OK, it could be the BF Sensor at fault. But more often it is simply due to a "mismatch" (bulb age, bulb socket condition), or corrosion at the ground wire for Left or Right light groups.
Contrary to the conventional wisdom, I have seldom found it necessary to replace both bulbs in a pair to cure a False Warning problem. In my experience (16+ years and as many cars, counting friends), False BF Warnings are VERY rare. And I've seen many systems work fine with different bulb types and brands. IMO, socket contacts and the common ground connection are prime suspects, if all 3 light groups are working.
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Bruce Young
'93 940-NA (current), 240s (one V8), 140s, 122s, since '63.
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