I have a '89 740GL and several months ago the light out indicator came on, but all of the lights are working. I have replaced the relay, but the indicator is still on. Has anyone had any experience with this? Any suggestions would be appreciated.

Thanks!

The basics of the BFS operation —

The Bulb Failure Sensor tests for equal current on each side of these three pairs of lights (includes 3 bulb brake lights):

• 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 as follows.
With the car running:
1 – Step on Brakes.... If no Warning light,
2 – Turn Parking (tail) lights on.... If still no Warning light,
3 – Turn on Low beams. Should be the cause of the Warning light.

If Warning light is ON when all these lights are OFF (or all test OK), it could be the BFSensor at fault. But more often it is simply due to a "mismatch" (bulb brand, age, or socket condition), or corrosion at the grounding wires for Left & Right light housings.

Despite advice to the contrary, I've almost never had to replace both bulbs in a pair to achieve a current balance between sides. IMO, it's another myth.

Note that there will be NO warning light if BOTH bulbs in a pair are out.

Also note that the only Front Lights tested are the Headlight Low Beams

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Bruce Young
'93 940-NA (current) — 240s (one V8) — 140s — 122s — since '63.

Thanks for that great description Bruce - didn't know it was just restricted to those 3 sets.

So does the rear window brake-light have 2 lights that have 2 separate wiring-lines going to them ? That's the only way I see the BFS monitoring it...

Noel

Thanks for asking Noel,

I love to explain this bit of Volvo trivia that had me stumped until I autopsied a BF Sensor several years ago, after installing an IPD Brake Light "improvement" that yielded contant false warning lights,

The basic BFS principle is that.an equal current thru two opposing coils (having the same number of turns) will have no magnetic effect on the reed switch 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, for example), 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 coil and the 2 x 10 coils cancel out. However, a failure of any one of the three bulbs will upset the 20=20 "magnetic balance" and the sensor switch will ground the BFS light.

Bruce
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Bruce Young
'93 940-NA (current) — 240s (one V8) — 140s — 122s — since '63.

Neat solution. Are there 2 models of BFS - one for that 3rd light (2:1 coils) and one for those without (1:1 coil) ? Or else there must be an unused coil(s) segment depending on whther that 3rd light is or isn't present...

Noel

Sometimes installing a bulb on one side (right or left) that dosn't match the resistance of the bulb on the other side will cause this. The other possibility is a bad ground on one side.

To isolate the bulb remove a pair (left and right) one at a time until the bulb-failure light goes out. e.g. Remove both brake-bulbs, then rear parking bulbs, then front parking-bulbs etc.

Noel