"REAR LAMP OUT" but all lights work??? - Taurus Car Club of America : Ford Taurus Forum
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post #1 of 7 (permalink) Old 04-01-2019, 01:01 AM Thread Starter
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Question "REAR LAMP OUT" but all lights work???

I did a thorough search and could not find an answer.

Trying to figure out an annoying issue:

The dash would show a warning "Rear Lamps Out" only when I press the brakes, off when I lift off. Double checked the bulbs, fuse and fluid level, all good.

I pulled off the switch's connector and no warning light or stop lights ( of course )

Question: Could the switch be faulty? I tried to spray some electronics parts cleaner to the switch but no change.

Wondering if anyone else has gone thru this?

TIA,


Jim

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Last edited by Draco911; 04-01-2019 at 01:15 AM.
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post #2 of 7 (permalink) Old 04-01-2019, 06:18 AM
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Brake light switch is not the issue.

For the heck of it, try replacing the brake light bulbs. There is very low ohm resistance wire in the circuit to the brake lights that has a small voltage drop across it that is proportional to the current the bulbs draw (Ohms Law). As bulbs age, their filaments evaporate, their resistance goes up, and the current draw drops. When the current drops below a set value, the lamp out module turns on the light.
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post #3 of 7 (permalink) Old 04-01-2019, 08:18 AM
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Quote:
Originally Posted by Jeff K View Post
Brake light switch is not the issue.

For the heck of it, try replacing the brake light bulbs. There is very low ohm resistance wire in the circuit to the brake lights that has a small voltage drop across it that is proportional to the current the bulbs draw (Ohms Law). As bulbs age, their filaments evaporate, their resistance goes up, and the current draw drops. When the current drops below a set value, the lamp out module turns on the light.
Different car but: burned out turn signal bulb and I replaced it. Then after a few days it quit but not out. Pulled the bulb and put it back all OK, few days and repeat no light. The bulb was one on my shelf. I went to the store and bought a pair. Replaced both and problem solved. More modern car with electronic turn signal with adjustable volume click. Electronic systems can be sensitive to un-balance. So I agree, start with new matched bulbs.


-chart-
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post #4 of 7 (permalink) Old 04-01-2019, 10:12 AM Thread Starter
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Oh Wow, these darn modern cars, they just suck you right into their money pit vortex :-(


Thank you gentlemen, I will give it a try and report back. Much Much Appreciated!


J

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post #5 of 7 (permalink) Old 04-01-2019, 11:32 PM Thread Starter
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Lightbulb

Update:

Masters Chart & Jeff!

Replaced Stop Light Bulbs 3457 and CHMBL Bulbs 921. BINGO, no more idiot light "Rear Lamp Out".

Yeap, all 4 old blubs, worked but they did look pretty sad, darken and aged. $14 buck later, all is well in the world ( 1st world issues )

Thanks again Fellas
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post #6 of 7 (permalink) Old 04-02-2019, 07:01 AM
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Quote:
Originally Posted by Draco911 View Post
Update:

Masters Chart & Jeff!

Replaced Stop Light Bulbs 3457 and CHMBL Bulbs 921. BINGO, no more idiot light "Rear Lamp Out".

Yeap, all 4 old blubs, worked but they did look pretty sad, darken and aged. $14 buck later, all is well in the world ( 1st world issues )

Thanks again Fellas
Bulbs have many years of history and refinement so their life span bulb to bulb is very VERY close. One does well to stay with name brand and replace in pairs. That black in the glass is molecules of metal that used to be filament. When one goes out the other is close behind. And HL are much worse. Average battery volts running 13.9V but Ford uses dropping resistor wire to the HL so they run at 11.8V at the same condition. Run those bulbs at bat volts and the life ~90 hours. At the lower volts ~1200 hours. OK, one bulb burns out and the volts on the other increase ~1.1V and that old bulb will quickly burn out also.


And for what is worth, I worked for a supplier of material to bulb makers and have been in their plants and seen their testing and certification.


Old rule: increase volts on a bulb by 5% and life drops to half. By math, V/V ^-16 for life.


-chart-
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post #7 of 7 (permalink) Old 04-02-2019, 11:27 AM Thread Starter
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Amazing info :-)

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