3rd brake light works but R & L tail lamps don't

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81cutlass

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Feb 16, 2009
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Wondering if anyone knows where to track this down. 86 Grand prix with a 3rd brake light.

My turn signals, hazard, and normal tail lamps work as normal, so that means all my wiring (at least from the column back) and bulbs are fine.

My brake lights (left and right tail lamps) don't work but the 3rd light does. That means the brake switch on the pedal is fine.

Is that something in the turn signal switch? I can't find an 86+ diagram to understand how the 3rd brake light works. I am having a hard time understanding how the 3rd brake light works but the tail lamps don't. Is there some diodes in the turn signal switch, or an additional harness that runs from the brake light switch back?
 

81cutlass

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Feb 16, 2009
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Since both my turn signals and hazard flashers work the wiring from pins 18 to 19 back is fine. Obviously they are getting power so the fuse is fine, and the 3rd brake light works so the switch is fine. That aluminum power wire must be fine?

I just can't find a diagram for 86+ 3rd brake light cars. Where does the 3rd brake light get its signal from? Post turn signal switch or pre?


1625980349264.png
 

ELCAM

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Jun 19, 2021
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The third brake light gets the signal before the 4 way and TS switch. See 17 in the above diagram it would come from there and have its own circuit. That way the 3rd still works if the 4 way/ TS switch fails.
 
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airboatgreg

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Oct 2, 2016
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Look at the connector for the turn light switch at bottom of steering column. As I recall a white wire will be burnt. You can just bypass the connector
 
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oldsmobile joe

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Nov 12, 2015
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Here's a pic from my 86 cutlass fsm. Should be similar
20210711_083647.jpg
 
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ELCAM

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Jun 19, 2021
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Look at the connector for the turn light switch at bottom of steering column. As I recall a white wire will be burnt. You can just bypass the connector
If you find this wire burned it usually means there are other problems in the circuit creating higher current draw. Sometimes it is the connector pins (there are several connectors in the circuit) getting corroded, corroded sockets or a bad ground or other wires.

Also trailer wiring can do this, pretty easy to overload the circuit with a whole other set of bulbs.
 
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81cutlass

Comic Book Super Hero
Feb 16, 2009
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13,542
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Western MN
Look at the connector for the turn light switch at bottom of steering column. As I recall a white wire will be burnt. You can just bypass the connector

If you find this wire burned it usually means there are other problems in the circuit creating higher current draw. Sometimes it is the connector pins (there are several connectors in the circuit) getting corroded, corroded sockets or a bad ground or other wires.

Also trailer wiring can do this, pretty easy to overload the circuit with a whole other set of bulbs.

Thanks for the help everyone. It helped a lot knowing the 3rd light is on a separate harness and bypassed the turn signal switch. That made a lot of sense.

I took a look at the plug at the base of the column and although the wires weren't obviously burnt that white wire going up to the turn signal switch was pretty corroded near the pin. I unplugged the connector and probed it and I'm pretty sure it was a bad connection.

The car does have a trailer hitch and used to pull a boat in the 80's according to the son of the original owner I bought the car from so it looks like that pin might have gotten warm decades ago from extra load and has corroded over the years.

I need to pull that pin out of the plug, possibly put a new pin on and use some electric cleaner.

It's working now.
 
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