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Discussion Starter · #1 ·
I'm in the possession of an 89 Thunderbird with the 3.8, and it has no brake lights or power going to the brake lights when the pedal is pushed. I also believe it has a bad dimmer switch as when the light fuse is in the corner lights stay on, but while trying to make sense of the wiring, and vacuum trouble shooting book I have I couldn't find an obvious reason why the brake lights don't work. As far as I can see the wiring is fine in the trunk where it all bunches together. If you have any suggestions as to where I should concentrate my efforts I'd be grateful.
 

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Do you have voltage TO the BOO (Brake On/Off) switch? Do you have voltage OUT? Are all three lights out in the back, or does the CHMSL (Center High Mount Stop Light) work?

If the CHMSL works but the others don't, that's normally a pointer to the MFS (Multi Function Switch).

Which is where you check after you check at the BOO if it's working.

If you're asking "How to check", next step is to pick up a trouble light and/or a voltmeter/multimeter. (I'd go for both, but a good trouble light can help you).

RwP
 

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Discussion Starter · #3 ·
Do you have voltage TO the BOO (Brake On/Off) switch? Do you have voltage OUT? Are all three lights out in the back, or does the CHMSL (Center High Mount Stop Light) work?

If the CHMSL works but the others don't, that's normally a pointer to the MFS (Multi Function Switch).

Which is where you check after you check at the BOO if it's working.

If you're asking "How to check", next step is to pick up a trouble light and/or a voltmeter/multimeter. (I'd go for both, but a good trouble light can help you).

RwP
I'm not sure where the BOO switch is, but it's all 3 lights that do not come on while braking. The 2 lower corner ones do work as blinkers, and running lights, but not as brake lights. I've got both, just don't know where to poke them at in the car.
 

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The BOO (Brake On/Off) is of course under the dash, activated by the brake pedal.

One thing that CAN happen is that the connector can work loose; this will cause a no-brake-light action.

So it's time to lay down and look up under the dash at the brake pedal rod.

Note: There's a white vacuum valve at the TOP of the brake pedal rod; that's for the cruise. The BOO is, IIRC, black in color and has wires from it.

RwP
 
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Discussion Starter · #5 ·
The BOO (Brake On/Off) is of course under the dash, activated by the brake pedal.

One thing that CAN happen is that the connector can work loose; this will cause a no-brake-light action.

So it's time to lay down and look up under the dash at the brake pedal rod.

Note: There's a white vacuum valve at the TOP of the brake pedal rod; that's for the cruise. The BOO is, IIRC, black in color and has wires from it.

RwP
Looks like the connector was worked lose. Thank you so much for your help. You are a saint.
 

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This is a pretty standard problem of the MFS.Take it out, take it apart and clean the copper contacts with 220 grit sandpaper, refill it with dielectric grease, it will last another 20 years.
The emergency flashers switch is in that circuit, so do it too.
 
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