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Discussion Starter #1 (Edited)
I have a 1997 F350 dually Power Stroke 7.3L 4X4 cab and chassi with a mason dump. Bought it new in 1998. I'm original owner. Recently I converted to LED tail lights and since then, the tail lights have been on dim when the key is on. Also I have noticed the ABS light has been on.

I checked the circuit with a digital volt meter and found 8 volts on the brake lights when the key is on and no brake pedal applied.

What could be causing this?
 

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Discussion Starter #2
Found the problem. Needed some resistance in the circuit. The third brake light on the top center of the cab is burnt out. After replacing the bulb, everything is working correctly. The ABS light went out, the 8 volt red glow from the brake lights went away, and the transmission is working fine. It was unlocking the overdrive everytime I let off the gas pedal.
 

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Electrical Ogre
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These trucks are touchy as you have discovered. The brake line feeds the computer so it knows when the brakes are applied. When the brakes are released, the computer receives a ground by way of the lamp filament as a redundant check to ensure the circuit is functioning. This will also KO the cruise operation if no ground is sensed. Of course the LEDs won't allow the reverse current, so it freaked out. If you convert the High mount brake to LED, add a ballast resistor.
 
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