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I am in the process of wiring in a universal trailer light controller into my 2014 Focus ST. In doing so, I've run into a very strange issue which is just beyond my electrical knowledge and I'm guessing it has something to do with CANBus so I've Googled around a bit but nothing seems to match my particular issue.

All the other input pins on the tail light are switched normally, L Turn, R Turn and Tail/Running light are all 0v when off and 12v when switched on (obviously intermittent 12v on the turn signals). There is NO PROBLEM with those three inputs.

The equivalent wire that goes to the brake light bulb, however, is ALWAYS 12v when grounded to the chassis, whether or not the brake is pressed. But yet the bulb correctly turns on and off with the brake.

I feel insane even writing that, but I don't understand how the brake light is switched. Is it a ground switch? I left my voltmeter's positive lead pushed into an open fuse location with +12v and got a complete circuit on the ground side of the brake light socket as well so I don't see how it can be. Can anyone explain what's going on?

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  • It sounds like the switch is on the ground side as you suggested. You may be measuring continuity through the bulb filament, could you remove the bulb and repeat your last test? Alternatively if you have someone to help you could measure for a circuit from the ground pin on the brake light to the chassis (or battery -ve post to be sure) and have them press the brake pedal, there should be continuity when the pedal is pressed.
    – Sam
    Nov 12, 2015 at 9:06
  • I believe it is a ground side switch as well. You may want to wire directly to your brake lights to make this work.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Nov 14, 2015 at 2:34

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I did finally figure this out. The newer Focus uses CANBus to detect bulb and circuit faults, so the brake light circuit ALWAYS has 12v going through it, with a clamped amperage to prevent the bulb from illuminating. When you press the brake pedal, the "clamp" goes away, allowing the bulb to glow.

The other factor, is the car's BCM detects load on the circuit, so wiring the trailer lights DIRECTLY to the brake like circuit also causes a fault as the BCM is seeing too much draw and not only faults, but shuts down the circuit.

Solution to both issues is to wire in a SPST relay and source 12v from the battery directly (actually there's a conveniently placed fuse box in the rear hatch).

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