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Discussion Starter #1
I'm in the process of wiring in the Tekonsha P3 trailer brake controller to add trailer braking for a race trailer. I've got it all hooked up, but have a question about powering it.

Since it wants a 20amp circuit I didn't want to tap into anything existing. I used an add-a-fuse from the glovebox fusebox, but no matter what I choose it looks like everything has constant power. I'm looking for switched power so it is only powered while the key is in 'on' position.

Any tips from how others have powered a trailer brake controller before? Safely/properly is what I'm looking for, I'd be willing to change how I'm doing it.
 

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2002-2005 Range Rover MkIII / L322
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what year is your truck?
I think you need to reread your brake controller wire diagram,
it gets power directly from the battery hot post and the power is routed via a 20 amp self resetting breaker.
No switched power here.
This is to prevent the controller shutting off due to a blown fuse thus losing the brakes function
 

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Check the manual for your brake controller, mine required constant 12v, it has a sleep mode where it puts itself to sleep and won't drain the battery. I just used the 12v cig. lighter circuit in the center cubby. I remove the cig lighter plug and installed 2 usb plugs so I don't have to worry about someone overloading the 20A circuit.

If you really insist on having 12v switched power, I found the easiest location is from the big waterproof electrical box under the hood, I believe this is where the main ignition relay is located and 12v switched power (red/white wires) is distributed from.
 

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etischer same for you,
please reread your wire diagram instructions and read the link I posted.
your system may work fine till the cig lighter socket gets shorted out then the brake function will be lost.

FWIW my center power socket had a penny fall into it, thus blowing the fuse.
installing the USB power ports can also cause a shorted outlet.
 

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Discussion Starter #6
The Range Rover is a 2003.

I've read the linked article which is very helpful. I'm looking for an alternative to power it with a pull from inside the cabin. The article also doesn't address switched power as they pull from the engine +12v post which is constant.

So, to try to rephrase, what's a good source to pull a switched +12v from inside the cabin? I can handle the fusing of it appropriately with a self resetting 20a fuse as per the instructions.
 

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Eric I am surprised that an engineering type person like yourself that works at an electrical powered car company has not followed the MFGs instructions for wiring a safety item into your machine.

I will say it one more time the MFG of the brake controller doesnt want the power supply to be fused, this is a critical safety item

Power is wired directly to + 12v source power, the WD diagram includes a self resetting 20 breaker in the circuit.

The reason for this is if you should be towing a big trailer,
and need the brakes and if it was on a circuit that was fused and the fuse blew,
then you will lose the brakes for the trailer, thats why a direct power source is required.

The 20 amp breaker will reset once it cools off this can take seconds,
and a blown fuse will require stopping and replacing the blown fuse,
if you dont have brakes on your trailer due to a non powered brake controller then you could have an accident
 

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Reach, you are missing the point. Here is a copy of the home page installation article on installing a brake controller in a 2003 L322. If you really are worried at all about safety you will have to accept that brake controllers require full power. To ignore this very basic concept is fool hearty and dangerous. If you are in accident due to trailer brake failure you are inviting insurance to fail to pay any claims due to improper installation of after market safety equipment, lawsuits and even worse injuries or death of innocent bystanders due to your negligence. You either need to install this properly or deliver your rig and controller to a trailer specialist.

Power Supply and Ground Lines
The +12 volt supply wire should be connected as directly to the battery as possible, via a 20-amp self resetting circuit breaker. (If you connect it to some vehicle circuit controlled by one of the interior fuse boxes, there is a risk of losing the trailer braking function when that circuit malfunctions or blows a fuse). Accordingly, it is desirable to run the power supply and ground lines through the firewall to access the battery. Fortunately, on the RR III one of the major wiring harnesses goes through the firewall just above above and to the left of the brake pedal. The harness is enclosed in a rubber boot where it goes through the firewall -- so I was able to expand it slightly to accmodate the additional wires (see photos below).
 
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