I frequently receive questions about using a vehicle’s power outlet to supply power to Braking Systems. Most "Box-style” systems require a dedicated power supply, terminating in a receptacle that will accept a standard "Cigarette Lighter” plug. On older vehicles with true "cigarette lighters”, the existing vehicle wiring was capable of carrying up to 30 amps since it was used to power the cigarette lighter heating element. Today these power outlets are designed to recharge cell phones or power low current items such as GPS’s. On many popular towed cars, the most convenient outlet is connected to a circuit for which a fuse needs to be removed to tow the car. This leaves no power at the outlet for the brake system. All makers of these portable braking systems have auxiliary power outlet kits which will provide sufficient power safely.
This Technical Tip will show you how to provide a suitable power connection. Begin by purchasing the following from an auto parts store:
• a suitable high current "cigarette lighter” receptacle (flat or screw-on bracket style)
• a 15-amp mini fuse and mini fuse holder,
• two appropriate (1/8” & 1/4” or 3 & 6mm) ring connectors
• 3-4 butt (blue) connectors and
• about 6-10 feet (1.8- 3m) of 14-gauge copper wire with yellow or red
• A similar length of another color insulated wire, or a shorter length if the ground connection can be made near the socket location.
• ¼” or 3/8” (5-7mm) wire loom, enough to cover the wire where it is exposed.
You will also need the following:
• A roll of electrical tape, wire ties, a crimping tool, wire cutters and wire stripper, a wrench or socket to remove the stud bolts, a screwdriver, a wire coat hanger or a portable drill with an 18”-24” (46-61cm) long installer bit of 3/16” (5mm) diameter.
It is best to supply power from the under-hood fuse box battery terminal which is under the fuse box cover and is connected to the positive battery cable. (see Photo) It provides power for the fuse box and connecting to it provides power directly from the car battery. Install the larger ring connector on one end of the fuse holder and install a butt connector on the other end to which you connect the red
or yellow wire. Install the ring connector onto the stud and reinstall the nut on the stud. Do Not insert the fuse yet! The Ground connection can often be found in this area. Place a ring connector onto the other colored (i.e. green
) wire and after removing the nut on the ground stud, insert the ring connector and replace, then tighten the nut. Note: the factory ground terminal which is a great location for the ground connection. (Green
arrow in photo). If the vehicle battery negative cable must be disconnected to tow, then it is recommended that a Battery Disconnect Switch be utilized for this purpose and this ground connection be made to the battery terminal side of the Disconnect Switch.
Run both wires through the large rubber grommet where the existing wiring harness goes between engine and passenger compartment. Typically, it is located to the right of the brake vacuum booster on the
driver’s side of the car when facing the engine compartment. It may be easier to locate it from the passenger compartment above the foot well area. Use a craft knife to make a small slit in the rubber (being careful to avoid the wire harness) and use a coat hanger that has been straightened out, with a small hook formed on one end. Place the end of the wire in this hook, tape it, and pull it through the firewall. Leave sufficient wire to easily install the loom and secure it to existing wiring.
On newer cars there often are two grommets that the vehicle wire harness goes through. The first is to the "firewall” (see photo) which can be seen from the engine compartment. The second is often hidden under some rubber and plastic trim that seals the driver’s side windshield area from sound and water as the wiring enters the passenger compartment. Often the grommet is a hard white or black plastic part with a formed area through which the wiring enters the passenger compartment. Often this is 10-12” (25-30 cm) away from the accessible firewall grommet and it is impossible to get a hand into this area, thus requiring a drill to provide a small access hole. (See large photo with all grommets marked.) After confirming that there is no wiring or obstructions in the passenger compartment where the drill will come through the grommet, choose the location to drill. It is recommended that an assistant observe as the hole is drilled in the grommet. Take an installer bit, which is longer than the distance between the grommets and long enough to allow a comfortable work space and place it into a portable drill. Use a good bright portable light, insert the drill bit into the space in the area of the wire harness, not too close to it. (see photo) SLOWLY drill the hole and gently push on the drill to go through the grommet.
With the hole drilled through the grommet drilled through the grommet, use the small hole in the drill bit to draw the two wires through the new hole in the grommet.
Connect the red
wire that was connected to the fuse holder to the Red
(+12V) side of the socket or the center connector on the socket, after determining where the socket can be safely mounted. The remaining wire which was connected to the Ground stud under the hood, is connected to the outer shell of the Power Socket or the White wire of the socket harness. Alternatively, the ground
connection can be made near the receptacle by placing a ring connector on the other wire and placing it under an existing screw. Take the other end of this wire and connect it to the White wire or the metal shell of the power receptacle.
Choose a location where is not going to be kicked or knocked. Some sockets can be mounted on the console by drilling a suitable hole in the console.
Ensure you can reach in behind to attach the wires and securely mount it. Often attaching a flat style connector using Velcro applied to the back of the flat style socket and attaching it using the mating material in a convenient location on the side of the center console. Other socket types such as the one shown in the photo require two or four small screws in the mounting bracket to be screwed into the plastic dash or kick panel. Route the wires away from the brake, accelerator and steering controls and any sharp metal up under the dash above the foot well area. Wire tie the wiring in place after installing wire loom on it for protection.
Install the 15-Amp fuse in the fuse holder and test the socket for power. Now you have a safe power connection for your Portable Braking System.