A quick and easy way to secure a vehicle is to install a battery cut-off switch. This switch interrupts the circuit between the battery, starter solenoid and alternator and prevents thieves from starting the car. Many battery cut-off switches are designed to be turned on and off with special keys, making it even more unlikely that the car will be tampered with. Provided that basic safety measures are followed, such as ensuring the car battery is disconnected during the job, anyone can install a battery cut-off switch.
Open the bonnet and disconnect the negative battery cable from the battery by loosening the lock bolt on the terminal with a box wrench and pulling it off the battery post.
Mount the battery cut-off switch inside the car near the driver's seat. Depending on the style of switch purchased, drill holes through the dash to accommodate screws to hold the switch frame in place or install the screw-on plate. Unscrew and remove the plate, drill a hole the size of the front of the switch and pass the switch through the hole from the backside. Screw the faceplate back on. The pressure from the plate will hold the switch in place.
Remove the locknuts from the two posts at the rear of the cut-off switch using a box wrench.
Measure out enough 2- or 4-gauge wire to reach from the cut-off switch to the positive terminal on the car's battery. Use 4-gauge wire if the distance is less then 8 feet and 2-gauge if it is more.
Cut the wire to the desired length using the cutters built into the electrician's pliers. Crimp ring terminals onto each end. Match the ring terminal size to the gauge of the wire used and ensure the ring will fit over the battery and switch post.
Remove the positive cable from the battery and the starter solenoid. Use a box wrench to loosen the locknuts at each end and pull the cable off the posts. Follow the cable from the battery to find the starter solenoid.
Attach one end of the wire created in step 5 to the positive post of the battery and tighten the locknut. Slip the other end onto the post of the battery cut-off switch that is closest to the battery, but do not tighten the locknut in place.
Disconnect the wire connecting the alternator and the starter solenoid at the solenoid end only. Check to see if the wire will reach from the alternator to the post of the cut-off switch that the positive cable from the battery is attached to. If it does not, disconnect the end attached to the alternator and replace the whole wire with an appropriate length of 14-gauge wire with ring terminals on each end. Reattach one end to the alternator.
Connect the alternator to the same post on the battery cut-off switch that the positive cable from the battery is attached to and tighten the locknut into place.
Splice a 200-amp inline fuse in the middle of the wire connecting the alternator to the cut-off switch. Cut the wire, strip 1/4 inch of insulation of the two ends and connect them to the inline fuse. Follow the directions for the style fuse purchased to complete the connection.
Measure, cut and crimp a ring terminal on each end of a 2- or 4-gauge wire to connect the second post of the battery cut-off switch (the one furthest from the battery) to the starter solenoid. Use 4-gauge wire if the distance is less then 8 feet and 2-gauge if it is more. Tighten the locknuts at each end to hold the wire in place.
Reconnect the negative battery cable to the battery. Tighten the locknut with a box wrench to secure the connection. Follow the directions for the battery cut-off switch to turn it to the "On" or "Off" position as needed. Some use toggle switches and others use keys to change modes.
Install the switch where it is easy for the driver to reach, but is not obvious to others to increase protection from theft.
Never perform electrical work on a car with the battery connected. This greatly increases the risk of a severe electrical shock and could cause a fire.