Automotive wiring 101
This article is a comprehensive guide for automotive wiring and will take you through all aspects of the parts, equipment and the knowledge required to wire and run all accessories in modern automobiles.
Types of wires/applications
The type of wire stranding is important when it comes to the maximum load capacity of a wire. With DC current, the more strands in a gauge of wiring the better it can carry the load without overheating and failing. Compare a lower grade of wire made of 19 strands of individual 21 gauge wire wound to form a 8 gauge wire, to the wire shown below, that is made up of 133 strands of 29 gauge wires to form the same 8 gauge wire.
Terminals and connectors
OEM terminals and connectors
OEM type hardware can be sourced from a junkyard. Use proper splicing techniques if going this route.
These OEM connectors are also available in the aftermarket. One such place is The Repair Connector Store
The crimped type terminal/connector is the most common type of connector in use in automotive wiring. They come in several different sizes to fit a given range of wire gauge size. The wire is slid into the connector and a wire crimper mechanically binds the wire to the connector by compressing a divot in the holder. Some connectors use a plastic skirt that insulates the wire/connector from shorting and the color identifies the average wire gauge (awg) to the user. Terminals/connectors are available in a wide array of styles from spade, bullet, butt, ring, etc.
These crimp-on types also are ganged together inside multiple holders or "terminal blocks" to form the basis for wiring circuits and harnesses used in automotive applications.
Screw-on type connectors are a compression type of connector whereby wire(s) are inserted in a insulated sleeve and a compression cap is screwed tight to lock in the wire to the sleeve to make a tight- and in some cases water resistant- connection.
Fuses / Circuit breakers
Switches / Relays / Sockets
Mounts / Isolators
Many of the functions on a vehicle are electrical, which requires a way for the current to get from the battery to the load and then another way for it to get from the load back to the battery. Oftentimes, fellows will be chasing around trying to find a problem when the real problem is the lack of proper grounding in the vehicle.
A framed car gets a line from the negative battery terminal to the motor, motor to frame and frame to body (3 separate cables). A unibody car gets a line from the negative battery to the motor and motor to body (2 separate cables).
Ground cables can be made from #4 battery cable (see your nearest welding supply store). Welding cable will carry much more current for the same size because the wire strands are much smaller inside the cable than the cheap cable from Auto Zone, etc. Cut the cable to length then sweat on copper lugs and cover them with heat shrink tubing.
- Battery negative to motor: There will almost always be a threaded hole somewhere on the block or head. If not, use a bolt that holds something else and is handy. Run a finishing tap down to the bottom of the hole and blow out the chips and other crud in the hole by spraying carb cleaner into the hole with the little red straw that comes with the can. Watch your eyes and do not breathe the vapor. Now, use a file or your pocket knife or other suitable tool to scrape off all crud, paint, rust, etc. from around the hole. You want the lug laying against bare metal when you tighten the bolt down. Cover the entire connection with clear silicone so that no moisture can get into it.
- Motor to frame: Same way, but with a plain lug on each end of the cable. If there is not hole in the frame to use that's convenient, drill the proper size hole for a self-threading bolt. Make sure there are no fluid or electrical lines running inside the framerail that would be breached by drilling a hole. You don't have to push the drill bit all the way in, just stop when you feel the bit clear the metal on the other side of the rail. Use a file or knife or sandpaper to make a large, clean, bare spot of shiny metal to attach your lug to, then seal it as above.
- Motor to body or frame to body: Find a place on the firewall where there is already a hole that you will have access to from the outside and from the inside of the car. If there is not one available, then look around for a good spot. You will want a spot on the inside where you can get to it with a socket or wrench to hold a bolt head. We're going to use a bolt and nut and maybe a couple of washers for this one. On the motor side, clean off an area down to bare, shiny metal where the lug will sit so that the lug will not be sitting on any painted part of the firewall. Run the bolt into the hole from inside the car and have a buddy attach the ground cable/washer/nut and tighten it while you hold the bolt head from inside. Once tightened, cover the connection thoroughly on the inside and the outside with clear silicone.
Batteries come in a wide array of of case sizes, amperages, and post arrangements. Choices of side or top post include at least 12 to 15 different case sizes, as well as more than 10 different amperage's to suit any given application. Automotive batteries are available as wet-cell, gel-cell, or AGM (absorbed glass mat) types.
See: Auto battery for more.
See: Troubleshooting alternators for more.
See: Starter motors for more.