Circuits

Automotive circuits are divided into three categories: power, signal (control or data), and ground circuits.

Power Circuits
Power circuits are classified as those cables that provide battery or mechanically switched battery voltages to electrical loads. Some examples of these circuits are:
^ Ignition circuit - 300
^ Battery circuit - 1
^ Headlamp circuit - 40
^ Defog circuits - 193, 1240, 292, 293

Signal (Control Or Data) Circuits
Signal circuits are classified as those cables that provide signal voltages from one electrical device to another device. Examples of these signals and devices are:
^ Evaporative Emission Canister Purge Solenoid to PCM - 428
^ Fuel Level Sensor to PCM - 30

Signal wires are usually current limited by one of the devices that is attached to the wire. In the above examples the PCM and fuel gage limit the current in these circuits through the use of electronics (pull up or pull down resistors, transistors, etc.). Signal wire current levels for short circuit conditions normally do not exceed operating levels by an appreciable amount.

Ground Circuits
Ground circuits are classified as those circuits that provide a ground (zero voltage level with reference to the battery) to the electrical system. Examples of these circuits are:
^ I/P Chassis ground - 350, 250
^ PCM ground - 551

Since ground circuits are already shorted to ground, the current level is based on the electrical load resistance. Like signal circuits, ground circuit cable selection is a function of operating current.