For 1990-2009 cars

### SIR System

Refer to Disabling the SIR System in SIR.

### How To Use This Section Of The Manual

The first part of this section provides:

• | General description of the type of circuits.

• | Explanation of the types of protection devices and fuse blocks.

• | Listings of the types of fuses and relays.

The second part of this section provides:

### Circuits

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

• | power circuits

• | signal (control or data) circuits

• | 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, 3C

• | Battery circuit - 1A, 1B

• | Headlamp circuit - 40A, 40B, 40C

• | Defog circuits - 193, 1240, 293, 293A

### 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, 30A, 30B

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.