Reading Switches on a Schematic

A switch is shown as a gap in the line with a movable contact that can bridge the gap. When the switch is off — the contact is shown open and current cannot pass. When the switch is on — the contact is shown touching both sides and current flows through.

Two terminals. Open or closed. The headlight switch in the off position shows the contact separated from the terminal — the circuit is open. Flip the switch to on and the contact bridges the two terminals — the circuit is closed and current flows to the headlights. On the schematic, the two terminals are labeled with numbers or letters. When you test the physical switch, check continuity between those two terminals with the switch in each position.

A switch with multiple positions — like a blower motor switch with off, low, medium, and high settings — shows multiple contacts and terminals. In the low position, terminals 1 and 2 might connect. In medium, terminals 1 and 3. In high, terminals 1 and 4. The schematic shows which terminals connect in each position. When diagnosing a switch that works in some positions but not others, test continuity between the correct terminal pairs for each position. No continuity in a specific position with correct continuity in others means the switch has a bad contact in that position.

The ignition switch schematic shows which circuits receive power in each key position — OFF, ACC, RUN, START. In the ACC position, the radio and accessories get power but the engine management does not. In RUN, everything gets power. In START, the starter circuit gets power but some accessories may temporarily lose power. The schematic shows these power distribution paths clearly — this is how you determine whether a no-power complaint is caused by a faulty ignition switch or something downstream.