In most cars, the ignition switch itself is not a relay. Instead, power to the ignition system and related circuits is delivered by one or more relays—typically an ignition relay (IG relay) or a main power relay—with a separate starter relay handling cranking. The exact arrangement varies by make and model.
The ignition switch acts as a control that routes battery voltage to different circuits depending on key position. In modern vehicles, the ignition circuit can be managed by the body control module or engine control unit, which may switch relays rather than feeding power directly through the switch. Understanding the wiring helps diagnose no-start or intermittent-start problems.
Where relays sit in the ignition circuit
Relays are used to deliver the high-current power that the ignition system and engine management rely on, while the ignition switch only signals these relays when you turn the key. The following outlines the common roles you will encounter in most cars.
Ignition and main relays
In many vehicles, two relays are involved in powering the ignition-related circuits.
- Ignition Relay (IG relay): When the key is in ON or RUN, the IG relay supplies power to the ignition system, ECU/PCM, fuel pump, and other essential circuits that must stay energized while the engine runs.
- Main/Power Relay: Some models include a main relay that feeds multiple circuits from the battery through the ignition switch, providing a centralized control point to protect and isolate critical loads.
Not every car uses both relays; some rely on a single IG relay or feed the ignition circuit directly through the switch. Always consult your vehicle's wiring diagram for exact details.
Starter circuit
The starter circuit is a separate path that enables the engine to crank. In most cars, a starter relay or the starter motor solenoid is energized by the ignition switch when you turn to START, providing the high current needed to crank the engine. In some older vehicles, the ignition switch may directly energize the starter solenoid without a relay.
- Starter relay or starter solenoid: Activates the starter motor when you hold the key in the START position or press the START button, via a dedicated circuit controlled by the ignition switch or BCM/ECU.
- Start button systems: In push-button start cars, the computer initiates with a similar relay or solenoid control to engage the starter.
These arrangements help spare the ignition switch from high current and enable centralized control by the vehicle's electrical system. If you’re troubleshooting, check these relays as part of a systematic diagnostic approach.
Tips for diagnosing relay problems
When dealing with an ignition problem, a few practical steps can help identify whether a relay is at fault or if the issue lies elsewhere in the ignition circuit.
- Check fuses and look for a blown fuse tied to the IG or main relay circuit.
- Listen for clicking sounds from the under-hood relay box when turning the key to ON or START; a lack of click can indicate a faulty relay or a power feed issue.
- Swap a suspected relay with a like-for-like, known-good unit to see if the problem follows the relay.
- Verify voltage at the IG relay input and output with a multimeter to confirm the relay is switching as commanded.
If the problem persists, the issue could be with the ignition switch itself, the BCM/ECU, or wiring harness connections, which may require a professional diagnostic.
Summary
In short, there is no universal “ignition switch relay.” The ignition switch is typically a control input, while relays such as the ignition (IG) relay, main relay, and a separate starter relay supply power to the ignition system and to the starter. The exact configuration varies by vehicle, so always refer to the factory wiring diagram for your make and model.