Ignition Switch Diagram

Ignition Switch Diagram — interactive diagram. Open it in the editor to customise components and wiring.

This is a free printable ignition switch diagram: download the diagram as SVG or open it and print to paper or PDF.

An ignition switch diagram shows the electrical contacts behind the key cylinder — how OFF, ACC, ON/IGN, and START positions connect and disconnect circuits for accessories, ignition, engine management, and the starter motor.

The vehicle ignition switch is a rotary multi-position switch that sequentially energises vehicle electrical circuits as the key is turned through its positions. Understanding the ignition switch wiring diagram is essential for diagnosing starting problems, installing aftermarket electronics, and tracing intermittent electrical faults.

A conventional ignition switch has four main positions:

OFF (position 0): All ignition switch outputs are de-energised. Battery power is maintained only at circuits fed directly from the battery, bypassing the ignition switch — typically the clock, alarm, central locking memory, and some ECU keep-alive memories.

ACC (Accessory, position 1): The ACC terminal is energised. This supplies power to accessories that can operate without the engine running: radio, windows, sunroof, some interior lighting. Designed to allow use of accessories while the vehicle is parked without energising the full ignition circuit. In the ACC position, the ignition system and engine management are not powered.

ON/IGN (Ignition On, position 2): The IGN terminal is energised in addition to ACC. This supplies power to the ignition system (coils, ECU, fuel pump, instrument panel, warning lights). The instrument warning lights illuminate at this stage as a bulb test and to indicate which systems are active before the engine starts. When this position is first selected, the fuel pump typically runs for approximately 2 seconds to pressurise the fuel rail before cranking.

START (Crank, position 3): A spring-loaded position that returns to IGN when released. Energises the starter motor terminal (ST). Depending on vehicle design, some accessories may be de-energised during cranking to maximise battery current available for the starter. The starter solenoid receives the signal from the ST terminal, which then closes the main starter circuit.

The switch itself contains physical wiping contacts, and the terminal labelling varies by manufacturer — common terminal designations include B (battery), ACC, IGN, and ST, though some older vehicles use numbers (15, 30, 50) based on DIN 72552 standards: 30 = battery, 15 = ignition (switched), 50 = starter, X = starter inhibit during cranking.

How to wire ignition switch diagram

Identify ignition switch terminal designations for the specific vehicle Obtain the vehicle's wiring diagram or service manual for the specific make, model, and year. Ignition switch terminal designations vary by manufacturer. Do not assume that a generic diagram matches the vehicle. At minimum, identify which terminals correspond to: permanent battery supply, ACC output, IGN output, and START output.
Test each terminal with a multimeter across all switch positions With the battery connected and the ignition key inserted, use a multimeter in DC voltage mode to probe each ignition switch output terminal in each key position. Build a simple table: for each terminal, record whether voltage is present in OFF, ACC, IGN, and START. This confirms the switch's functional map before any wiring work begins.
Identify the source of each output terminal The ignition switch's battery input terminal (B+ or terminal 30) must come from the battery positive through a fuse or fusible link. Verify this by tracing the wire. Do not connect anything directly to the battery without overcurrent protection. In most vehicles, the ignition switch input is fed from the main fusible link or underbonnet fuse box.
Connect accessories to the appropriate switch terminal Connect accessories that should operate when the key is in ACC or IGN to the ACC terminal or the IGN terminal as appropriate. Do not connect high-current loads directly to the ignition switch terminals — the switch contacts are rated for relatively low currents (typically 10–20 A per terminal). For high-current accessories, use the ignition switch output to trigger a relay whose contacts carry the load current.
Use relays for any load above approximately 5–10 A Wire a relay coil between the appropriate ignition switch terminal and ground. The relay's normally open contacts carry battery power to the high-current load. This protects the ignition switch contacts from excessive current and extends their service life. It also allows placement of the high-current wiring closer to the battery and load, reducing resistance losses.
Verify the starter circuit separately The starter control wire (ST terminal) must connect to the starter solenoid's trigger input. This wire carries only the solenoid coil current (typically 5–15 A), not the full starter motor current. Verify that the wire gauge, connections, and routing are sound — a high-resistance starter control circuit can cause slow engagement, failure to crank, or a chattering solenoid.

Specifications

DIN 72552 terminal 30	Battery positive, permanently live, unfused at switch (primary fuse upstream)
DIN 72552 terminal 15	Ignition (IGN) switched positive — live in IGN and START positions
DIN 72552 terminal 50	Starter control — live only in START position
Accessory (ACC) terminal	Live in ACC, IGN, and some START positions depending on vehicle
Typical ignition switch contact current rating	10–30 A per terminal depending on design
Starter solenoid control wire current draw (typical)	5–15 A (solenoid coil only; not the main motor current)
Ignition switch key positions	OFF (0), ACC (1), IGN/ON (2), START (3) — spring-loaded from START back to IGN
SRS (airbag) capacitor discharge time after battery disconnection	Vehicle-dependent — typically 30 seconds to several minutes; always consult service manual

Safety warnings

Ignition switch wiring connects directly to the battery via a fusible link or main fuse. The battery supply to the ignition switch is permanently live. Disconnect the battery negative terminal before working on ignition switch wiring to prevent short circuits. Arc flash from accidentally shorting battery connections can cause severe injury.
Vehicles with passive immobilisers, push-button start, or engine management systems have additional circuits beyond the basic ignition switch. Incorrectly bypassing or modifying these circuits can disable the vehicle permanently or trigger fail-safe modes — consult vehicle-specific service information.
Some ignition switches are part of a combined steering column lock assembly. Removal or replacement of this assembly requires the steering column to be accessible and the vehicle stationary. Follow the manufacturer's procedure — improper removal can damage the steering lock mechanism.
Do not connect high-current loads (above approximately 10 A) directly to ignition switch terminals. The switch contacts are not designed for continuous high-current operation and will overheat, arc, and fail — potentially melting surrounding wiring and creating a fire risk inside the steering column.
In any vehicle with airbags, work near the steering column requires disabling the SRS (Supplemental Restraint System) per the manufacturer's procedure. Airbag circuits retain energy for several minutes after battery disconnection.

Tools needed

Digital multimeter (DC voltage, continuity)
Wiring diagram (vehicle-specific service documentation)
Insulated test probes and extension leads for multimeter
Trim and steering column cover removal tools
Insulated screwdrivers
Personal protective equipment: safety glasses, insulated gloves

Common mistakes

Connecting high-current loads (aftermarket audio amplifiers, additional lighting) directly to ignition switch terminals rather than through relays, overloading the switch contacts and causing them to arc and fail.
Connecting an aftermarket accessory's power wire to the START terminal rather than the ACC or IGN terminal — the accessory then only operates during cranking, which is both useless and confusing to diagnose.
Assuming that all vehicle ignition switches follow the same terminal layout — variations between manufacturers and model years mean a diagram for one vehicle may be completely incorrect for another.
Bypassing the ignition switch for convenience (e.g., always-on power for a device) by tapping a permanently live source and labelling it as switched — this drains the battery when the vehicle is parked and can be a theft risk.
Failing to fuse aftermarket additions to the ignition circuit individually — the ignition switch is already protected by the main fusible link, but individual circuit fuses are required to protect the wiring to each additional load.
Leaving wire splices into the ignition switch harness exposed and taped with unsuitable tape — in a confined steering column environment, vibration and heat cause tape joints to fail; use proper connectors or solder-and-heat-shrink terminations.

Troubleshooting

Engine cranks but ignition system and accessories lose power as soon as the key is released from START: Cause: Ignition switch IGN contact has failed — provides power in START position only, not in the IGN position Fix: Test the IGN terminal voltage with the key in IGN position (not START). If no voltage is present in IGN but is present during START, the ignition switch internal contact for IGN has failed. Replace the ignition switch.
Accessories remain on after the key is removed and vehicle is switched off: Cause: An accessory's switched power wire is connected to a permanently live source, or the ignition switch is failing to open its ACC contact when turned to OFF Fix: Test whether the ACC terminal on the ignition switch drops to 0 V when the key is turned to OFF. If it does, the accessory in question is wired to the wrong source. If the ACC terminal remains live in the OFF position, the ignition switch has a failed contact.
Starter does not engage when key is turned to START: Cause: No voltage reaching starter solenoid trigger terminal, open circuit in starter control wire, or failed ignition switch START contact Fix: Measure voltage at the starter solenoid trigger terminal with the key in START. If 12 V is present and the solenoid does not engage, the solenoid is at fault. If no voltage is present, trace back through the starter control wire to the ignition switch — test the ST terminal on the switch with the key in START.
Intermittent starting failure or instrument cluster losses at random: Cause: Worn or corroded internal ignition switch contacts causing intermittent open circuit in the IGN or START circuit Fix: Wiggle the key in the ignition while monitoring dashboard power. If symptoms appear or disappear with key movement, the ignition switch contacts are worn. Ignition switch replacement is the definitive repair — cleaning internal contacts on most modern switches is not a lasting solution.

Frequently asked questions

What do the DIN 72552 terminal numbers on an ignition switch mean?

DIN 72552 is a German automotive wiring standard that assigns numbers to electrical functions: terminal 30 is battery positive (unfused, permanently live); terminal 15 is the ignition-switched positive (live in IGN and START); terminal 50 is the starter control (live only in START position); terminal X is a terminal that cuts power to certain accessories during cranking to prioritise starter current. This numbering is common on German vehicles and in technical service documentation.

Why does my vehicle start briefly then immediately die?

A likely cause is that the ignition switch's IGN contact is faulty — it energises during START but fails to maintain power when released back to IGN. The engine starts on cranking power but dies when the key returns to IGN because the IGN terminal goes open. Test by holding the key in the IGN position and attempting to sustain the engine. A failing ignition switch internal contact is the common diagnosis.

Can I wire an aftermarket push-button start using the ignition switch terminals?

Yes, using relays. A relay for the ACC circuit, a relay for the IGN circuit, and a relay for the ST circuit — all controlled by the push button and a latching circuit or an immobiliser control module. The start relay must be a momentary-action type that disengages when the engine starts. Many aftermarket keyless entry and start systems include these relay circuits in their kits.

Why does my radio stay on after I remove the key?

Either the radio's constant 12 V (memory) wire and the switched 12 V (ACC) wire are both connected to permanently live sources, or the radio has an accessory delay feature that keeps it powered for a set period after the ignition is switched off. Check that the radio's red (ACC) wire is connected to a source that goes off when the key is removed — if it is on a permanently live source, the radio will never switch off automatically.

What happens electrically when the starter is engaged?

Turning to START sends 12 V to the starter solenoid's trigger terminal (S terminal on the solenoid). This energises the solenoid coil, which simultaneously pulls the starter pinion gear into mesh with the ring gear on the flywheel and closes the main solenoid contact, connecting the battery positive directly to the starter motor armature. The motor draws cranking current (typically 100–400 A) and rotates the engine.