H4 Headlight Wiring Diagram: P43t Three-Pin Connector, Beam Circuit, and Relay Wiring Reference

H4 Headlight Wiring Diagram: P43t Three-Pin Connector, Beam Circuit, and Relay Wiring Reference — interactive diagram. Open it in the editor to customise components and wiring.

This is a free printable h4 headlight wiring diagram: download the diagram as SVG or open it and print to paper or PDF.

An H4 headlight wiring diagram maps the three-pin P43t connector with its dedicated low-beam, high-beam, and ground terminals, plus relay wiring to protect the vehicle's headlight switch.

The H4 is a dual-filament halogen bulb used for both low-beam and high-beam in a single headlamp unit. It was introduced with the P43t base specification (also referred to as P45t in some markets — the 't' denotes a three-pin configuration), and remains one of the most widely used automotive headlamp bulbs worldwide. Its three-contact base is the defining characteristic that distinguishes it from single-filament H1, H7, and H11 types.

The P43t base has three contacts positioned at specific angular intervals to prevent incorrect bulb installation. The three pins serve dedicated functions: one pin connects to the low-beam filament supply, one to the high-beam filament supply, and one serves as the common ground for both filaments. No current flows through the bulb housing or mounting bracket — the ground return is through the dedicated ground pin.

On the vehicle side, the matching three-pin H4 connector (commonly a Superseal or similar weatherproof plug) carries three wires: the low-beam positive supply, the high-beam positive supply, and the common ground. The headlight switch routes battery-positive voltage to either the low-beam or high-beam supply wire depending on switch position; both filaments share the single ground return.

Halogen H4 bulbs operate at 12 V DC with a typical wattage of 55 W (low beam) and 60 W (high beam). Current draw is approximately 4.6 A (low) and 5.0 A (high). If the vehicle routes this current through the headlight switch without relay assistance, the switch contacts carry sustained current of nearly 5 A per headlamp, per side — totalling up to 10 A through the switch for both headlamps on the same beam. Over time, switch contact degradation is the predictable outcome.

For this reason, relay wiring is standard practice on any performance or reliability-focused H4 installation. A relay kit uses the existing headlight switch wires merely to trigger relay coils (drawing approximately 80–150 mA per relay), while the high-current supply to the headlamp bulbs is drawn directly from the battery through the relay contacts, via a dedicated inline fuse. This approach also reduces voltage drop to the headlamps, resulting in noticeably brighter light output — a 0.5 V drop on a 12 V halogen circuit reduces luminous output by more than 10%.

H4 bulbs are sensitive to glass contamination. Skin oils from bare-hand contact on the quartz envelope cause localised hot spots that crack the envelope or dramatically shorten bulb life. Always handle H4 bulbs with clean gloves or by the base only.

How to wire h4 headlight wiring diagram

Disconnect the vehicle battery negative terminal Before working on any headlight wiring, disconnect the negative (−) battery terminal. Headlight circuits are typically live at battery voltage and not fused by the ignition switch. Wait 60 seconds if the vehicle has airbag systems.
Identify the three H4 connector wires using a verified source Locate the existing H4 three-pin connector at the headlamp. Using the vehicle's wiring diagram or a reliable vehicle-specific reference, identify the ground wire, the low-beam supply wire, and the high-beam supply wire. Typical colour conventions vary by manufacturer — do not assume without verification. With the battery reconnected and multimeter in DC voltage mode (and great care near a live circuit), verify which wires carry voltage in each switch position to confirm identification before disconnecting.
Plan the relay circuit layout For a relay-assisted installation, you need: one relay per beam circuit (or one combined relay if using a single-relay kit), one inline fuse per relay load circuit (close to the battery positive terminal), dedicated cable from the battery positive terminal through each fuse to each relay's load contacts, and trigger wires from the existing H4 low-beam and high-beam supply wires to the relay coil inputs.
Wire the relay power supply from the battery Run a length of 12 AWG (or 14 AWG for single lamp) cable from the battery positive terminal through an inline fuse holder (10 A fuse for single headlamp, 15 A for both headlamps combined) to the relay's contact power input terminal (relay pin 30). Secure this cable away from exhaust components and mechanical hazards. Use a ring terminal at the battery post and at any chassis connections.
Wire the relay coil from the existing switch wiring Connect the existing vehicle low-beam supply wire to the relay coil positive terminal (relay pin 86) for the low-beam relay. Connect the existing high-beam supply wire to the high-beam relay coil positive terminal. Connect relay coil ground terminals (relay pin 85) to a reliable chassis earth point near the headlamp. Fit a flyback diode across each relay coil (pin 85 to 86) if using relays without built-in suppression — particularly important in vehicles with modern ECUs sensitive to voltage spikes.
Wire relay output to H4 connector Run a new cable from the relay's normally open output contact (relay pin 87) to the corresponding pin on the H4 connector — low-beam relay pin 87 to the H4 low-beam pin, high-beam relay pin 87 to the H4 high-beam pin. Use the H4 connector's ground pin for the headlamp ground, running a dedicated earth cable from the connector ground pin to a clean chassis earth stud or directly to the battery negative terminal.
Reconnect battery, test all functions, and verify beam aim Reconnect the battery. Test low beam on all headlamps — verify correct beam pattern projects from the low-beam filament. Test high beam — verify high-beam pattern. Test flash-to-pass if applicable. Verify no relay buzzing, no flickering, and battery voltage within 0.5 V at the headlamp connector with beams on. After any headlamp connector or bulb work, verify headlamp aim is within legal limits using a beam setting screen or workshop beam setter.

Specifications

Bulb Base Type	P43t (three-pin keyed base, also referenced as H4 / 60/55 W)
Rated Voltage	12 V DC
Low-Beam Filament Power	55 W (approximately 4.6 A at 12 V)
High-Beam Filament Power	60 W (approximately 5.0 A at 12 V)
H4 Connector Pins	3: Low-beam positive, High-beam positive, Common ground (shared by both filaments)
Recommended Relay Contact Rating	30 A minimum (per relay) for headlamp switching
Recommended Inline Fuse (single headlamp per relay)	10 A blade fuse
Maximum Voltage Drop to Headlamp	Less than 0.5 V from battery positive to H4 supply pin under load

Safety warnings

Always disconnect the vehicle battery negative terminal before working on headlight wiring. Headlight circuits carry battery voltage at rest, and accidental short circuits can ignite wiring insulation. Automotive electrical fires spread rapidly in an engine bay environment. Reconnect the battery only when all connections are made, insulated, and secured.
Do not touch the glass envelope of an H4 halogen bulb with bare hands. Skin oils cause localised thermal stress on the quartz glass at operating temperature, leading to premature bulb failure or cracking of the envelope. Handle bulbs with clean cotton or nitrile gloves, or grip only the plastic base.
Ensure all relay wiring uses correctly rated cable, connectors, and fuses. Undersized cable between the battery and relay load contacts is a fire risk if the fuse is oversized or absent. The inline fuse must be located as close to the battery positive terminal as practical — within 450 mm is the standard recommendation for automotive fusing.
After any headlamp connector work, bulb replacement, or aim adjustment, verify headlamp beam aim meets the legal requirements of the applicable vehicle regulations (ECE Regulation 112, FMVSS 108, or national equivalent). Misaimed headlamps are both a safety hazard and an MOT/inspection failure.

Tools needed

Digital multimeter (DC voltage, resistance, and continuity)
Non-contact voltage tester
Automotive wire crimping tool (for ring terminals, blade terminals, and weatherproof splice connectors)
Wire strippers
Insulated screwdrivers and nut drivers for relay mounting and terminal screws
Clean cotton or nitrile gloves (for H4 bulb handling)
Headlamp beam setting screen or aiming gauge (for post-installation beam verification)

Common mistakes

Transposing the low-beam and high-beam supply wires at the H4 connector — results in the opposite filament activating for each switch position, incorrect beam pattern aim, and a roadworthiness failure.
Omitting the relay and running headlamp current through the original switch and harness wiring — particularly problematic on vehicles where the wiring harness is 20+ years old and connector resistance has increased; results in low headlamp voltage, dim output, and accelerated switch failure.
Running the relay load power feed from an accessory fuse or relay box rather than directly from the battery — additional resistance in long wiring runs from a remote source defeats part of the purpose of the relay installation and may introduce a voltage drop through shared wiring.
Fitting an undersized fuse in the relay load circuit — a 5 A fuse on a 55 W + 60 W combined headlamp circuit (approximately 9.6 A total) will blow on high beam; size fuses for the actual load with appropriate margin.
Touching the H4 bulb glass envelope during installation and not cleaning it — the bulb will develop a hot spot at the fingerprint location during first operation and either fail immediately or have significantly shortened life.

Troubleshooting

H4 bulb not illuminating despite power present at connector: Cause: Open-circuit filament (blown bulb), poor ground connection at H4 ground pin, or incorrect pin identification resulting in voltage on wrong pin Fix: Test resistance across the low-beam filament pins (ground pin to low-beam pin) with a multimeter — infinite resistance confirms blown filament. Test resistance from the H4 ground pin to chassis earth — should be less than 0.5 Ω. If resistance is high, clean or replace the chassis earth connection. Verify which connector pin carries voltage with a meter if filament identity is uncertain.
Low-beam works but high beam does not (or vice versa): Cause: One filament blown, one relay not triggering (in relay-assisted system), or one switch output wire broken Fix: Test the non-working beam by measuring voltage at the H4 connector on the relevant supply pin with the switch in the appropriate position. In a relay system, test trigger voltage on the relay coil positive terminal. If trigger voltage is present but relay does not click, replace the relay. If trigger voltage is absent, trace back to the headlight switch output.
Headlights dim and voltage at bulb connector is well below battery voltage: Cause: High resistance in headlamp supply wiring, corroded H4 connector pins, high-resistance switch contacts (non-relay system), or undersized cable in relay power feed Fix: Measure voltage at the battery positive terminal with headlights on (reference). Then measure at the relay load input, relay load output, and H4 connector in sequence. Identify where voltage drops most severely — that section of wiring or connector is the high-resistance fault. Clean corroded connectors, replace damaged wiring, and ensure relay load cable is minimum 14 AWG for single-lamp circuits.

Frequently asked questions

What are the three pins on an H4 bulb connector and which is which?

The H4 P43t base has three pins: one for the low-beam filament positive supply, one for the high-beam filament positive supply, and one common ground shared by both filaments. The physical pin positions are keyed so the bulb can only be inserted in one orientation, preventing transposition. On the vehicle's three-wire connector, the ground wire (typically black or earth-coloured) connects to the common ground pin; the remaining two wires connect to low-beam and high-beam as identified in the vehicle's specific wiring diagram.

Why should I install a relay when wiring H4 headlights?

Without a relay, the full headlamp current (approximately 4.6–5 A per bulb, per beam, multiplied by the number of headlamps) flows through the headlight switch contacts and the often-aged wiring harness. A relay kit allows the switch to trigger only the relay coil (approximately 100 mA), while the headlamp current is drawn directly from the battery through a dedicated fuse and relay contacts rated for the load. Benefits are switch longevity, protection of the harness, and noticeably brighter light output from reduced voltage drop.

What happens if I connect the H4 low-beam and high-beam wires to the wrong pins?

If the low-beam and high-beam supply wires are transposed at the connector, the headlights will illuminate the wrong filament on each switch position — the high-beam filament will fire when the low-beam switch position is selected and vice versa. The beam pattern projected will be incorrect for each switch position, and the beam aim will be wrong. In most H4 lamps the beam patterns differ significantly between filaments; this is both an MOT/safety test failure and a road safety risk.

What fuse rating should I use for each H4 headlamp relay circuit?

Each H4 relay output circuit fuse should be rated for the maximum lamp current plus a margin but no higher than the rating of the cable used. For a standard 55/60 W H4 at 12 V, maximum current is approximately 5 A. A 10 A fuse on 14 AWG (2.0 mm²) cable provides adequate protection for a single headlamp per relay, with margin for cold-start inrush. Do not use a single relay and fuse for both headlamps unless the fuse and cable are rated for the combined current.

Can I install LED or HID replacement bulbs in an H4 housing?

LED and HID replacement bulbs are available in the H4 base format. However, their optical performance depends entirely on the reflector geometry and projection lens of the headlamp housing, which was designed for the specific filament position of the halogen H4. Many LED retrofits produce scattered, non-compliant beam patterns even if they produce high luminous flux. In many jurisdictions, LED or HID bulbs in headlamp housings not type-approved for that technology are not roadworthy. Check local vehicle lighting regulations before installing any non-halogen H4 replacement.

Related diagrams

Free electrical calculators

Edit this diagram free in the online editor