H4 Pinout
This is a free printable h4 pinout: download the diagram as SVG or open it and print to paper or PDF.
The H4 headlight bulb has three pins on a P43t base: low beam filament, high beam filament, and a shared ground return, making it the most common dual-filament headlight standard worldwide.
The H4 is a dual-filament halogen headlamp bulb defined by ECE Regulation 37 and IEC 60061-1 (sheet 7004-29-1). It uses the P43t bayonet-type base with a 43 mm diameter and three offset pins that index the bulb into the reflector housing in the correct orientation. The 't' in P43t indicates that the three pins are at different heights — 9 mm, 15 mm, and 21 mm from the base flange — so the connector can only be inserted in one orientation, preventing incorrect installation.
The three electrical connections are: - Pin 1 (low beam): supplies the low-beam (dipped beam) filament. The low-beam filament is positioned lower in the glass envelope and is accompanied by a metal shield inside the bulb that prevents direct light from entering the reflector above the cut-off line, creating the characteristic flat-top beam pattern. - Pin 2 (high beam): supplies the high-beam (main beam) filament. The high-beam filament is positioned at the focal point of the reflector for maximum throw distance. - Pin 3 (ground/earth): the common ground return for both filaments. In a vehicle, this pin connects to chassis ground. It is the shortest or most easily identified pin on most H4 connectors.
The H4 bulb is typically rated at 60 W (high beam) / 55 W (low beam) at 12 V DC, giving a main beam current of approximately 5 A and a dipped beam current of approximately 4.6 A. For 24 V truck applications, a separate H4 24 V variant exists with different wattages.
The connector housing is a three-cavity female connector, commonly made of nylon or high-temperature plastic, with terminals crimped onto the vehicle harness wires. Most vehicle manufacturers use a standardised connector body that accepts the P43t base, though the connector body shape may vary by manufacturer.
H4 bulbs produce considerable heat. Touching the glass envelope with bare skin deposits oils that create thermal stress concentrations, leading to premature bulb failure. Always handle with clean gloves or a clean cloth.
How to wire h4 pinout
- Disconnect the vehicle battery Before working on headlight wiring, disconnect the negative battery terminal to eliminate the risk of short-circuit while handling connectors and exposed terminals. Allow any capacitive loads (such as airbag systems) time to discharge per the vehicle manufacturer's specification.
- Access the headlight connector Remove the headlight unit or access the rear of the headlight assembly. The H4 connector is the three-wire female connector pressed onto the P43t base of the bulb. Note the wire colours before disconnecting — they vary by manufacturer but commonly use: low beam (pin 1) on one colour, high beam (pin 2) on another, and ground (pin 3) on black or brown.
- Identify pin positions With the connector facing you (female socket side visible), identify the three cavity positions. The pin at 12 o'clock (top) is typically pin 2 (high beam), and the arrangement of pin 1 (low beam) and pin 3 (ground) varies by connector orientation — always refer to the vehicle wiring diagram or use a multimeter to confirm.
- Test filaments With the bulb removed from the reflector, use a multimeter in continuity or resistance mode. Measure between pin 1 and pin 3 — the low-beam filament should show a low resistance (typically 2–4 Ω cold). Measure between pin 2 and pin 3 — the high-beam filament similarly. An open circuit (infinite resistance) indicates a failed filament.
- Fit replacement bulb Wearing clean gloves or holding the bulb with a clean cloth, align the P43t pins with the reflector housing locating slots. Seat the bulb fully until it clicks or locks. Reconnect the three-pin connector, ensuring it seats fully. Do not touch the glass envelope with bare fingers.
- Reconnect the battery and test Reconnect the negative battery terminal. Switch on low beam and verify the headlight illuminates. Switch to high beam and verify both filaments operate correctly. Check the beam pattern for correct aim before driving.
Specifications
| Bulb base type | P43t (3-pin bayonet, 43 mm diameter, offset pin heights: 9 mm, 15 mm, 21 mm) |
|---|---|
| Standard wattage (12 V) | 60 W high beam (pin 2) / 55 W low beam (pin 1) |
| High beam current at 12 V | Approximately 5.0 A |
| Low beam current at 12 V | Approximately 4.6 A |
| Colour temperature (standard halogen) | Approximately 3 200 K |
| Applicable standards | ECE Regulation 37, IEC 60061-1 sheet 7004-29-1 |
| 24 V variant | Available for heavy vehicles; different wattage — confirm before purchase |
Safety warnings
- Always disconnect the vehicle battery negative terminal before working on headlight wiring. Vehicle electrical systems operate at low voltage but can deliver extremely high short-circuit currents that cause burns, fires, and damage to electronic modules.
- H4 halogen bulbs operate at very high temperatures (bulb surface exceeds 250 °C when lit). Allow the bulb to cool completely before handling. Never touch the glass envelope of a halogen bulb with bare skin — skin oils cause localised hot spots that crack the quartz glass and fail the bulb prematurely.
- Ensure all fuses in the headlamp circuit are correctly rated. An oversized fuse does not protect the wiring and can allow the harness to overheat and catch fire in the event of a wiring fault.
- When fitting LED H4 replacement lamps, verify that the resulting beam pattern complies with local road traffic regulations. Incorrectly aimed or non-compliant LED beams can dazzle oncoming drivers and create a road safety hazard.
- This pinout reference is for illustrative purposes. Always cross-reference with the specific vehicle manufacturer's wiring diagram. Wire colours and pin positions vary between vehicle models and production years.
Tools needed
- Digital multimeter (resistance and continuity modes)
- Terminal release tool (for extracting crimp terminals from connector housing)
- Crimping tool (for replacing damaged terminals)
- Clean lint-free gloves or cotton cloth (for handling halogen bulb glass)
- Flashlight or workshop light
- Vehicle-specific wiring diagram
Common mistakes
- Touching the halogen bulb glass with bare fingers — oils from skin cause quartz glass to crack from localised thermal stress when the bulb heats up, dramatically shortening bulb life.
- Fitting the replacement bulb without fully seating the bayonet/lock ring — a loosely seated bulb will vibrate out of the reflector housing focus point, causing incorrect beam aim and premature failure.
- Not fitting a relay for the headlight circuit — running the full 5 A+ headlight current through the original steering column switch causes switch contact wear and voltage drop at the bulb, reducing light output.
- Confusing pin 1 (low beam) and pin 2 (high beam) in a repair — low beam on the high beam circuit positions the filament at the wrong point in the reflector and the beam aim will be incorrect.
- Using an incorrect voltage rating bulb — fitting a 24 V bulb into a 12 V system results in very dim output; fitting a 12 V bulb into a 24 V system immediately burns it out.
Troubleshooting
- One beam (low or high) does not illuminate
- Cause: That specific filament has failed, the corresponding fuse has blown, or the switch, relay, or wiring for that beam is open-circuit Fix: Check the fuse for the non-working beam first. If intact, remove the bulb and test filament resistance between the relevant pin and pin 3. If open-circuit, replace the bulb. If the filament is intact, use a multimeter to check for supply voltage at the connector pin with the beam switched on.
- Both beams do not illuminate
- Cause: Common fuse blown, ground connection (pin 3) open-circuit, battery disconnected, or both filaments have failed simultaneously (rare) Fix: Check the common fuse and ground connection at pin 3. Measure continuity from pin 3 terminal to chassis ground. A poor ground is the most common cause of both beams failing together — clean and tighten the ground strap or terminal.
- Bulbs fail repeatedly after short service life
- Cause: Overvoltage from a faulty alternator or charging system, vibration from a poorly seated bulb, or contamination of the glass envelope from handling with bare hands Fix: Measure charging system voltage at the battery with the engine running — should be 13.8–14.4 V for a 12 V system. Higher voltage accelerates bulb evaporation rate. Check bulb seating and lock ring. Replace with a correctly handled new bulb using clean gloves.
Frequently asked questions
Which pin on an H4 bulb is the ground?
Pin 3 is the ground (earth) return on the H4 P43t base. It is the common return for both the low-beam and high-beam filaments. On most three-pin connectors and wiring diagrams, the ground pin is the third pin and connects to chassis ground in automotive applications.
What wattage is a standard H4 bulb?
A standard H4 halogen bulb is rated 60 W high beam / 55 W low beam at 12 V DC, drawing approximately 5 A on high beam and 4.6 A on low beam. A 24 V variant for heavy vehicles uses different wattages — always check the vehicle specification and bulb marking.
Why does the H4 connector only fit in one way?
The P43t base uses three pins at different heights (9 mm, 15 mm, and 21 mm from the base flange). These offset heights act as polarisation keys so the connector can only seat in one orientation, ensuring the low-beam and high-beam filaments align correctly with the reflector optics and the shield is positioned correctly.
Can I replace an H4 halogen with an LED equivalent and use the same wiring?
LED H4 replacement lamps that use the same P43t base and three-pin connector exist and are designed for plug-in replacement. However, the LED filament position, beam pattern, and EMC compatibility vary considerably between products. Many aftermarket LED H4 units do not replicate the OEM beam cut-off correctly and may dazzle oncoming drivers. Check local road regulations before fitting LED replacements.
What causes an H4 bulb to fail on only one filament?
Each filament (low and high beam) is an independent tungsten element. Filament failure is usually caused by vibration fatigue, end-of-service-life thinning of the tungsten wire, or a voltage spike. Measuring continuity between pin 1 and pin 3 tests the low-beam filament; between pin 2 and pin 3 tests the high-beam filament. A reading of infinite resistance (open circuit) confirms filament failure.
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