Signaling Systems and Wiring Diagrams: relays, horn, turn signals, tail, brake and reverse lights, headlamps

The automotive signaling and lighting system is essential for road safety, allowing communication between drivers and pedestrians. Understanding how these conventional circuits work and the standard pin designations based on the DIN standard is essential for any diagnosis or customization.

0. The language of wiring diagrams: DIN 72552 terminal designations

Before wiring any relay, you need to speak the same language as the wiring diagram. The German standard DIN 72552 assigned standard numbers to the terminals of automotive components. These numbers are printed on relays, fuse boxes and switches and appear in the wiring diagrams of many manufacturers, especially European ones. Anyone who masters this numbering can read a diagram from any brand that uses it.

0.1 How a relay works

A relay is an electromagnetic switch. A small current flowing through the coil between 85 and 86 creates a magnetic field that pulls the moving contact. This contact connects terminal 30 to 87 and lets a much larger current flow. This way, a delicate button or switch controls a heavy load without overheating or wearing its contacts.

• 4-pin relay: 30, 85, 86 and 87. The contact is normally open: it only closes when the coil is energized.
• 5-pin (changeover) relay: adds 87a, connected to 30 at rest. It is used to switch between functions (for example, turning one load off when another turns on).
• Diode or resistor across the coil: many relays have a component in parallel with the coil to absorb the voltage spike when it switches off. On these relays the polarity of 85 and 86 matters: if reversed, the diode can short. Check the diagram printed on the housing.
• Coil resistance: on 12 V relays it is usually in the range of a few tens of ohms. An open coil (infinite resistance) prevents operation; check the manufacturer’s value for comparison.

1. Horn System

The horn produces a continuous warning sound by electromechanical vibration. Because of its considerable current (about 5 A), the conventional circuit uses a 4-pin auxiliary relay to protect the push button on the steering wheel.

How It Works:
When the driver presses the horn button, it closes the circuit, sending a low current to energize the relay’s internal coil. This coil creates a magnetic field that pulls a high-power internal contact, closing the main circuit and sending current straight from the battery to the horn.

Commercial Wiring Diagram (4-Pin Relay):

• Pin 30: Connected to the Battery Positive (+), direct (fuse-protected).
• Pin 85: Coil supply input (Switched Positive / Line 15).
• Pin 86: Negative (-) control coming from the horn button on the steering wheel.
• Pin 87: Power output connected directly to the horn’s positive terminal.
• Note: The horn housing or second terminal must be properly grounded to the vehicle chassis.

• Horn types: the electromagnetic disc type (sharp, dry sound) and the snail type (spiral trumpet, fuller sound). Both work with an electromagnet that makes a diaphragm vibrate by opening and closing an internal contact.
• Steering wheel button: on vehicles with an airbag, the connection between the rotating steering wheel and the fixed column goes through the clock spring (spiral cable). A broken clock spring can leave the horn silent and turn on the airbag warning light at the same time.
• Quick diagnosis: a silent horn with a clicking relay points to the fuse, the 30-87 contacts, wiring or horn ground. A silent horn with no click points to the button, clock spring, line 15 or the relay coil. A horn that will not stop usually means a stuck button or a relay with welded contacts.

2. Position Lights (Tail Lights)

Position lights (front and rear) show the presence and actual width of the vehicle. In robust conventional systems, the relay prevents overloading and overheating the main dashboard switch.

How It Works:
Turning the light switch to the first position sends a low-current electrical signal to activate the auxiliary relay. The relay closes its high-capacity internal contacts, lighting the rear lamps, front lamps and instrument panel illumination at the same time.

Commercial Wiring Diagram:

• Pin 30: Battery Positive (+), direct (Line 30 with fuse).
• Pin 85: Positive (+) signal sent by the light switch on the dashboard.
• Pin 86: Connected to the vehicle ground / chassis (-).
• Pin 87: Output that distributes the positive (+) to all position lamps at the same time.

• Tail lights use low-power bulbs, typically 5 W (such as W5W and the smaller filament of P21/5W bulbs), plus license plate and dashboard illumination.
• On many vehicles, the position light circuit uses designation 58, and the dashboard illumination shares the same supply, which is why the dashboard also lights up with the tail lights.
• When a whole side goes out, check that side’s fuse and the common tail light ground. When a single bulb goes out, check the bulb, the socket and contact corrosion.

3. Warning Lights (Hazard Lights and Turn Signals)

The turn signal and hazard light system manages intermittent direction and emergency signaling through a specific component: the flasher relay (usually 3-pin).

How It Works:
The flasher relay works by generating timed electrical pulses. When the hazard lights are switched on, the circuit does not need the ignition to be on and powers all 4 (or 6) signal lamps at the same time.

Commercial Wiring Diagram (3-Pin Relay):

• Pin 49 (or B/X): Positive (+) supply input. It receives Switched Positive (Line 15) for the turn signals or direct Battery Positive (Line 30) when the hazard button is pressed.
• Pin 31 (or E): Connected directly to the vehicle ground (Negative).
• Pin 49a (or L): Pulsed signal output going to the turn signal switch (steering column) or to the hazard switch.

3.1 Thermal relay, electronic relay and blown bulbs

• Thermal relay (old type): a bimetallic strip heats up with the lamp current, bends, opens the contact, cools and closes again. Because it depends on current, with a blown bulb it flashes more slowly or not at all.
• Electronic relay: an integrated circuit drives a transistor at a calibrated frequency. Many monitor the current and, when they detect a blown bulb, start flashing fast (for example, 120 flashes per minute or more) to warn the driver.
• Normal frequency: vehicle type-approval regulations set a range of about 60 to 120 flashes per minute; a frequency outside this range indicates a fault in the relay or the load.
• LED bulbs: they draw much less current, and the relay “reads” this as a blown bulb, causing hyperflash. The correct solution is an LED-compatible relay or the specified load resistor, always with approved products.

Automatic turn signal cancellation: the lever has a detent position. As the steering wheel returns, lobes (the canceling cam), usually molded into the clock spring rotor, push an actuator that releases the lever. If the turn signal does not cancel by itself, inspect the lever actuator and the cam.

4. Reverse Light

This system has a simple mechanical-electrical operation and warns those behind the vehicle that it is about to move backward.

How It Works:
Unlike the other systems, the reverse light rarely needs a relay because of the low consumption of the rear signal lamps (usually two 21 W bulbs). The circuit is controlled by a plunger-type switch screwed directly into the gearbox.

Commercial Wiring Diagram:

1. Current comes from Line 15 (Switched Positive) and passes through a protection fuse.
2. The wire goes to one of the terminals of the mechanical switch on the gearbox.
3. When reverse gear is engaged, the gearbox shift rod or selector mechanically presses the switch, closing the internal contact.
4. Current flows to the switch’s second terminal and through the wires to the positive poles of the reverse lamps at the rear. The lamp sockets are in contact with the local body ground.

• The reverse switch is a normally open contact, closed mechanically by the shift linkage or selector fork in the reverse position.
• In automatic transmissions, this function is performed by the selector position switch (multifunction switch) or by the transmission control unit.
• Test: with the connector unplugged, there must be continuity between the terminals only with reverse engaged. If the switch is good, check the fuse, the voltage at the socket and the lamp ground.
• Many vehicles use the same reverse signal for parking sensors and the rear camera: a dead reverse light may come with faults in those systems.

5. Other Systems: Brake Light

Brake signaling has priority and works regardless of whether the ignition is on (Line 30).

How It Works:
An electrical switch is strategically positioned at the brake pedal (it may be mechanical or hydraulic). When the driver presses the pedal, the circuit closes instantly, lighting the higher-intensity red rear lamps and the high-mounted stop light.

Commercial Wiring Diagram:

• A constant (+) supply from the battery passes through a dedicated fuse and reaches the pedal switch.
• When the brake is applied, the circuit closes and sends voltage directly to the higher-power filament (21 W) of the dual-filament rear bulbs (whose smaller 5 W filament is used by the position light).

• The brake light has priority and is therefore supplied by line 30: it works even with the ignition off.
• The high-mounted stop light is wired in parallel with the main brake lights.
• On modern vehicles, the pedal switch also informs the engine control unit, the stability control and the cruise control. Many use two contacts: one normally open (lamps) and one normally closed (signal to the control unit). A poorly adjusted switch can keep the brake lights permanently on and drain the battery, or cause faults in cruise control and stability control.
• A tail light that comes on with the brake or flashes with it usually indicates a dual-filament bulb with shorted filaments or a bad ground, which makes the current “back-feed” through the other filament.

6. Headlamps: bulb types, construction and aiming

A conventional headlamp uses a mirrored parabolic reflector : the bulb at the focus produces a parallel beam, and the lens or the reflector itself shapes the beam. The low beam is asymmetric: it reaches farther on the curb side and cuts off the light on the side of oncoming traffic to avoid glare.

• H4 bulb: two filaments in the same bulb (high and low). The low-beam filament has a small shield that blocks part of the light and creates the beam cutoff.
• H3, H1, H7 and similar bulbs: a single filament; used in fog lamps, auxiliary lamps or headlamps with one bulb per function.
• Halogen cycle: tungsten that evaporates from the filament combines with the halogen gas and is redeposited on the filament. This prevents bulb blackening and allows a hotter filament, producing whiter, more intense light. That is why the bulb is made of quartz and must not be touched with your fingers: skin oil creates hot spots that shorten bulb life.
• Off-spec bulbs: bulbs with higher power than the original (for example, 100 W instead of 55/60 W) overheat the headlamp, overload the wiring and switch, can yellow the lens and are illegal on public roads. Use approved bulbs.

Aiming: ideally use a headlamp aiming device (optical beam setter). Without one, the reference handout describes the wall method shown below. Before aiming, inflate the tires to the correct pressure, set the fuel and load to the specified condition and set the electric leveling control, if fitted, to zero.

• Aim one headlamp at a time, covering the other, always using the low beam.
• Weak or yellowish headlamps almost always have a bad ground, a corroded terminal or a voltage drop in the relay or switch. Measure the voltage at the socket with the headlamp on: it should be close to battery voltage.
• Bulbs that burn out often indicate high charging voltage (check the alternator), excessive vibration (headlamp mounting) or a loose socket contact.
• Headlamps with a hazy lens, oxidized reflector or poor sealing lose range; if cleaning does not help, the optical unit must be replaced.

7. Fuses and circuit protection

The fuse protects the wiring, not the load: it is chosen to blow before the wire heats up and melts its insulation. That is why a fuse must never be replaced with a higher-rated one.

• Blade fuses follow a color code linked to the rated current (for example, 10 A red, 15 A blue, 20 A yellow, 30 A green, in the most common standard). Always check the value printed on the fuse.
• Test without removing it: with the circuit on, measure the voltage at the fuse’s two test points relative to ground. Voltage on both sides = good fuse. Voltage on one side only = blown fuse.
• A fuse that blows right after replacement indicates a short to ground in the circuit. Disconnect the loads on that fuse one by one until you find the shorted section, instead of fitting a larger fuse.
• High-current circuits (cooling fan, rear defroster, headlamps) use their own relays and fuses, usually in the engine compartment box.

8. How to read an automotive wiring diagram

• Supply lines at the top: in “current flow” diagrams (like those used by VW), lines 30, 15 and X are at the top and ground (31) at the bottom. The current “flows down” the drawing, through fuses, switches, relays and loads.
• Component identification: each part has a code (letter and number) explained in the legend. The diagrams are divided by system and by page, as shown in manufacturers’ indexes: battery and starting; instruments; turn signals and hazard lights; light switch; headlamps; tail, brake and reverse lights; horn; wipers; and so on.
• Wires: the diagram shows the color (sometimes with a second stripe color) and the conductor cross-section in mm². This helps you find the right wire in the harness and repair it with the correct gauge.
• Ground points and connectors: they are numbered, with their location on the vehicle. Many lighting faults are found exactly at these points.

9. Multimeter diagnosis: voltage drop

Measuring continuity with the circuit off is misleading: a wire with only a single strand of copper intact shows continuity but cannot carry the current of a lamp. The professional method is to measure the voltage drop with the circuit operating.

• Positive side: red lead on the battery (or fuse) positive and black lead on the socket positive, with the lamp on. The reading shows how much is lost along the way.
• Ground side: red lead on the socket ground and black lead on the battery negative. Readings above about 0.2 V on each side indicate excessive resistance that must be corrected.
• Test light: prefer an LED test light or the multimeter. In circuits with electronic modules, an incandescent test light can draw too much current and damage electronic outputs.

10. Common fault table

11. Assessment — 10 questions

1. In the DIN 72552 standard, which terminal is the direct battery positive? A) 15 · B) 30 · C) 31 · D) 50
2. Why does the horn use a relay? A) To flash · B) So that the steering wheel button only switches a low current · C) To reduce the horn voltage · D) To eliminate the fuse
3. In a 5-pin relay at rest, terminal 30 is connected to: A) 87 · B) 85 · C) 87a · D) 86
4. The hazard lights work with the ignition off because they are supplied by line: A) 15 · B) X · C) 50 · D) 30
5. An electronic flasher relay that starts flashing fast usually indicates: A) weak battery · B) a blown bulb in the circuit · C) faulty alternator · D) blown headlamp fuse
6. In a P21/5W bulb, the 21 W filament is used for: A) tail light · B) brake light · C) reverse light · D) license plate light
7. A fuse mainly protects: A) the bulb · B) the wiring · C) the relay · D) the battery
8. Does a relay “click” guarantee it is good? A) Yes · B) No: the contact may be burnt, and the drop between 30 and 87 must be measured
9. A weak, yellowish headlamp with a new bulb points first to: A) wrong bulb · B) voltage drop in the circuit or a bad ground · C) new battery · D) aim set too high
10. Before aiming the headlamps you should: A) inflate the tires and park on level ground · B) remove the spare wheel · C) switch on high beam · D) disconnect the battery

Answer key with comments

• 1 — B. Terminal 30 is the permanent positive; 15 is switched.
• 2 — B. The button only controls the coil; the horn current flows through the relay contacts.
• 3 — C. 87a is the normally closed contact.
• 4 — D. Line 30: always live, to signal a stopped vehicle.
• 5 — B. The drop in current is detected and the relay warns by flashing fast.
• 6 — B. The higher-power filament indicates braking.
• 7 — B. The fuse is sized to protect the conductor.
• 8 — B. The coil may work with a worn contact; the voltage drop reveals the fault.
• 9 — B. Resistance in the circuit reduces the voltage at the bulb, so the light is weak.
• 10 — A. Tires, load and floor change the vehicle’s inclination and therefore that of the beam.