Relay S3 12A - one of those small but critical elements of automotive electrical systems, the existence of which the driver remembers only when it breaks down. This device is responsible for switching circuits with a current load of up to 12 amps, and its failure can paralyze the operation of the headlights, cooling fan, fuel pump or other systems. Depending on the car model, the relay may be located in a mounting block under the hood, in the passenger compartment, or even under the dashboard - and finding it without a diagram can be difficult.

In this article we will look at pinout of contacts relay S3 12A, we will show typical connection diagrams for different cars (from VAZ up to Toyota), we will explain how to check it with a multimeter and replace it if necessary. We will pay special attention common mistakes during diagnostics - for example, why the relay may “click” but not work, and what to do if the problem remains after replacement. If you are faced with non-working headlights, a faulty starter or electronics glitches, this instruction will help identify the culprit.

Note: The article discusses standard size relays 90.3747-10 (analogue 98.3777, 902.3747), which are installed on most domestic and foreign cars. For accurate identification, check the markings on the body of your relay with the photo in the section below.

1. Purpose of relay S3 12A: where used in the car

Relay S3 12A belongs to the category control relays with normally open contacts. Its main task is to relieve low-current circuits (for example, buttons on the instrument panel) from high currents, protecting them from overheating and melting. Typical Applications:

  • 🔦 Lighting circuits: low/high beam, fog lights, dimensions. The relay turns on when the light switch is turned, supplying current directly from the battery to the lamps.
  • 🚗 Fuel system: fuel pump control (in some models VAZ, Renault, Kia/Hyundai). When you turn the ignition key, the relay closes the pump circuit, creating pressure in the fuel line.
  • 🌀 Cooling fans: in electrically driven systems (e.g. GAZelle, Chevrolet Niva). The relay is triggered by a signal from the temperature sensor, turning on the fan.
  • 🔋 Starter and generator: in the starter blocking circuit when the engine is running or to disconnect the generator field winding.

On some vehicles (for example, Ford Focus 2 or Opel Astra H) relay S3 12A can be duplicated or paired with a similar relay on 20A or 30A - it depends on the load of the circuit. Important: if your car has a relay with markings S3 20A, it cannot be replaced by 12A - this will lead to melting of the contacts!

To determine exactly which system the relay in your car is responsible for, check electrical diagram (usually included with the repair manual) or use the table below:

Car make Relay installation location Purpose
VAZ 2110-2115 Mounting block (under the hood) Low beam (relay K4), cooling fan (K6)
Renault Logan/Sandero Relay block in the cabin (behind the glove compartment) Fuel pump, heated rear window
Toyota Corolla (E12) Block under the steering wheel (left side) Fog lights, air conditioning compressor
Chevrolet Niva Additional relay block (under the hood) Cooling fan, heated seats
📊 Where is the S3 12A relay installed in your car?
In the mounting block under the hood
In the cabin behind the dashboard
Under the glove compartment
I don't know, I haven't looked yet

2. Pinout of relay contacts S3 12A: diagram and designations

Standard relay S3 12A has 5 contacts, numbered on the bottom of the case. Their purpose is universal for most cars, but may vary depending on the manufacturer. Below is a typical pinout:

  • 85 - manager"minus" (connects to a button or ECU). When voltage is applied to this contact, the relay is activated.
  • 86 - manager"plus" (from the ignition switch or fuse). Often connected to +12V through a resistor.
  • 87 — normally open contact (output to a load, for example, a lamp or pump).
  • 87a - normally closed contact (rarely used, usually missing or not used).
  • 30 - general"plus" (direct connection to battery or fuse).

Scheme of work: When voltage is applied to the contacts 85 and 86 A magnetic field appears inside the relay, which attracts the armature and closes the contacts 30 and 87. Thus, current from the battery flows to the load (for example, headlights).

Visually, the pinout can be represented as follows:



[85] [86]



| |


30 | | 87


|_______|


[87a]

Attention! On some relays (for example, from Bosch or Hella) contacts may be numbered differently - 1-5 instead of 85-87a. Always check the markings on the case!

How to distinguish a 12A relay from a 20A or 30A?

The main difference is the thickness of the contacts and markings on the case. A 12A relay usually has thinner terminals (wire thickness ~1 mm), while 20A and 30A are thicker (1.5–2 mm). The maximum current strength may also be indicated on the case (for example, “12A 12V DC”).

3. How to test the S3 12A relay with a multimeter: step-by-step instructions

If the relay stops working (for example, the headlights do not turn on or the fan does not work), check its functionality. For this you will need multimeter (in continuity or resistance measurement mode) and power source 12V (you can use a battery or power supply).

☑️ Checking relay S3 12A

Done: 0 / 5

Step 1. External inspection

Before checking, inspect the relay for:

  • 🔥 Melted or blackened contacts (a sign of a short circuit).
  • 💧 Traces of oxidation or corrosion on the legs.
  • 🧲 Cracks on the case (can lead to sticking of contacts).

Step 2. Check the winding

Connect the multimeter probes to the contacts 85 and 86. The winding resistance must be within 50–120 Ohm. If the device shows:

  • 0 ohm - short circuit in the winding (relay must be replaced).
  • ∞ (infinity) - winding break.

Step 3. Checking contacts

Connect 12V to contacts 85 (minus) and 86 (plus). You should hear a click, which means the relay has activated. Now call the contacts 30 and 87:

  • 🔌 If the resistance is ~0 Ohm — the contacts are closed, the relay is working properly.
  • 🚫 If resistance ∞ — contacts do not close (oxidation or mechanical wear).

Step 4. Check for sticking

Remove power from contacts 85-86. Contacts 30-87 must open (resistance ∞). If they remain closed, the relay is “stuck” and requires replacement.

⚠️ Attention: If the relay clicks, but the contacts 30-87 do not close, the problem may be oxidized pad legs. Try cleaning the contacts with an eraser or WD-40.

4. Typical malfunctions of the S3 12A relay and their causes

Relay S3 12A fails for several reasons, most of which are related to circuit overload or bad contact. Let's look at the most common breakdowns and their symptoms:

Malfunction Signs Possible reason
Winding break The relay does not click, the load does not turn on Overheating, mechanical damage, manufacturing defects
Winding short circuit The relay gets hot and the fuse blows. Moisture ingress, insulation breakdown
Oxidation of contacts The relay clicks, but the load is unstable High humidity, long-term operation
Contacts sticking The load does not turn off (for example, the fan runs constantly) Overvoltage, wear of contact materials

The most insidious malfunction - when the relay clicks but doesn't complete the circuit. This occurs due to burnt contacts 30-87 at high currents (for example, when using powerful halogen lamps instead of standard ones). In this case, the relay must be replaced, since cleaning the contacts will give a temporary effect.

Another common problem is false positive relay due bad mass. For example, if the contact 85 poorly connected to the body, the relay may turn on spontaneously. Check the reliability of the grounding!

💡

If the relay fails frequently, check the circuit for overload. The current may be greater than 12A due to a load fault (such as a short circuit in a lamp or pump).

5. Replacing the S3 12A relay: step-by-step instructions with photos

If the relay is faulty, it must be replaced. The procedure is simple, but there are nuances that will help you avoid mistakes. Let's look at the process using an example VAZ 2114 (for other cars the steps are similar, only the location of the relay box is different).

Tools:

  • 🔧 Flat screwdriver (for removing the block cover).
  • 🔍 Flashlight (to see the markings).
  • 🧴 WD-40 or alcohol (to clean contacts).

Step 1: Power off

Remove"minus" terminal from the battery to avoid short circuit when manipulating the relay.

Step 2. Finding a relay

On VAZ 2114 relay S3 12A (low beam) is located in the mounting block under the hood, marked as K4. On other cars, look according to the diagram (see section 1).

Step 3: Removing the Relay

Gently pull the relay towards you, rocking it from side to side. If it gets stuck, do not use force - spray WD-40 on the contacts and wait 1-2 minutes.

Step 4: Installing a New Relay

Insert new relay all the way, making sure that the legs fit into the block without distortion. Attention: If the relay is upside down, it will not work!

Step 5. Check

Connect the battery and check the operation of the circuit (for example, turn on the low beam). If the problem persists, look for the cause in fuse, wiring or load (lamp, pump, etc.).

⚠️ Attention: On some vehicles (for example, Renault) the relays are fixed with latches. Don't break them - gently press them out with a screwdriver!
💡

When replacing a relay, always check the condition of the contacts in the block. Oxidized or burnt sockets can cause repeated failure.

6. Analogs of relay S3 12A: which ones can be used for replacement

If the original relay S3 12A out of stock, it can be replaced with analogues from other manufacturers. The main thing is to comply three conditions:

  1. Coincidence standard size (standard 5-pin relay housing).
  2. Coincidence rated current (12A or higher).
  3. Coincidence work schemes (normally open contacts).

Here is a list of proven analogues:

Manufacturer Article Notes
Bosch 0 332 019 150 High quality, suitable for most foreign cars
Hella 4RA 003 568-031 Reinforced contacts, recommended for high load circuits
VAZ (JSC Avtoelektronika) 90.3747-10 Budget option, suitable for domestic cars
Siemens V23134-A1019-A401 Used in VW, Audi, Skoda

Can I use a 20A relay instead of a 12A one? Yes, but only if the current in the circuit does not exceed 12A. For example, for headlights or a fan this is acceptable, but for a fuel pump (where the current can reach 15A) it is better to install a relay with a reserve - 20A or 30A.

What not to do:

  • ❌ Install relays with normally closed contacts (marking 87a instead of 87).
  • ❌ Use relay with another trigger voltage (for example, 24V instead of 12V).
  • ❌ Installing a relay without a housing ("bare" contacts) - this will lead to a short circuit.

7. Frequent errors when working with the S3 12A relay

Even experienced car owners sometimes make mistakes when diagnosing or replacing a relay. Here are the most common of them:

  1. Contacts 85 and 86 are confused.

    If you reverse the polarity (minus on 86, and plus on 85), the relay will not work. In some cases this may lead to diode breakdown (if it is installed parallel to the winding).

  2. Ignore the fuse.

    If the relay fails, always check corresponding fuse (usually it is in the same chain). Its burnout may be the cause of the malfunction, and not the consequence.

  3. They don't check the load.

    If the new relay burns out quickly, the problem may be short circuit in the lamp, pump or wiring. Before replacing the relay, check the load resistance!

  4. Use a relay from another circuit.

    For example, they install a relay fan instead of a relay headlights. This may work, but does not guarantee reliability - relays on different circuits may have different response timing characteristics.

How to avoid mistakes? Always follow the principle:"First diagnosis, then replacement". Check:

  • 🔋 Availability 12V on control contacts (85-86).
  • 🔌 Integrity of wiring from relay to load.
  • 💡 Serviceability of the load itself (lamp, pump, etc.).
What to do if the relay works, but the load does not turn on?

Check mass circuits - often the problem lies in poor grounding. Also make sure to contact 30 comes +12V (sometimes the wire from the battery burns out).

FAQ: Frequently asked questions about the S3 12A relay

Is it possible to repair the S3 12A relay or just replace it?

Theoretically, the relay can be disassembled and the contacts cleaned (for example, with sandpaper or alcohol), but this will give a temporary effect. In 90% of cases, it is better to replace the relay with a new one - its cost is low (from 50 to 300 rubles), and the risk of repeated breakdown is high.

Why does the relay click but the headlights don't turn on?

This means that the relay coil is working properly (it works), but power contacts 30-87 do not close due to burning. Also check:

  • Integrity of lamps and their contacts.
  • Availability 12V on contact 30 (there may be a broken wire from the battery).
  • The fuse in the headlight circuit is faulty.
Which relay is better to choose: original or analogue?

For domestic cars (VAZ, GAS) an inexpensive relay will do 90.3747-10. For foreign cars (Toyota, VW) better take Bosch or Hella - they last longer under high load conditions. Among budget analogues, relays have proven themselves well Ween (China).

How to test a relay without a multimeter?

Can be used control lamp or continuity test from wire and battery:

  1. Connect the lamp (or battery with lamp) to the contacts 85-86 - If the relay is working properly, you will hear a click.
  2. Call contacts 30-87 the same lamp: when the relay is activated, the lamp should light up.

Attention! Do not use powerful lamps (for example, from headlights) for testing - this may burn the relay contacts.

Where can I buy the S3 12A relay if it is not available in local stores?

The relay can be ordered online:

  • 🛒 Wildberries or Ozon (article 90.3747-10).
  • 🔧 Exist.ru or AutoDoc (for original analogues).
  • 📦 AliExpress (cheap Chinese relays, but quality varies).

Delivery time ranges from 1 day (at local auto parts stores) to 2-3 weeks (when ordering from China).