Have you ever looked under the hood and seen little plastic boxes with strange numbers on them? Or they tried to replace a burnt-out relay, but got lost in the designations 12V/30A or 87a/87b? Numbers and letters on car relays are not just random symbols, but the key to understanding which relay you need and how it works in the machine's electrical circuit. An error in selection can lead to a short circuit, overheating of the wiring, or even a fire.

In this article we will figure out what all these mysterious numbers mean: from the rated voltage (12V or 24V) to maximum current (20A, 30A, 40A). You will learn how to read the markings on the relay body, how they differ 4-pin and 5-pin model, and why you can’t install a relay with a current reserve β€œjust in case.” And also - how to check a relay with a multimeter and what to do if all the markings on it have been erased.

1. Basic numbers on the relay: voltage and current

The first thing you should pay attention to is rated voltage and maximum current. These parameters are usually indicated in large numbers on the top or side of the case. For example, the inscription 12V 30A means:

  • πŸ”‹ 12V β€” the relay is designed to operate in a network with a voltage of 12 volts (standard for passenger cars).
  • ⚑ 30A - the maximum current that the contact group can pass through without overheating.

For trucks and special equipment, relays are often used 24V, and in some cases - even 48V (for example, in electric buses). Important: If you put a 12-volt relay into a 24-volt network, it will either not work or burn out. The reverse replacement (24V instead of 12V) is also unacceptable - the coil will not receive enough voltage to operate.

With maximum current the situation is more complicated. Many car owners mistakenly believe that they can take a relay with a reserve - for example, 40A instead of 20A. However, this is fraught with:

  • πŸ”₯ Overheating of the wiring (if the cross-section of the wires is not designed for increased current).
  • πŸ’₯ False positives due to increased sensitivity.
  • πŸ›‘ Accelerated wear of contacts (under low loads, contacts can β€œburn” due to insufficient pressure).
⚠️ Attention: If the relay indicates current 10A, and you connect the load to 15A, the contacts will spark, heat up and quickly fail. Always select a relay rated to match the load (or slightly higher, but no more than 20%).
πŸ“Š Which relay do you most often change in your car?
Starter relay
Fan relay
Turn signal relay
Fuel pump relay
Other

2. Decoding contacts: what do the numbers 30, 85, 86, 87, 87a mean?

On the bottom or side of the relay you will see numbers indicating contact numbers. This is a standard marking accepted in auto electrics. Here's what they mean:

Contact number Designation Purpose
30 General contact Connects to positive wire from the battery or fuse. This is the "input" of power.
85 Control minus Connects with mass (body) or negative wire from the button/sensor.
86 Manager Plus Connects to positive wire from a button, relay or ECU that sends a signal to operate.
87 Normally open contact Output to load (for example, fan, lights). Closes with 30 when the relay is activated.
87a Normally closed contact Used in relays with switching contacts. Opens when triggered.

For example, in standard 4 pin relay (most common type) pins are numbered as 85, 86, 30, 87. When voltage is applied to 86 and closure 85 to ground, contacts 30 and 87 close, passing current to the load.

Relay with 5 contacts (designation 87a) are used less frequently - for example, in alarm systems or for switching circuits. Here when triggered 30 disconnects from 87a and connects to 87.

How to test a relay without a diagram?

If the markings on the case are erased, use the multimeter in dialing mode:

1. Connect the probes to 85 and 86 β€” the resistance should be ~50–120 Ohms (this is a coil).

2. Connect the probes to 30 and 87 - without power, the resistance should be infinite (open), when 12V is supplied to 85/86 β€” close to 0 (closed).

3. Additional symbols: what letters and symbols mean

In addition to numbers, you can find letters and symbols on the relay. Here are the most common:

  • πŸ”Ή SPST (Single Pole Single Throw) - single-pole relay with one pair of contacts (normally open).
  • πŸ”Ή SPDT (Single Pole Double Throw) - relay with switching contacts (for example, 30-87-87a).
  • πŸ”Ή NO (Normally Open) - normally open contact (analogue 87).
  • πŸ”Ή NC (Normally Closed) - normally closed contact (analogue 87a).
  • πŸ”Ή DC - direct current (as opposed to AC - a variable that is not used in the car).

The following may also be indicated on the body:

  • πŸ“ Dimensions (for example, 28.5x19x20 mm) - important for installation in the fuse box.
  • 🌑️ Temperature range (for example, -40Β°C..+85Β°C) - critical for the relay in the engine compartment.
  • πŸ”§ Mounting type β€” in the block, on DIN rail or with screw terminals.

On some relays premium (for example, HELLA or Bosch) you can find the marking IP67 - this means that the relay is protected from dust and temporary immersion in water. Such models are installed in places with high humidity (for example, in headlights or under a bumper).

4. How to select a relay by numbers: step-by-step instructions

Let's say your cooling fan relay is burned out and you want to replace it. Here's how to choose the right analogue:

Take a photo of the old relay from both sides|Write down all the numbers and letters from the body|Check the number of contacts (4 or 5)|Check the current rating (look at the fuse in the circuit)|Check the wiring diagram of your car model

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Step 1: Determine the voltage.

For passenger cars this is almost always 12V. For trucks (eg MAN, Scania) β€” 24V. If you are not sure, look at the battery voltage or check with a multimeter.

Step 2. Check the maximum current.

He must be no lessthan the fuse current in this circuit. For example, if the fan circuit has a fuse at 20A, the relay should be on 20A or 30A (but not on 10A!).

Step 3: Check the contact type.

Count the number of legs:

  • 4 contacts - standard relay (30, 85, 86, 87).
  • 5 contacts - relay with switching group (30, 85, 86, 87, 87a).

Step 4. Compare dimensions.

The relay must physically fit into the socket. The most common sizes:

  • Standard β€” ~25Γ—20Γ—15 mm (for example, Bosch 0 332 209 150).
  • Mini β€” ~19Γ—15Γ—10 mm (for example, for alarms).
  • Cube β€” ~28Γ—28Γ—28 mm (for high-current circuits).
⚠️ Attention: If you are replacing a relay in the fuse box (for example, Mondeo or Passat B6), please note the shape of the legs. Some relays have asymmetrical contacts and cannot be reversed during installation!

5. Frequent errors when working with relays

Even experienced auto electricians sometimes make mistakes when working with relays. Here are the most common:

  • ❌ Ignoring polarity. Contacts 85 and 86 Polarity sensitive! If you mix up the plus and minus, the relay will not work (or the coil will burn out).
  • ❌ Using a relay with a current reserve. As already mentioned, β€œin reserve” take the relay for 40A instead of 20A - bad idea. The contacts may not fit tightly at low currents, which will cause sparking.
  • ❌ Soldering wires directly to the contacts. The relay is designed for crimp terminals. Soldering leads to overheating and cold contacts.
  • ❌ Installing a relay without a block. Dangling wires on the relay legs are a risk of short circuit due to vibration.

Another typical problem is contact oxidation. If the relay periodically β€œglitches” (for example, the fan turns on every once in a while), try:

  1. Remove the relay and inspect the legs for green deposits.
  2. Clean your contacts WD-40 or fine sandpaper (1000 grit).
  3. Check the tightness of the fit in the block - sometimes the contacts unbend.
πŸ’‘

If the relay clicks, but the load does not turn on, the problem may be burnt contacts 30-87. Try to slightly move the relay in the socket - if it works, it means a replacement is needed.

6. How to test a relay with a multimeter: step-by-step guide

If the relay does not operate, it can be checked without removing it from the machine. You will need a multimeter and a screwdriver (or a piece of wire).

Checking the coil (winding):

  1. Switch the multimeter to resistance measurement mode (200 Ξ©).
  2. Connect the probes to the contacts 85 and 86.
  3. The resistance must be in the range 50–120 Ξ©. If 0 Ξ© - short circuit if ∞ - break.

Checking the contact group:

  1. Connect the probes to 30 and 87.
  2. Without voltage applied, the resistance should be ∞ (open).
  3. Serve 12V on 85 and 86 (can be from a battery through a light bulb).
  4. When triggered, the resistance between 30 and 87 should become 0 Ξ©.

If the relay 5-pin, additionally check:

  • Resistance between 30 and 87a - must be 0 Ξ© up to supply voltage and ∞ after.
  • Resistance between 30 and 87 - on the contrary, ∞ before and 0 Ξ© after.
πŸ’‘

If the relay clicks, but the contacts 30-87 do not close, which means the contact group has burnt out. Such a relay must be replaced; cleaning will not help.

There are cheap relays on the market no-name by 50–100 β‚½ and branded 300–800 β‚½. What's the difference?

Brand Features Where is it used? Average price
Bosch High quality contacts, long service life, moisture protection. Critical circuits (starter, fuel pump). 400–700 β‚½
HELLA Vibration resistant, wide temperature range. Trucks, special equipment. 500–900 β‚½
TE Connectivity Precision contacts, low resistance. Audio systems, alarm systems. 300–600 β‚½
Sonar Budget option, suitable for secondary circuits. Turn signals, heated seats. 100–250 β‚½
No-name (China) Low quality plastic, contacts oxidize quickly. Temporary replacement, circuit testing. 50–150 β‚½

For critical circuits (starter, fuel pump, cooling fan) it is better to take Bosch or HELLA. For secondary (turn signals, horn) will do Sonar. Cheap Chinese relays can only be used for temporary replacement - they often fail in cold weather or under high loads.

Frequently Asked Questions

Is it possible to replace a 4-pin relay with a 5-pin one?

No, these are different types of relays. The 4-pin is making (normally open), and the 5-pin is switching (has an additional contact 87a). If the circuit does not provide a normally closed contact, a 5-pin relay will not work.

What happens if you put a 24V relay in a 12V network?

The relay will not work because the voltage on the coil (85-86) will not be enough to attract the core. In some cases a faint click may be heard, but the contacts 30-87 don't become isolated.

How to determine which relay has burned out if there are several of them in the block?

The easiest way is to swap the relay with a known working one (for example, with a turn signal relay). If after replacement the problematic circuit starts working, but the other one stops working, the culprit has been found. You can also test each relay with a multimeter (see section 6).

Why does the relay get hot?

The reasons may be the following:

  • The current in the circuit exceeds the rated current (for example, a relay on 20A, but it goes 25A).
  • Poor contact in the block (oxidation, poor fit).
  • Burnt internal contacts 30-87.

Solution: replace the relay with a more powerful one (if the wiring allows) or check the circuit for a short circuit.

Can the relay be repaired?

Theoretically, yes, but in practice it is impractical. The relay is worth 100–500 β‚½, and its disassembly and cleaning of contacts takes time and does not guarantee long-term operation. An exception is a relay with burnt legs (they can be cleaned with sandpaper).