Introduction: Why the term "normally closed" causes confusion
Have you ever come across the inscription «NO/NC» on a relay or switch in a car and were at a loss as to what it means? If yes, then you are not alone. Terms "normally open" (NO) and “normally closed”** (NC) often become a source of confusion even among experienced car owners. Especially when it comes to car electrical repairs, where misunderstanding these concepts can lead to a short circuit or system failure.
In this article we will look at what is actually hidden behind the term “normally closed”** (NC), how it works in the context of automotive electricals, and where exactly it can be found under the hood or in the interior of your car. We will not delve into theoretical electrical engineering - only practical examples, diagrams and tips that will be useful when diagnosing or upgrading electrical equipment.
First, remember: "normally closed"** means that the contact closed by default, when no control signal is supplied to the device. This is the opposite of "normally open" (NO), where the circuit is open in its initial state. But why is this so important for a car? Let's figure it out.
What is a “normally closed” (NC) contact in simple words
Imagine a regular light switch in your apartment. In the off position the circuit is open - this is normally open (NO) contact. Now imagine a switch that, on the contrary, passes currentwhen not touched, and opens the circuit when pressed. This is it normally closed (NC) contact.
In automotive electrical engineering, NC contacts are used where it is necessary to guarantee safe default state. For example:
- 🔋 Fuel pump relay: When the ignition is off, the circuit is open (NO), but some systems use NC logic for emergency shutdown.
- 🚨 Oil pressure sensors: When the pressure drops, the contact closes, turning on the lamp on the dashboard.
- 🔒 Central lock: in some models, NC contacts block the doors if there is no signal from the key fob.
The key difference between NC and NO is logic of work. If the device fails (for example, a relay coil burns out), the NC contact will remain closed and the NO contact will remain open. This is critical for safety systems where a "closed-state failure" can prevent an accident.
Where NC contacts are used in a car: 5 key examples
NC contacts are not as common as NO contacts, but they can be found in critical parts of a machine. Here are the typical places where they are used:
| Car assembly | Example of NC contact | Why is NC needed? |
|---|---|---|
| Ignition system | Starter Interlock Relay | Prevents the engine from starting when the engine is running (closes the circuit only when the key is in the "Start" position) |
| Brake system | Brake fluid level sensor | When the level drops, it closes the lamp circuit on the panel (danger signal) |
| Lighting | Side light relay (on some models) | Ensures that the headlights are turned on when the ignition is turned off (for example, in “parking” mode) |
| Climate control | Interior temperature sensor | Closes the fan circuit when the temperature threshold is exceeded |
| Security | Door switches | In some cars, NC contacts turn on the interior lights when the door is open |
Interesting fact: in old carburetor cars (for example, VAZ-2106) NC contacts were often used in the forced idle economizer circuit (EFI). If the sensor failed, the contact remained closed, and the engine did not stall at idle - it was protection mechanism against sudden engine stop.
If your car's circuit has an NC contact, always check it with a multimeter in dial mode with power off - this way you will avoid false positives.
How to check the NC contact with a multimeter: step-by-step instructions
Diagnostics of NC contacts requires care, since their condition depends on the presence of a control signal. Here's how to properly check such a contact:
- Turn off the power device (remove the fuse or disconnect the battery).
- Connect the multimeter in mode
calls(or resistance measurements) to the NC contacts. - If the device shows
0 ohm(or emits a beep) - the contact is operational and closed by default. - Apply a control signal (for example, turn on the ignition or press a button) - the resistance should become
endless(open circuit).
Example for relay:
1. Remove the relay from its seat.
2. Connect the multimeter probes to the terminals 87a (NC) and 30 (general).
3. Without supplying 12V to the coil (pins 85 and 86), the resistance should be ~0 Ohm.
4. Apply 12V to the coil - the resistance should become ∞.
Device power is turned off|Multimeter in continuity mode|Checked initial state (0 Ohm)|Checked state when triggered (∞)|Compared results with documentation-->
⚠️ Attention: Never test the NC contacts for sparks (short circuit) - this can damage sensitive sensors, for example in the ABS system or airbags.
NC vs NO: when to use which contact in car tuning
If you are modifying electrical equipment (for example, installing additional lighting or an alarm), it is important to choose the right type of contact. Here are the basic rules:
- 🔦 For emergency systems (for example, a sound signal when the hood is opened) use NC - this way the circuit will be closed if the sensor malfunctions.
- 💡 For comfort functions (foot lights, heated seats) suitable NO - they should only turn on when commanded.
- 🚗 In motor control circuits (injection, ignition) NO is safer - if there is a break, the circuit will open and the engine will stall, rather than go wild.
Case Study: During Installation additional brake light for the rear window it is better to use a relay with NO contact (pin 87). If the relay fails, the lights simply won't come on, but the brakes will still work. But in start blocking system (immobilizer) is often used NC logic: if the circuit breaks, the engine will not start, which protects against theft.
What happens if you confuse NO and NC in the alarm system?
If you use NO instead of NC in the alarm siren circuit, then when a sensor (for example, a shock) is triggered, the siren won't turn on, since the circuit will open. And if the sensor fails, the siren can work constantly, draining the battery.
Common mistakes when working with NC contacts
Even experienced auto electricians sometimes make mistakes related to NC logic. Here are the most common of them:
- Ignoring the connection diagram: for example, connecting the NC output of the relay (87a) instead of NO (87) in the cooling fan circuit. The result is that the fan runs constantly, even on a cold engine.
- Test under voltage: Testing the NC contact without turning off the power can lead to false alarms (for example, in ABS sensors).
- Ignoring "reverse logic": in some Japanese cars (for example, Toyota Corolla E120) NC contacts are used in the fuel pump control circuit. If you replace the relay with NO, the car will not start.
Real life example: owners Nissan Almera N16 They often encounter a problem when, after replacing the cooling fan relay, it stops turning off. The reason is that the standard circuit uses an NC contact, while the new relay uses a NO contact. Solution: either look for an original relay, or resolder the contacts.
⚠️ Attention: In systems xenon lighting NC contacts are used in the ignition unit to control voltage. If NO and NC are mixed up when replacing the unit, the lamps will flicker or not turn on at all.
How to read circuits with NC contacts: analysis of symbols
On vehicle electrical diagrams, NC contacts are designated differently depending on the standard. Here are the main options:
| Standard | Designation NC | Example (where found) |
|---|---|---|
| Russian (GOST) | Contact with overbar (─| |─) | Schemes VAZ, GAZ, UAZ |
| European (DIN) | 87a (for relay) | Schemes Volkswagen, BMW |
| American (SAE) | NC or "Normally Closed" | Schemes Ford, Chevrolet |
| Japanese (JIS) | b-contact (eg b1, b2) | Schemes Toyota, Mitsubishi |
For example, in a relay circuit Bosch 0 332 019 150 (applied in Audi A4 B6) the conclusions are designated as follows:
85, 86- control coil,30- general contact,87— NO (normally open),87a— NC (normally closed).
If your car uses such a relay, and you see a line from the output in the diagram 87a to the check engine light, this means that the light comes on when no signal from the ECU (for example, if a wire breaks).
In Japanese cars (for example, Subaru Impreza) NC contacts are often marked with the letter "b". If the diagram indicates a contact b1, most likely this is a normally closed pin.
FAQ: Frequently asked questions about normally closed contacts
Is it possible to replace a relay with an NC contact with a regular one with an NO contact if the wires are re-soldered?
Technically yes, but it depends on the purpose of the circuit. For example, in the system alarm (where NC guarantees that the emergency lights will turn on in the event of a break), such a replacement will lead to a failure of the function. In chains comfort (for example, heated seats), replacement is possible, but you will need to reprogram the control logic (for example, invert the signal from the button).
Why do some cars have an NC contact for the oil pressure sensor, while others have a NO contact?
It depends on the manufacturer's philosophy:
- NC (for example, in Renault Logan): when the pressure drops, the circuit closes and the lamp lights up. If the sensor fails, the lamp will light constantly - this is a signal for diagnostics.
- NO (for example, in Kia Rio): when the pressure drops, the circuit opens and the lamp goes out. Here, sensor failure will result in the lamp not lighting up even if there is a real problem.
NC logic is considered safer because sensor failure is immediately visible.
How to check whether an unknown relay has NC or NO contact?
The algorithm is simple:
- Call contacts
30(general) and87awithout applying voltage to the coil. If the circuit is closed, it is NC. - Apply 12V to the coil (terminals
85and86). If the contact is open, confirm NC. - Check contact
87- he should behave in the opposite way (NO).
If both contacts (87 and 87a) are closed without voltage - the relay is faulty (contacts are stuck).
My car uses an NC relay in the brake light circuit. Is it possible to replace it with NO by swapping the wires?
No, it's dangerous! In brake lights, the NC relay is usually used to circuit duplication: If the main brake pedal sensor fails, the relay will close the circuit via the NC contact and the brake pedals will still light up. If you replace it with NO, if the sensor breaks, the brake lights will stop working altogether, which will create an emergency situation. In this case, you need to look for a relay with the same logic or upgrade the circuit by adding a second sensor.
Why does the alarm manual say to use an NC contact to block the start?
This is made for fault tolerance. If an attacker cuts the interlock control wire, the NC contact will remain closed and the engine will not start. When using a NO contact, a wire break, on the contrary, unlocks launch, which reduces protection. The exception is alarms with two-stage locking (for example, StarLine A93), where both types of contacts are combined.