12 Volt Car Relays Used In Automotive Industry

A car relay is a electro-magnetic switch that uses a small amount of current to control a larger amount of current. How they work is by a wire wound magnetic coil that when excited by a electric current, moves a mechanical spring contact inside the relay, completing a circuit. Car relays are usually controlled by another switch such as your car horn button, headlight switch circuit or power window / door lock switches. Relays are located all over a car, truck, van, suv or hybrid. They are placed in the engine compartment, fuse box or fuse panel, under the dash board or behind door panels and kick panels. Check your owners manual or service manual for exact placement.

Types Of Automotive Relays

The most common relays used to control electrical accessory devices in the automotive industry are ISO Mini and Micro relays. 4 and 5 pin designs, SPST / SPDT are used in both styles of automotive relays. Normally open or normally closed types. Look for relay diagrams or a relay schematic that is printed on the side of the relay to determine it's function. 20, 30 and 40 amp relays are common in automotive use. Below are some examples of basic 12 volt DC car relays.

Basic 4 Pin Mini Relay

Case size: 1 inch x 1 inch x 1 inch
Pin 30: Power into the relay for the accessory device
Pin 87: Relay switched power out to the accessory device
Pin 85: Positive hot line to activate the relay coil
Pin 86: Ground line to the relay

Basic 4 Pin Micro Relay

Case size: 1 inch x 1 inch x 1/2 inch
Pin 3: Power into the relay for the accessory device
Pin 5: Relay switched power out to the accessory device
Pin 2: Positive hot line to activate the relay coil
Pin 1: Ground line to the relay

Basic 5 Pin Mini Relay

Case size: 1 inch x 1 inch x 1 inch
Pin 30: Power into the relay for the accessory device
Pin 87: Relay switched power out to the accessory device
Pin 87a: Powered hot when relay is at rest. Open when relay is activated
Pin 85: Positive hot line to activate the relay coil
Pin 86: Ground line to the relay

Basic 5 Pin Micro Relay

Case size: 1 inch x 1 inch x 1/2 inch
Pin 3: Power into the relay for the accessory device
Pin 5: Relay switched power out to the accessory device
Pin 4: Powered hot when relay is at rest. Open when relay is activated
Pin 2: Positive hot line to activate the relay coil
Pin 1: Ground line to the relay

Note: In a basic or resistor protection relay, pins 1 and 2 of the micro relays and pins 85 and 86 of the mini relays can be interchanged positive or negative. In a diode protected relay, Mini relays must be pin 85 (+) power, pin 86 (-) ground / Micro relays must be pin 2 (+) power, pin 1 (-) ground. Look for a relay diagram or a relay schematic that is printed on the side of the relay to determine.

Protection Diode And Spike Suppression Resistor For Automotive Relays

Diode Protection For A Relay (4 pin mini shown)

Resistor Protection For A Relay (4 pin mini shown)

Current flowing through a relay coil creates a magnetic field which collapses suddenly when the current is turned off. The sudden collapse of the magnetic field induces a brief high voltage across the relay coil that wants to flow in the opposite direction which can cause damage to a ECM, PCM or ICs. A quenching protection diode or suppression resistor allows the induced voltage a path to block and dissipate. This prevents spiked voltage becoming high enough to cause damage to a electronic control module or a IC.

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