Relays

There are two commonly used relay configurations on automobiles:

• 4-Pin Relay - This relay has one set of contacts. The contacts are normally open; however, the relay could be constructed to have normally closed contacts instead.  Most automotive 4-pin relays have normally open contacts.  In the photograph shown below, the relay has a schematic printed on the case which represents its electrical configuration.  Notice that pins 85 and 86 are attached to the coil of wire.  Notice that pins 87 and 30 are connected to a normally open set of contacts; a normally open switch.

• 5-Pin Relay - This relay has two sets of contacts. One set of contacts is normally open and the other set is normally closed.  This configuration allows one relay to be used to control two different circuits.  In the photograph shown below, the relay has a schematic printed on the case which represents its electrical configuration.  Notice that pins 1 and 2 are attached to the coil of wire.  Notice that pins 3 and 5 are connected to a normally open set of contacts; a normally open switch.  Also notice that pins 3 and 4 are connected to a normally closed set of contacts; a normally closed switch.

Relay operation is as follows:

• A small amount of current, typically in thousandths of an amp, is allowed in the coil of wire.

• As the Danish physicist Hans Christian Oersted discovered, the current in the coil of wire produces a strong magnetic field around the wire.

• As English physicist Dominique Francois Jean Arago discovered, the strong magnetic field from the current in the coil of wire magnetizes the iron core.

• The iron core has a set of electrical contacts (a switch) next to it.

  • One contact is attached to the end of the iron core. 

  • The other contact is suspended in the air next to the other contact.

  • The circuit is electrically open; no current is allowed through the contacts.

• When the iron core is magnetized, it pulls the two contacts together to close the electrical circuit.

• When the electrical contacts (switch) close, a large amount of current, typically in amps, can flow through the switch contacts to operate another device.

This is how a very low current on-board computer can control a very high current device like a cooling fan motor.

REMEMBER - Relays use small amounts of current to control large amounts of current.

Examples of commonly used relays on automobiles include:

• Cooling fan relay - The cooling fans typically draw about 20 amps on high speed.

• Air-conditioning clutch relay - The air conditioning clutch typically draws 4 amps when engaged.

• Fuel pump relay - A fuel pump can draw 8-12 amps while operating.