Relays & Switches

What it is

A relay is a switch worked by a small coil. A low-current control circuit energizes the coil, and the relay contacts carry the heavy load current to lamps, horns, fans or motors. A switch is worked by hand or by a mechanism. Together they let a dash switch control a load without running heavy wire to the cab.

How to identify

Standard automotive relays are cube-shaped ("mini" or ISO size) or smaller ("micro"). The terminal numbers are molded next to the blades and usually shown in a diagram on the case. A 4-pin relay has one set of contacts. A 5-pin relay adds a normally closed contact.

Standard relay terminals

Switch terms

• SPST (single pole, single throw): simple on and off.
• SPDT (single pole, double throw): one input switched between two outputs.
• DPDT (double pole, double throw): two separate circuits switched together, used for motor reversing.
• Momentary switches return when released, like a horn button. Maintained switches stay where set.
• Rocker, toggle, push-pull and rotary describe the handle, not the circuit.

Key points

• Some relays have a diode or resistor across the coil to stop voltage spikes. With a diode relay, polarity on 85 and 86 matters. Check the case diagram.
• Contact ratings are printed on the case or in the maker's data. Use a relay rated for the load and for inductive loads such as motors.
• A relay that clicks but does not power the load may have burned contacts. Test voltage at 87 with the coil energized.
• Same footprint does not mean same internal circuit. Match the case diagram, not just the shape.
• Sealed relays are better in exposed locations.

Common mistakes

• Swapping a diode relay into a circuit with reversed coil polarity.
• Using a 4-pin relay where the circuit needs 87a.
• Running load current through a dash switch that was meant to drive a relay coil.
• Assuming every relay with the same pin layout has the same contact rating.

Governing standards: DIN 72552 (terminal numbering); ISO 7588-1 (plug-in relays); vehicle and relay maker's data.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.