Relay

📖 YOUCANIC Automotive Glossary

A relay is an electromagnetic switch that allows a low-current control circuit (such as a signal from the ECU or a dashboard switch) to activate a high-current circuit that powers heavy electrical loads like the fuel pump, starter motor, cooling fan, headlights, horn, and A/C compressor clutch. Inside the relay, a small coil of wire creates a magnetic field when energized by the control circuit, which pulls a metal contact arm to close (or open) the high-current circuit. Most automotive relays are the standard ISO micro relay with four or five pins: pin 85 and 86 are the coil control circuit, pin 30 is the common (battery power input), pin 87 is the normally open output (closed when relay is energized), and pin 87a is the normally closed output (open when energized). Relays are typically located in the under-hood fuse box or an interior fuse panel and are often interchangeable with other relays of the same pin configuration in the same fuse box.

Relay failures cause complete loss of function for the circuit they control — for example, a failed fuel pump relay means the fuel pump will not run and the engine will crank but not start, while a failed cooling fan relay means the fan will never turn on, leading to overheating. Common symptoms include an audible click from the relay but the component not activating (stuck internal contacts), no click at all when the circuit is commanded (open coil), or a component that stays on permanently even with the key off (welded/fused contacts causing parasitic battery drain). DIYers can diagnose relay issues by swapping the suspected relay with an identical relay from another circuit in the same fuse box — if the problem follows the relay, it is confirmed bad. You can also test a relay with a multimeter by checking coil resistance across pins 85 and 86 (typically 50-120 ohms) and checking for continuity between pins 30 and 87 while applying 12V to the coil pins. Relays are inexpensive and should be replaced rather than repaired.