Relay Logic

📖 YOUCANIC Automotive Glossary

Relay logic describes how automotive relays function as electrically controlled switches, allowing low-current control circuits (switches, ECU outputs) to control high-current power circuits (motors, pumps, heaters) without routing heavy current through delicate switches or control module pins. A standard ISO micro relay has a control side (coil, pins 85/86) and a power side (contacts, pins 30/87/87a). When the control circuit energizes the coil (typically 150-300 mA), the magnetic field pulls the internal contact arm, connecting pin 30 (battery power input) to pin 87 (normally open output). De-energizing the coil lets the spring return the arm, disconnecting pin 87 and reconnecting pin 87a (normally closed output) if present.

Understanding relay logic is essential for diagnosing circuits controlled by relays — fuel pump, cooling fans, A/C compressor clutch, horn, headlights, fog lights, starter, and many others. When a relay-controlled component fails, the relay itself may be the problem (stuck contacts, open coil), or the fault may be in the control circuit (no signal to the coil from the ECU or switch) or the power circuit (no power at pin 30, or load-side fault). DIYers can test relays by checking coil resistance (50-120 ohms between pins 85/86), verifying contact continuity when manually energized, or simply swapping with an identical relay from another circuit in the fuse box.