Relays - General Information and Operational Check

Relays are used to supply power or control signals to some electrical loads in the vehicle (such as fuel injection components, horn, starter, cooling fan(s), fog lamps, etc.). A relay is essentially an electrical switch that closes the high-current circuit in response to a low-current signal. If a relay fails, the corresponding load will not operate. Most relays are located in the underhood fuse/relay box; others are located throughout the vehicle. The operational check procedure for relays is described below. Defective relays must be replaced.
INSPECTION
PROCEDURE

1. If the connection of a relay to its electrical circuit cannot be determined from the wiring diagrams, note that the approach to checking any relay is basically the same in all cases.
2. On most relays, the control circuit is always connected to two of the terminals. When low-current voltage is applied to these terminals, current flows through the relay control coil, which closes the high-current contacts of the load circuit. The remaining terminals are the high-current circuit terminals.
3. To help identify the relay terminals, a diagram showing the terminal connections is usually printed on the relay case.

4. Connect a fused jumper wire between one of the relay control terminals and the battery positive terminal. Using a second jumper wire, ground the other control terminal; the relay should click. Some relays require correct polarity; if no click is heard, try reversing the polarity of the control terminal connections.
5. With the jumper wires connected, check the continuity between the high-current circuit terminals using an ohmmeter, referring to the terminal diagram on the relay case.
6. If the test result is negative, replace the relay.