Several electrical accessories in the vehicle, such as the fuel injection system, horns, starter, and fog lamps, use relays to transmit electrical signals to the components. Relays operate a low-current control circuit to open and close a high-current power circuit, and if a relay is defective, the associated component will not function properly. Various relays are mounted in the engine compartment and throughout the vehicle, and if a faulty relay is suspected, it can be removed and tested or taken to a dealer service department or repair shop, with defective relays needing replacement as a unit. To test a relay, it's helpful to consult the wiring diagram for proper hook-ups; however, if that's not possible, the following information can assist. Most relays have two terminals for the control circuit, which connect to the relay coil that, when energized, closes the large contacts to complete the circuit, while the other terminals are for the power circuit, which connect internally when the control circuit coil is energized. Relays are typically marked to help identify the control and power circuit terminals. To test, remove the relay and check for continuity between the power circuit terminals, ensuring there is no continuity between terminal 30 and 87. Next, connect a fused jumper wire from one control circuit terminal to the positive battery terminal and another from the other control circuit terminal to ground; the relay should click. If it doesn't, polarity may be critical, so swapping the jumper wires may be necessary. With the jumper wires connected, check for continuity between the power circuit terminals, which should now show continuity between terminals 30 and 87. If the relay fails any of these tests, it should be replaced.
Posted by FordPartsGiant Specialist