Some of the electrical appliances in the vehicle that applies relay include the fuel injection system
Horns, starter and fog lamps where a relay is used to switch the signal to the appliance. Relays switch a low current control circuit to cause an opening and closing of a high current power circuit. If there is a defective relay then it means that that certain component connected to the relay will not work accordingly. Majority of relays are confined to the engine compartment fuse/relay box and few specific relays are to be found in the under hood box mounted near the fender. In the case of a suspect relay, it can be replaced, which requires knowing a particular procedure or visiting a dealer service department or repair shop, and relay failures are expected to be a group of relays. The circuit that is managed by the relay can be scratched on the upper part of the relay while other numbers may be written on a decal or the imprint at the inner portion of the relay cover. The checks consist of verifying correct connections with the wiring diagrams; if the latter are unclear, then additional information can help understand the test connections. There are four fundamental categories of relays; including, and not limited to normally open and normally closed relays. Usually, the two terminals are assigned for the relay control circuit, which connects to the relay coil and other two terminals are used for power circuit that connected when the control circuit is activated. There may also be markings on the relays to facilitate identification or the control and power circuit terminal locations. To check a relay, ohmmeter should be connected across the power circuit terminal and no point continuity should be obtained. In a typical normally open four terminal relay, continuity should exist when the relay is on and on the other hand, it should not exist when the relay is off. To get polarity right, touch the fused jumper wire to one of the control circuit terminals then touch this terminal to the positive battery terminal; with another jumper touch the ground from the other terminal of the control circuit; There should be a click sound, and continuity should be present. If polarity is important, the wires may have to be cross connected and the correct position of the jumper wires could be as follows. If the relay does not pass the test it means it should be replace.