How to Check if a Relay is Bad. A relay is a low-current switch used to control a high-current circuit. Relays enable the use of low-power switches and wiring to control high-current devices or circuits. A five-terminal relay works in the same fashion, except the source power is directed to a different circuit when the relay isn't energized. When the relay is energized, the normally closed circuit opens, while the open circuit closes.
Open the hood or under-dash fuse box and locate the relay in question. In most cases, there is a small description of each relay and fuse on the inside or outside of the fuse box cover. Grab the relay with two fingers and gently pull it out of the fuse box.
Examine the relay case and terminals. You should see a number next to each terminal. Terminals 85 and 86 are the control or coil circuit. Terminal 30 is the main power source and terminal 87 is the switched power that is controlled by the circuit. A 5th terminal will be labeled 87a.
Connect a 12-volt battery's positive terminal to terminal's 30 and 86, using wire and alligator clips. Clip the test light wire to the negative battery terminal. Touch the test light to terminal 87a. If the test light illuminates, the normally closed circuit is working.
Connect terminal 85 to the battery ground terminal with a piece of wire and alligator clips. You should hear the relay click as soon as you touch the ground terminal. This sound indicates that the arm has been pulled down to power terminal 87.
Connect your test light between terminal 87 and ground. If the test light illuminates, the relay is working correctly. Disconnect the positive wires from terminals 30 and 86. Remove the ground wire from terminal 85. Install the relay into its proper receptacle in the vehicle fuse box.