Today I am testing the starter relay circuit using a test light to check for power and ground being present. A relay is basically an electro-magnetic switch. It works by energizing a coil with power and ground with an open (not connected) switch adjacent to the coil being energized. When current passes through the coil a magnetic field is created, the open switch is positioned close enough to the coil to be within it's magnetic field. While energized the magnetic field pulls the switch to the closed (connected) position to complete the circuit. Terminal 85 and 86 provide power and ground for the coil and terminal 30 supplies the power that is to be sent to the electrical load, which is grounded. The vehicle being tested is a 2002 Chevy Monte Carlo. To do this first I gain access to the starter relay also known as crank relay. It is located in the fuse and relay box under the hood on the right side of the vehicle. Next turn the key in the ignition to the key on engine off position (koeo), remove the crank relay, and attach your test light clip to battery ground in order to check the appropriate relay terminals for power. At terminal 86 and terminal 30 power should be present. With the test light connected to ground touch it's lead to these terminals and the bulb should illuminate which indicates that power is present at terminal 86 and 30. Next check for the ground being present at terminal 85 and 87. With the test light connected to battery power touch the lead to terminal 87 and the bulb should illuminate to indicate the starter motor is grounded. Then touch the lead to terminal 85 and turn the key to the run position koer (key on engine running) and the bulb should illuminate indicating that the computer is grounding the coil for the starter relay.