A Bosch relay contains a copper wire coil and a switch. When the coil accepts current from one electrical source it forms an electromagnet. The electromagnet pulls on the internal switch. When the switch closes it completes the secondary electrical circuit and the device the relay operates turns on. A Bosch relay connects to a 12-volt DC electrical circuit. Many cars and trucks use Bosch relays in the audio and alarm systems. A Bosch relay uses the numbers 30, 85, 86, 87 and 87A for terminal identification.
Switch a multimeter to its resistance setting. Choose the highest resistance setting if the multimeter has more than one setting.
Touch the ends of the multimeter's probes together and observe its readout screen. An analogue multimeter's needle moves across the dial to infinity when the probes touch. A digital multimeter beeps and the screen reads infinity when the probes touch. An infinity readout means the probes have continuity between them.
Place one multimeter probe on the Bosch relay's 85 terminal and place the other probe on the 86 terminal. If the multimeter reads infinity, then the Bosch relay has continuity through its coil. If not, then replace the relay due to an electrical short in the coil.
Hold one multimeter probe against the Bosch relay's 30 terminal and set the second probe against the 87A terminal. If the multimeter reads infinity, then the Bosch relay's normally closed terminal, 87A, has continuity to the 30 terminal. If the multimeter does not read infinity, then replace the relay because the switch has failed to return to its normally closed position.
Apply power to the Bosch relay's 85 and 86 terminals with a 12-volt battery. This powers the coil. Usually a Bosch relay makes a clicking noise as the internal switch moves from the normally closed terminal to the normally open terminal -- the internal switch moves from the 87A terminal to the 87 terminal.
Hold one multimeter probe on the Bosch relay's 30 terminal and the other probe on the 87 terminal. If the multimeter reads infinity, then the relay works. If not, then discard the relay.