Starter relays have appeared in various forms since the first electric starter motors were installed in automobiles. A relay's purpose is to interpose between the ignition switch and the powerful, current-hungry starting motor. This allows a minimal 12-volt signal from the ignition switch to activate a magnetic coil in the relay that pulls in a very sturdy contactor switch that is capable of supplying the full battery current to the motor. Wiring a relay is straightforward.
Mount the starter relay. There are two major types. The simpler one is used for inertial engagement starter motors, and it is usually mounted on the vehicle's wheel well. The second type also acts as a mechanical solenoid to engage the starter's pinion gear with the flywheel when the car is being started. This one is mounted directly on the starter motor itself. Both operate and are wired similarly.
Connect the starter motor lead to the switched output. The output terminal and the battery connection are usually much heavier than the other terminals on the relay, since they conduct so much more current between the battery and the starter motor. They might be 10 gauge or larger wires depending on the vehicle and starter size.
Connect the smaller terminal or terminal post marked SIGNAL, SWITCH or IGN to the ignition switch. This terminal goes to one side of the magnetic coil inside the relay. The other side of the coil is connected to the other smaller terminal which may be marked negative or ground. This should be connected next. Normally it is connected to the frame or to the battery's negative terminal. Use 16 gauge wire and quality insulated crimped terminals if performing this wiring for the first time. Sometimes the starter relay negative is routed through a neutral safety switch and then to ground, particularly on vehicles with automatic transmissions.
Connect the battery positive terminal to the other large terminal last, which may be marked BATTERY or BAT. Sometimes there may be an interposing fuse between the battery and the starter solenoid to protect against burning cables or starting a fire if the starter motor develops a direct short circuit.
Check the starter relay wiring one more time and then test by attempting to start the vehicle. If the vehicle won't start, check for a strong click from the relay when it is initiated. Check for 12 volts at the relay input to the coil with the meter's negative on the frame of the vehicle. Make sure the negative relay coil lead is connected to ground with the vehicle in neutral. Finally, check to make sure 12-volts goes to the starter lead when the ignition switch is turned to START.
Many starting difficulties in vehicles are caused by corrosion, loose cables, or broken cables where the outside insulation of the cable looks good, but the conductor inside is broken.
Never wear rings, watches, necklaces or other metal jewellery when working with electricity as they may short the battery and cause severe burns. Exercise extreme caution when working on starter solenoids and starters that are accessed underneath the vehicle.