Relays have become very common items in production cars by offering a way to trigger high-amperage components like electric fans using a low-amperage trigger signal. The nomenclature illustrated on a typical Bosch relay can be confusing but the design is actually very simple. This popular style of relay consists of two circuits, the load side and the control side.
If you study the enclosed drawing, note that the high-amperage input feeds power to the Number 30 terminal. If all you know is that one, everything will fall into place. The terminal directly across from the 30 power input is the power output side – marked on the Bosch relays as Number 87. This is wired to the load such as an electric fan or fuel pump.
The second circuit is the trigger or switch side of the relay. This operates as indicated by the connections located perpendicular to the power side as shown in the illustration. Switched, low-amperage 12-volt power comes from the right side in this illustration connected to Number 86 and this switched circuit is completed with the ground side on the left side marked as Number 85. This side of the circuit can be controlled by a permanent ground with switched 12-volt power. Or, it will also work if constant power is applied to Number 86 and the ground side is switched.
Grounding the control side of the circuit is how most electric fans are triggered when controlled by an engine ECU. This way, the ECU is protected from high amperage draw by the relay. If you just remember that the load or power side runs across one side of the relay starting at the 30 terminal while the control or switched side operates perpendicular across the relay on the opposite side, it is really easy to remember how to wire the relay without having to search for a wiring diagram.
The fifth or 87a terminal is located in the middle of the relay and connects to the high side Number 30 terminal as a normally closed connection. This means that the relay circuit is normally live rather than dead as in the previous configuration with the switch off. When switched power is applied to the control side of the circuit this will disconnect the circuit.
With regard to amperage ratings, the most common Bosch relay is a 30 to 40-amp unit. You can find these at many automotive outlets but also at Summit Racing under PN BCH-0332209150.This is the most commonly used relay for most automotive components.