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Automotive relays - Which pin does what



Automotive relays, of the type found on auto parts store shelves, are sometimes referred to as "ice cube" relays because of their shape. They come in 4-pin and 5-pin configurations, and all plug into the same socket. Four-pin relays are single pole, single throw. The terminals are 30, 87, 85, and 86. These terminal designations compare to a Lucas designations as: 85 and 86 equal Lucas W1 and W2, and are the relay coil terminals. 30 and 87 equal Lucas C1 and C2, and are the switch contact terminals.

All five-pin relays are NOT alike. The terminals on a five-pin relay are either 30, 87, 87, 85, & 86, or 30, 87, 87a, 85, and 86. The four outer pins on these relays are the same as the four outer pins on a five-pin relay, and any five-pin relay can be used as a four pin relay provided there is nothing wired to the center pin of the relay socket.

The difference between the two types of five-pin relays is in the center pin - either 87 or 87a. If this center pin is labeled 87, then it is a single pole, single throw relay with an extra output terminal. The center 87 pin acts exactly like the other 87 pin, and both 87 pins are connected to pin 30 when the relay is energized. Both 87 terminals are connected together inside the relay.

If the center pin is labeled 87a, then the relay is a single pole, double throw relay. In this relay, pin 87a is connected to pin 30 when the relay is OFF, and pin 87 is connected to pin 30 when the relay is ON.

Whenever you buy a relay, be sure to check the pin designation on the case of the relay before using it, as it is not uncommon to get the wrong relay when taking them from the shelf - they are often mislabeled on the shelf, but very rarely on the relay case.


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