Rick's Free Auto Repair Advice

Relay layout and operation

Relay layout and operation

Auto manufacturers try to use standard relays. They’re referred to as ISO (International Standards Organization) relays. To test these relays you need to understand what the numbers mean.

Here are the layout for the relay and the relay socket along with illustrations of what happens when the relay is energized.ISO relay, relay layout, terminal 30, terminal 87, terminal 87A

A relay consists of an electro-magnet and a switch. When the electro-magnet is energized, the magnetic field pulls the arm of the switch so it makes disconnects electrical flow from one set of contacts and moves it to another set.

In the case of an ISO relay, terminal 30 is usually connected to battery voltage. In the de-energized state, battery voltage come in on terminal 30 and flows out on terminal 897A (Figure 2).

The electro-magnet portion of the relay can be activated two ways: by switching power on and off, or by switching ground on and off. Most car makers prefer to switch the ground using either a body control module (BCM) or the powertrain control module (PCM). When the electro-magnet is energized, it pulls the switch arm over and power now flows from terminal 30 to terminal 87 (Figure 4).

If you’re testing the relay itself, refer to Figure 1 for the layout.

If you’re testing the relay socket, refer to Figure 3 for the layout.


© 2012 Rick Muscoplat


Posted on by Rick Muscoplat

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