How To Test Relays

(How to Repair Relays)

What is a Relay

4 pin relay

A basic automotive relay is a switch operated by an electromagnet, this is controlled by either a small switch (headlight switch etc.) or a control module/ECU (fuel pump etc.).
This takes the load off the switch etc. and only small wires are required behind the dashboard. If you look at the diagram you will see that the relay connects pin 30 and 87 together.
So for a quick test, just bridge them together with a fused wire see picture. If you want to test further connect a meter between 85 and 86 you should see battery voltage when you operate that circuit (note fuel pump relays only work for a few seconds to prime or when cranking).

Testing Vehicle Relays

So for a quick test, just bridge them together with a fused wire see picture. If you want to test further connect a meter between 85 and 86 you should see battery voltage when you operate that circuit (note fuel pump relays only work for a few seconds to prime or when cranking).

relay testing

Relay Pin Terminal Numbering

car relay

Many automotive relays now have continental markings and diagram on side to help with pin identification.

1 = 15 or 86
2 = 85
3 = 30
4 = 87A
5 = 87

To bridge this relay you would connect 3 and 5

Click Here for Relay Terminal List

More Relay Types

5 pin  relay change over

These are 5 pin relays the left one is change over and the right one is a dual output.

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