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How to Test a Relay Switch?

Medha Godbole
A relay switch is of paramount importance to the functioning of the car. Read the following story that provides useful information regarding testing the same, in case you encounter any problem with it.
Where there is an electrical device or a vehicle, there is one thing that will be present by default. It is similar to a miniature circuit breaker and does almost the same work. These are called relay switches. Just like spark plugs and ignition coils, these are an important part of vehicles in general.
These are important to regulate the electrical overloads in the car's electrical system and open, when triggered by an electronic signal. Now, if these are not working properly, you can imagine what the car or any electronic device will go through. The need to test their functioning is justified.


The fuel pump relay is typically a switch that is operated electrically. These need an electrical signal to be activated as mentioned previously. What this component does after it gets activated is that it connects an electrical supply to a particular accessory.
In the case of a car, it is the fuel pump. There are two parts of this device: the first part utilizes an electromagnet and then there is the secondary electrical circuit inside.
Now, the primary half of the relay closes its secondary half. It is powered in a manner akin to a light bulb circuit. Typically, when testing this switch, the relay contacts, operation, and activation circuits are tested.

Testing Procedure

For this, what you need is high-impedance test light, an ohmmeter, and two jumper wires. Considering that you have got hold of the relay switch which is not working properly, here is the actual process.
  • If required, turn the ignition on to energize the specific circuit you intend to test. What you have to do next is get the connection of the alligator clip from the testing light with a good ground on your vehicle.
  • Now, probe the wire, which will be jutting out of the relay switch, which is reaching the component. Do this with the help of the tip of test light. One indication that this device is working properly is that the bulb of the test light will glow. This also implies that there is proper voltage.
  • The wires feeding voltage to the relay have to be checked next in the same way as mentioned earlier, with the help of a test light. There is voltage if the light glows and if this does not happen, consider zero voltage there. In this case, the voltage source may have to be checked.
Power and control terminals of this switch have to be identified next. After this, check if the two power terminals are connected with the help of an ohmmeter. If there is a continuity between the two, it is an indication that the device needs to be replaced.
  • In the next step, you have to connect a jumper wire, which should be between the positive terminal of the battery and one of the control circuit terminals on the relay. Using another jumper wire, connect the other control terminal to the ground.
Ideally, you should have a 'click' on making the second connection. If this does not happen, try interchanging the connections. If even after reversing the connections you don't get that click, replace the device.
Now, you are almost done here. As the finishing stroke, connect the jumper wires akin to the connections earlier. Use an ohmmeter to check for continuity between the two terminals. If there is continuity, consider the relay to be in good shape. If it does not happen, then replace it.
Hopefully, you would have preserved the vehicle manual, in case you need to see color codes for identifying power and control circuit wires on any relay and knowing how to wire the same in the first place. If you feel this is too much of a hassle, the best way is to consult a mechanic and get things done.