How to Test a Four Terminal Relay

The four terminal relay is vital to the performance of a vehicle.

They are everywhere: under the hood and around the vehicle; hiding behind seats, the steering wheel, around the engine compartment and junction block; even inside the trunk. Relays work as control switches, allowing or interrupting current flow to the fuel pump, radiator fan, intake throttle, air conditioner and many other circuits. They are very reliable, but the constant work and heat they are subjected to finally break their coil or make their contact points stick or burn, preventing current from going through the circuit. Since they are a common and important component of your vehicle operation, you should learn how to test a suspected relay, concentrating on one of the most common types: the four terminal electro-mechanical relay.

Difficulty:

Moderately Easy

Instructions

Things You'll Need

• Relay
• Multimeter or test light
• Two jumper wires, 1 to 2 feet long

1. Locate the particular relay you want to test.

2. Test the relay incoming voltage using a multimeter or test light, to make sure it is receiving power. Turn the ignition key on or start the engine if necessary to activate the particular circuit you are testing. If there is voltage, go to the next step.

3. Unplug the relay from its harness. Be careful, as some relays have locking pins that must be pulled off before removal.

4. Find the two power terminals and two control terminals on the relay. Usually, the two thicker wires that connect to the relay hook up to the power terminals, the other two go to the control circuit terminals. Many relays are marked on top for easy identification; otherwise, refer to your particular vehicle service manual.

5. Use your multimeter to check for continuity between the two power relay terminals. There should be no continuity. If there is continuity, replace the relay.

6. Test the control circuit terminals on the relay by connecting a fused jumper wire from one of the terminals to the positive terminal of the car battery and the other relay terminal to ground using a separate jumper wire. You should hear a click sound coming from inside of the relay. If you do not hear the click sound, reverse the jumper wires connections and listen for the click sound again. If you still do not hear the click sound, replace the relay.

7. Leave the jumper wires connected to the relay control circuit terminals, power and ground. Now, using your multimeter, check for continuity between the two power relay terminals. If there is no continuity, reverse the jumper wire connections again and test once more for continuity. If you still find no continuity between the two power terminals, replace the relay.

Tips & Warnings

• Always verify that fuses or circuit breakers are in good condition or properly set. Many circuit problems originate at this point.

• The service manual for your particular vehicle will tell you how to locate relays and identify their power and control circuit terminals, along with pin numbers and wire color codes.

• You can buy a vehicle service manual at any auto parts store or consult one for free at most public libraries.

• When replacing a relay, make sure to take it with you to the auto parts store to match it up.