Before you test a relay, locate it and determine its function. Also, determine if it is computer-intergrated, this will determine the type of testing you do.
A relay can be checked with a jumper wire, voltmeter, ohmmeter, or test light. if the terminals are accessible and the relay is not controlled by a computer, a jumper wire and test light will be the quickest method.
Check the wiring diagram for the relay being tested to determine if the control is through an insulated or ground switch. If the relay is controlled on the ground side, follow this procedure to test the relay:
- Use the test light to check for voltage at the battery side of the relay. If voltage is not present, the fault is in the battery feed to the relay. If there is voltage, continue testing.
- Probe for voltage at the control terminal. If voltage is not present, the relay coil is faulty. If voltage is present, continue testing.
- Use a jumper wire to connect the control terminal to a good ground. If the relay works, the fault is in the control circuit. If the relay does not work, continue testing.
- Connect the jumper wire from the battery to the output terminal of the relay. If the device operated by the relay works, the relay is bad. If the device does not work, the circuit between the relay and the device's ground is faulty.
If the relay is controlled by a computer, do not use a test light. Rather use a high impedance voltmeter set to the 20-volt DC scale, then:
- Connect the negative lead of the voltmeter to a good ground.
- Connect the positive lead to the output wire. If no voltage is present, continue testing. If there is voltage, disconnect the ground circuit of the relay. The voltmeter should now read 0 volts. If it does, the relay is good. If voltage is still present, the relay is faulty and should be replaced.
- Connect the positive voltmeter lead to the power input terminal. Close to battery voltage should be there. If not, the relay is faulty. If the correct voltage is present, continue testing.
- Connect the positive meter lead to the control terminal. Close to battery voltage should be there. If not, check the circuit from the battery to the relay. If the correct voltage is there, continue testing.
- Connect the positive meter lead to the relay ground terminal. If more than 1 volt is present, the circuit has a poor ground.
If the relay terminals are not accessible, remove the relay from its mounting and test it on a bench. Use an ohmmeter to test for continuity between the relay coil terminals. If the meter indicates an infinite reading, replace the coil. If there is continuity, use a pair of jumper wires to energize the coil. Check for continuity through the relay contacts. If there is an infinite reading, the relay is faulty. If there is continuity, the relay is good and the circuits need to be tested carefully.
Be sure to check your service manual for resistance specifications and compare the relay to them. It is easy to check for an open coil; however, a shorted coil will also prevent the relay from working. Low resistance across a coil would indicate that it is shorted. If the resistance is too low, the transistors and/or driver circuits could be damaged due to the excessive current flow that would result.