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Troubleshooting relays

This artical explans hwo to troubleshoot a generic relay is a SPDT, Single Pole Double Throw. SPDT relays have a single set of NO (Normally Open) and NC (Normally Closed) contacts. The NO set will close and the NC set will open upon energizing the coil.


The relay innards consist of an electromagnetic coil, that when current flows through it, will attract a steel arm that is connected to a copper/brass contact. When the contacts close, electricity will flow . Some relays include a resistor wired in parallel with the coil, so that the computer can remotely check to see if the relay coil is operating correctly.

relay coil

To test the coil, place your multi-meter in the resistance or Ohms mode, and connect the test leads to pins 85 and 86. A functional coil will measure approximately 70 to 100 Ohms.

A burnt out coil will either measure open or possibly 680-Ohms, if the relay is equipped with a resistor across the coil leads. The PCM can determine the status of a relay coil by measuring the resistance across it. If the PCM sees approximately 70 to 100 Ohms, the PCM assumes the relay is functional. If the resistance is 680-Ohms, the relay coil is burnt open, and if the resistance is infinite (open), there is a disconnected wire in the circuit, or the relay has been removed.

testing relay

To test the contacts, you will need a 12VDC power source. If you do not have one conveniently available, sometimes a fresh 9v battery will suffice. You will need to attach your meter to the set of contacts in question, and apply power to the coil to activate the electromagnet. Your meter should be set to the continuity mode, or beep mode. The meter will sound when the contacts being tested are closed. You can also check the resistance of the contacts to see if they are burnt, or are making a good connection.

The alligator clips pictured top left, and bottom right go to the DC power source, and the other two mini-grabber clips go to the meter.


relay powered