Wednesday, May 9, 2012

The 25+ - How do I check a relay?


Moving into some more general questions:

#22. How do I check a relay to make sure that it’s not bad?
Good relays very seldom go bad, although poor diagnosticians seem to often point to a relay as a last ditch answer for another electrical problem. Almost all relays “click” when they turn ON and again when they turn OFF. If you activate the switch that controls the relay and you can hear the relay “click”, then the odds are good that the relay is good. If there’s no “click”, the relay could be bad, or it just might not be receiving the electrical signal. It’s best to pull the relay out of the circuit and do a bench test.

Both Single-Poll/Single-Throw and Single-Poll/Double-Throw relays should be marked with terminals 85, 86, 87 and 30. Ground terminal 86 and connect terminal 85 to +12 vdc power. The relay should click ON. Disconnect terminal 85 and the relay should click OFF. If it does that but you’re still suspicious of it working right, connect terminals 30 and 87 to a continuity tester and repeat. When terminal 85 is energized, 30 and 87 should be connected (continuous). When terminal 85 is de-energized, 30 and 87 should be disconnected (open). Cycle the relay on and off a few times until you are sure that it is good or bad. This test will check for 99.44% of possible relay malfunctions.

Better yet, let’s be Mr. Practical and leave the bench test for later when we have more time. Instead, just put a new relay in the circuit (you do have spare relays, don’t you?). If that cures the problem, throw the old relay away and go on about your business. If the problem still exists after changing relays, you need to look beyond the relay for the source of the problem.


Steve Watson, Watson's StreetWorks,  http://www.watsons-streetworks.com

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