Wednesday, October 19, 2011

Troubleshooting a Relay

This maintenance tip applies to relays in general including tilt switches. 
  • While troubleshooting vehicle or equipment electrical systems, you will eventually run across the need to test a relay. There are a couple types of relay, however they typically work similarly.
  • Within the housing is a coil and a switch (basically imagine an on/off switch). When the coil is energized, the switch changes from contact through the normally closed terminals to the normally open ones (to turn something off). It can also change from contact through the normally open terminal to the normally closed ones (to turn something on). An example would be to use a tilt switch to turn on an alarm when the vehicle angle is approaching a potential roll over situation.
  • To confirm a relay is working correctly, touch an ohm meter lead to the common terminal and the other lead on the normally closed terminal (depending on which relay you have will determine the terminal number). If the coil is not energized your meter should indicate continuity (or 0 ohms). Then move the second lead to the normally open terminal and your meter will indicate no continuity by showing infinite resistance (or OL).
  • Check continuity in the coil by attaching the leads across the coil terminals. Most relay coils have a resistance of approximately 90 ohms.
  • To test the relay contacts, energize (or power) the coil terminals by touching them to the positive negative posts on a 9V battery. When the relay clicks, the normally open lead should show continuity (0 ohms) and the normally closed lead should have no continuity (OL).
Source: Sullivan Training Solutions
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