Checking Relays

By using a relay, a heavy current can be turned on and off by a switch using much less current. For example, in the circuit shown here, when the switch is turned on (closed), current flows to the coil of the relay. Then, its contact is turned on (closed) and the light comes on. The current flowing through the switch is much less than that for the light.

Fig 1: Power Supply Circuit Diagram
Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.
When current flows through the coil of a relay, its core is magnetized to attract the iron piece, closing (ON) the contact at the tip of the iron piece. When the coil current is turned off, the iron piece returns to its original position by a spring, opening the contact (OFF).

Fig 2: Normally Open (NO) Type Relays
Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.
Relays may be classified as the normally open-type or the normally closed-type, depending on their contact construction.
NOTE: The de-energized state means that no current is flowing through the coil. The energized state means that current is flowing through the coil.
1. The normally open-type
  When a normally open relay as illustrated here is checked, there should be no continuity between terminals 3 and 4 when the relay is de-energized. There should be continuity between terminals 3 and 4 when battery voltage and ground are applied to terminals 1 and 2. The relay condition is determined by this check.
  
  Fig 3: Normally Open (NO) Type Energized And De-energized State
  Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.
  
  NOTE: Check the relay in both situation which is energized and is not energized.
2. The normally closed-type
  When a normally closed relay as illustrated here is checked, there should be continuity between terminals 3 and 4 when the relay is de-energized. There should be no continuity between terminals 3 and 4 when battery voltage and ground are applied to terminals 1 and 2. The relay condition is determined by this check.
  
  NOTE: Check the relay in both situation which is energized and is not energized.
  
  Fig 4: Checking Relays - Relay In Both Situation
  Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.