In the above describe relay.
For the not so technically inclined. Using some sort of continuity tester (light, sound, VOM), apply leads across 3 & 4. You should get a reaction from your test device. If not the contacts are bad. Leads across 6 & 7, no reaction from device. Now apply 12V DC across 1 & 2. Shouldn't really matter on polarity. If you hear a click then it's working. If you don't hear a click then reverse the leads. If you hear a click then it working. If you don't hear a click then the coil is shot. Once you hear the click, apply your test device across 3 & 4. The test device should do nothing. Apply the test device across 6 & 7. The test device should make an indication. If not the contacts are bad.
