For simple relays, just find out the terminals that go to the relay coil and hit it with 12V and see if you hear it click. That'll tell you that the coil is pulling in the contacts. Then check the contacts for continuity with a meter. For more complex relays it may be a bit harder to set up a test fixture, see how the relay is wired in Bentley and then try to duplicate the circuit to test it. For example I made a test fixture for the diesel glow plug relay, using a variable resistor to simulate the temp sender, a few switches to simulate the ingintion switch and a few light bulbs to simulate the glow plug load and the dash LED. It worked well to test and identify problems with the various relays I tested.
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