How to Check Auto Relays
Relays are used in a wide variety of applications in automobiles, from intermittent windshield wipers to cruise controllers, flashers, foglamps, stereo equipment and air-conditioning units. Their principle of operation is very simple, and they are fairly easy to test whether they are good or bad. A relay is an electrically operated switch used to isolate one electrical circuit from another. In its simplest form, a relay consists of a coil used as an electromagnet to open and close switch contacts. The switch contacts can be "normally open" (NO) or "normally closed" (NC), that is, when the coil is at rest and not energized (no current flowing through it), the switch contacts are given the designation of being NO or NC. In an open circuit, no current flows. In a closed circuit, metal switch contacts touch each other to complete a circuit, and current flows.
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Instructions
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Place the test leads from an ohmmeter across the coil terminals (shown as points A and B in the schematic diagram). Refer to your vehicle's repair manual or the relay manufacturer's schematic. Sometimes, the pin configuration will be drawn on the side of the relay itself. Check for continuity of the coil. The ohmmeter should show a very low resistance if the coil is good. Use this procedure to check the coil in any type of relay, regardless of the number of "throws" and "poles," or the voltage needed to trip the coil..
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Test the relay's switch contacts by first identifying the configuration of the switch contacts. Relay switch configurations are commonly "single pole singe throw" (SPST), "single pole double throw" (SPDT) and "double pole double throw" (DPDT).
The accompanying schematic diagram shows a relay with a SPST switch. Points A and B connect to the coil. Points C and D connect to the switch. When a voltage is applied across the coil at points A and B, an electromagnetic field is created which attracts a lever in the switch, causing it to make or break contact in the circuit at points C and D (depending if the design is NO or NC). The switch contacts remain in this state until the voltage to the coil is removed.
Test a single throw double pole relay's switch contacts with an ohmmeter. Place the test leads across points C and D. Measure the resistance, which will either be infinity or a dead short, depending on whether the relay configuration is normally open or normally closed. A reading of infinity indicates an "open" circuit, and a reading of zero ohms (short) indicates a "closed" circuit. -
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Apply the proper voltage to the coil (points A and B) to energize it. Apply the voltage either with a battery or power supply set to the proper voltage specified by the relay, such as 12 volts DC (direct current). With the test leads still attached to points C and D, once the coil is energized, the ohmmeter should give the opposite reading as it did when the coil was at rest and no voltage was applied. If the ohmmeter does not show a change between the on and off states of the coil, then the switch contacts in the relay are bad.
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Measure the contacts on a double pole single throw relay by placing one test from an ohmmeter on the center switch contact (point E on the schematic diagram), and the other lead on point C. Note the ohmmeter reading, either open or closed. Apply the proper voltage with either a battery or a DC power supply to energize the coil. Again, note the ohmmeter reading. It should be opposite the reading when the coil was at rest.
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Check the switch contacts between E and D. Use the same procedure, checking the open/closed status during the two states of the coil, energized and at rest. If any of these readings do not show a difference between the times when the coil is energized and at rest, then the switch contacts are defective. Note also that the at rest/energized coil state should give opposite readings for switch C E as it does for switch D E. For example, if the contact between C and E is normally open when the coil is at rest, then D and E should be normally closed, and vice versa.
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Attach an ohmmeter test lead to point D (as shown) and one on point C of a double pole double throw relay. Check for open and closed conditions with and without voltage is applied to the coil. Then move the one test lead from C to E and repeat the test. Note that this time the results should be opposite what they were for D and C. For example, if the contact between C and D is normally open when the coil is at rest, then D and E should be normally closed. When the coil is energized, the opposite readings should be present.
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Use the same procedure to test the other section of the switch, shown here as F, G and H.
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