What a voltmeter tells you about a relay

A relay is an electromagnetic switch that uses a small electrical signal to control a larger one. When you test it with a voltmeter, you are checking whether voltage is present at specific points — the coil terminals and the switch contacts — to determine if the relay is responding to commands and passing current through as it should.

A voltmeter shows you whether electricity is reaching the relay's coil (the electromagnet that triggers the switch) and whether the contacts are actually closing and opening. This is different from testing continuity with a multimeter's ohms setting; a voltmeter test tells you the relay is working under real operating conditions, with actual current flowing through the circuit.

The test takes about five minutes and requires only a voltmeter set to DC volts. You do not need to remove the relay from the socket — you can probe the terminals while it is installed.

Key Takeaways

  • Set your voltmeter to DC volts (usually 20V or 50V range) and connect the black probe to a known ground point on the engine or chassis.
  • Probe the relay's coil terminals first to confirm voltage is reaching them when the relay should be active.
  • Probe the switch contact terminals (the output side) to verify voltage appears there only when the relay is energized.
  • If voltage reaches the coil but the output contacts show no voltage, the relay contacts are stuck open and the relay has failed.
  • If no voltage reaches the coil at all, the problem is upstream — a blown fuse, bad switch, or broken wire, not the relay itself.

Locate the relay and identify its terminals

Most relays are cylindrical or rectangular modules that plug into a socket on the fuse panel, engine bay, or under the dash. The socket itself is usually labeled with the relay's function — fuel pump, cooling fan, starter, horn — or you can find the relay location in your vehicle's service manual or fuse panel diagram.

Once you have located the relay, look at the socket or the relay body itself. Relays typically have four or five terminals, and they are usually numbered. The coil terminals (which receive the control signal) are often labeled 85 and 86. The switch contact terminals (which carry the main current) are often labeled 30 and 87. Some relays also have an 87a terminal, which is the normally-closed contact — it carries current when the relay is not energized.

If the numbers are not visible, consult the relay schematic in your manual or search online for the specific relay part number. Knowing which terminals are which is essential; testing the wrong pair will give you false results.

Set up your voltmeter and establish ground

Turn your voltmeter dial to DC volts. Most voltmeters have a 20V or 50V range for DC; either will work for automotive relays, which typically operate on 12V systems. If your meter has an auto-range setting, you can use that instead.

Connect the black (negative) probe to a solid ground point on the engine or chassis — a bare metal bolt, the negative battery terminal, or a known ground wire. This gives your voltmeter a reference point. Do not hold the probe in your hand or let it float; it must make firm contact with metal.

Leave the black probe connected to ground for the entire test. You will move only the red (positive) probe from terminal to terminal.

Test voltage at the relay coil terminals

Have someone turn on the system that the relay controls — the headlights, fuel pump, cooling fan, or whatever circuit you are testing. The relay should receive a signal and energize.

While the system is on, touch the red probe to terminal 85 on the relay or socket. You should see voltage on the meter — usually 12V or close to it. If you see 0V, the control signal is not reaching the relay. This means the problem is not the relay itself; it is a blown fuse, a bad switch, a broken wire, or a corroded connector upstream.

Now touch the red probe to terminal 86. You should again see voltage (or close to 12V). If one terminal shows voltage and the other shows 0V, the coil circuit is broken at that point — likely a bad connection or wire.

If both terminals 85 and 86 show voltage when the system is on, the coil is receiving power correctly. Move to the next step.

Test voltage at the relay switch contacts

While the system is still on and the relay is energized, touch the red probe to terminal 30 on the relay. This is the input to the switch contacts. You should see 12V (or whatever voltage your system runs on).

Now touch the red probe to terminal 87, the main output contact. When the relay is energized and working, you should see the same voltage here — usually 12V. If terminal 87 shows 0V while the relay is on, the contacts inside the relay are stuck open and the relay has failed.

If terminal 87 shows voltage, the relay is passing current correctly. Turn off the system and move to the final check.

Verify the relay de-energizes when the system is off

Turn off the system that powers the relay. The relay should click or release, and the control signal should stop.

Touch the red probe to terminal 87 again. Now you should see 0V. If voltage is still present at terminal 87 after the system is off, the relay contacts are stuck closed — another sign of relay failure.

If voltage disappears from terminal 87 when you turn the system off, and reappears when you turn it back on, the relay is functioning correctly. The problem you are troubleshooting is elsewhere in the circuit.

Interpret your results

If the coil received voltage (terminals 85 and 86 both showed 12V when on) and the output contacts switched voltage correctly (terminal 87 showed 12V when on, 0V when off), the relay is working. Look for problems in the wiring, connectors, fuses, or switches that control the relay, or in the load circuit that the relay powers.

If the coil received voltage but the output contacts did not switch (terminal 87 showed 0V even when the relay was on), the relay contacts have failed internally. The relay must be replaced.

If the coil received no voltage at all (terminals 85 and 86 both showed 0V even when the system was on), the relay is not the problem. Check the fuse for that circuit, the switch that controls it, and the wiring between them.

Frequently Asked Questions

What if my voltmeter shows 12V at terminal 87 even when the relay is off?

The relay contacts are stuck closed. This usually means the relay has failed and needs replacement. In rare cases, a short circuit in the wiring downstream of the relay can also cause this; check for damaged insulation or a shorted component in the load circuit before replacing the relay.

Can I test a relay without removing it from the socket?

Yes. You can probe the socket terminals directly while the relay is plugged in. This is actually safer because the relay stays in its normal operating position and you do not risk damaging pins or connectors by removing it.

Why does my voltmeter show 6V instead of 12V at the coil terminals?

Low voltage usually means a weak battery, a corroded connection, or resistance in the control wire. Check that your battery is fully charged and that all ground and power connections are clean and tight. If voltage is still low, there may be a break or corrosion in the control circuit.

What if the relay clicks but the voltmeter shows no output voltage?

The relay coil is energizing (you hear the click), but the contacts are not closing properly. This is a failing relay — the electromagnet works, but the mechanical contacts are corroded or stuck. Replace the relay.

Do I need to disconnect the battery before testing a relay?

No. Testing with a voltmeter while the system is running is actually the correct method because it shows you how the relay behaves under real operating conditions. Never disconnect the battery just to test a relay with a voltmeter.