There are a few ways to test a relay using a multimeter to determine if it is functioning properly or if it is bad and needs to be replaced. Some key things to check include the coil resistance, contact resistance, and performing an actuation test by applying power to the coil.
Check the Coil Resistance
The first thing to check with a multimeter is the resistance across the relay coil terminals. Use the ohms setting on the multimeter and connect the probes across the coil terminals. Consult the relay specifications or data sheet to find the specified coil resistance. A typical relay may have a coil resistance somewhere between 50-1000 ohms.
If the measured resistance drastically differs from the specified value, such as being very high or near 0 ohms, that indicates there is likely a problem with the relay coil like an open coil or shorted wires. The coil resistance should measure very close to the specification.
Check the Contact Resistance
The other important resistance check to perform is to measure the contact resistance. This is done by setting the multimeter to the ohms setting and connecting the test probes across the common and normally open contacts.
Again, consult the specifications for the expected contact resistance. This may be specified anywhere from 50 milliohms up to 1 ohm for high current relays. Measure the contact resistance a few times by disconnecting and reconnecting the test probes to see a range of values.
If the contact resistance is drastically higher than the specifications, that indicates oxidized or burnt contacts and the relay likely needs to be replaced. Properly working contacts will have very low resistance, often in the fraction of an ohm range. Abnormally high resistance indicates faulty contacts.
Perform an Actuation Test
Another key test is to actually actuate the relay by applying power to the coil and testing for proper switching operation. This confirms the coil is energizing and the contacts are properly opening and closing.
To perform this test, connect the relay coil terminals to a power source like a battery or DC power supply that matches the coil voltage rating. Then connect the multimeter in ohms mode across the switch contacts and monitor the resistance.
With no power applied the resistance should indicate an open circuit. When coil power is applied, the contacts should close and the multimeter should show continuity between the contacts indicating near 0 ohms. The contact resistance may bounce around a bit as the relay actuates but you should see the basic transition between open and closed circuit.
If the relay does not switch or exhibits abnormal behavior like sticky contacts, that indicates a bad relay that should be replaced.
Key Indications of a Bad Relay
Here are some key things to watch out for when testing a relay that indicate it is bad or faulty:
- Coil resistance drastically different from specifications
- Open coil circuit with no continuity
- Shorted coil indicating near 0 ohms resistance
- High contact resistance much higher than specifications
- Erratic or intermittent contact operation
- Contacts do not open or close properly when actuated
- Coil energizes but does not pull in contacts
Tips for Testing Relays
Here are some useful tips when using a multimeter for testing relay operation:
- Refer to the manufacturer specifications for resistance values
- Measure coil resistance first before applying power
- Check contact resistance in both open and closed states
- Use a current limited power source to energize the coil
- Listen for an audible click when actuating
- Repeat the actuation test multiple times
- Compare measurements between multiple relays of the same model
Checking the coil and contact resistance along with performing an actuation test are the key steps in determining if a relay is functioning properly or is faulty using a multimeter. Drastic variance in expected resistance values or issues with contact switching indicate a bad relay that requires replacement. Following the proper relay testing procedures with a multimeter allows quick diagnosis of relay problems.
|Open or shorted coil
|Very low, fraction of an ohm
|Oxidized or burnt contacts
|Clicks and switches properly
|Coil or switch failure
Testing relays is an important diagnostic skill for automotive technicians, HVAC repairmen, electricians, and anyone who regularly works with electrical equipment that utilizes relays for control and switching. Taking the time to proficiently test relays using a multimeter can greatly assist troubleshooting electrical problems and prevent unnecessary replacement of properly functioning relays.
Developing sound relay testing methodology with a digital multimeter enables quick determination of good versus faulty units. Use the outlined coil resistance, contact resistance, actuation and inspection steps to confidently evaluate relay condition.
With some basic tools and testing knowledge, diagnosing faulty relays is a straightforward process. Catching problematic relays early on helps avoid major issues down the road and unnecessary costs from replacements. Take the time to periodically inspect and test relays using these techniques to prolong electrical system reliability.
In summary, here are the key steps for checking a relay with a multimeter:
- Check coil resistance – should match relay specifications
- Check contact resistance – should be very low when closed
- Actuate the relay and check for audible click
- Actuate the relay and check for proper switching operation
- Inspect relay for signs of overheating or damage
- Repeat tests a few times for consistency
Following a thorough relay testing procedure with a multimeter provides clear evidence of any issues and is the most efficient way to evaluate relay condition. Use relay specifications as a reference and understand what resistance measurements indicate about coil and contact integrity. Proper relay testing saves time and money by isolating failures and preventing unnecessary replacements.
Frequently Asked Questions
What resistance reading indicates a bad relay coil?
A coil resistance value that is dramatically different from the relay’s specifications often indicates a problem with the coil wiring. An open coil will show a very high resistance, while a shorted coil will read near 0 ohms.
What should you do if the relay clicks but does not switch properly?
If the relay actuates and clicks but does not properly open or close the contacts, that indicates a problem with the switching mechanism or contacts. The relay will need to be replaced.
What causes a relay’s switch contacts to become oxidized and increase resistance?
Oxidation on the contacts is typically caused by arcing or excessive heat at the switch contact point. This can happen over time from normal switching or from operating the relay at higher than rated loads.
Can you use an ohmmeter to test relays without applying power?
Yes, you can perform resistance checks on both the coil and contacts with just an ohmmeter without powering up the relay. This allows you to easily test relays outside of the circuit.
How often should relays be tested to identify potential failures?
For relays in frequent use or in critical applications, it’s wise to test them at least annually. Relays that cycle on and off thousands of times daily should be tested monthly or quarterly.
Regular testing at suitable intervals helps detect bad relays before they lead to equipment failure. Always test a suspect relay right away if abnormal system operation is noticed.
Can a partially shorted relay coil still allow the relay to work intermittently?
Yes, a relay with a partially shorted coil can exhibit intermittent operation. The coil may energize enough to actuate the relay occasionally but not reliably. A partially shorted coil will measure lower resistance than normal.
What tools do you need to properly test an automotive or DC relay?
The only tools required are a digital multimeter capable of measuring ohms for the resistance tests and a 12V source such as a car battery to actuate the relay during testing.
Should you hear an audible click when a good relay is energized?
In most standard relays, you should hear a distinct audible click or clunk noise when the coil is energized and the relay actuates. No click indicates the relay is not properly switching.
Can you use a multimeter to test a solid-state relay?
Solid-state relays require slightly different testing procedures than electromechanical relays. It’s difficult to isolate failures with a standard multimeter. Most solid-state relays require device-specific tools for testing and diagnostics.
How can you determine the coil resistance specification for an unlabeled relay?
If the relay has no markings or you don’t have the specification sheet, the coil resistance can sometimes be roughly estimated based on the relay voltage and size. However, the safest option is to replace with a new properly labeled relay.
Properly labeling and documenting relays is important to allow for easy future testing and replacement.
Summary of Key Relay Testing Procedures
Here is a summary checklist of the key steps to test a relay using a multimeter:
- Identify the relay to be tested and gather specifications.
- Set multimeter to ohms setting.
- Test coil resistance – check against spec.
- Test closed contact resistance – should be very low.
- Energize relay coil and check for audible actuation click.
- Energize relay coil and test contact switching operation.
- Repeat energizing test multiple times.
- Inspect for signs of overheating or arcing.
- Compare measurements against identical model relays if available.
Following this full test procedure will confirm relay condition and identify any out-of-spec or intermittent operation. Properly testing relays before installation prevents premature failures and ensures optimal electrical system performance.