Testing Automotive Relays Explained

This is a defective relay

Testing automotive relays before replacing them is a good idea and not to difficult. In most cases if you order or purchase a relay and it doesn't fix the car your stuck with it. Most auto parts stores have a policy that electrical parts are non-refundable.

The good news is that there are several ways to test automotive relays to verify that it’s bad before you replace it. The picture on the right shows a bad relay that I discovered during a no start diagnoses. This starter relay actually had a material failure in the copper contact blade.

It cracked causing an open condition resulting in no current flow to the starter motor solenoid. This was the first time I have ever seen a failure like this. Sidebar: This relay was made in China where as for a long time most where made in Germany. We might see more failures like this in the future as car-makers cut costs to stay competitive?

To me this is an unusual failure because in most failed relay cases I've seen, when the cover is removed from a malfunctioning relay you find burned contacts or melted windings. The contacts can also become corroded even green in color to the point it will not flow current or complete the circuit as intended.

So the first way of testing automotive relays is by actually removing the cover and performing a visual inspection. Note that some companies actually glue the cover on to prevent tampering and seal out moisture.

In some cases you can take a utility knife and carefully cut the adhesive seal to remove the cover. I should also mention that in some cases the cover is really hard to get off and might actually be impossible. Luckily there are two more good ways of testing automotive relays.

Swapping Out  the Relay

In many cases you will need an automotive wiring diagram or at least a professional grade car repair manual that will indicate exactly where the relay is located. When you find the one you suspect as defective you might see other ones just like it in the relay center.

Take a close look at the part number, example 17008-4915. You can locate another one with the same exact part number and swap locations. This way you can retest your malfunctioning circuit and verify that it is now working with the swapped part in place. You can also verify that the circuit that you swapped from is now not working.

If this is the case you have definitely nailed down a bad automotive relay. Car makers are always trying to hold down the cost of manufacturing and often use the same part number relays for many different circuits in the vehicle. This allows them to mass produce these electrical parts and just plug them in during assembly.

Use this to your advantage and swap them out to test for bad like components. The way that an automotive relay should be tested is to actually break out an automotive test meter and confirm proper operation of the relay. When testing relays it is important to remember that they are not as complicated as they may seem and neither is the test procedure.

Most relays have four or five terminals. The job of this electrical component is to physically connect two of those terminals on command to allow electricity to flow to the load in the circuit. The load would be something like an ac compressor, starter motor, fuel pump and so on. Again you will probably need an automotive wiring diagram to determine which terminals connect to which wires.

Basically two terminals are used to energize the magnetic coil that pulls a contact closed, allowing the electrical current to flow. When properly testing automotive relays you will need a 12 V battery to energize the coil. Take 12 V and apply it to one side of the coil and a good ground to the other side. I have a car battery for bench testing but you can disconnect the one in the car and use it for this test.

meter testing automotive relay

With the coil energized the switch should be opened or closed and you can verify the flow using a meter set on the continuity check setting. In this mode your meter will beep when the circuit is closed and working properly.

If this test fails and the switch does not open and close by energizing the magnetic coil there is one more test that can be performed with an automotive meter. Physically check the resistance of the magnetic coil by selecting the resistance mode or Ohms on your meter.

One lead gets touched to one terminal of the coil and the other lead as well. A magnetic coil in an automotive relay should have just about no resistance and your meter should read close to zero ohms. Testing automotive relays takes a little practice but is well worth the effort.

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These types of repairs can be very expensive at your auto repair center because they require a good amount of skill level. This is also an automotive repair that is clean and exercises the brain. This next link will take you to some additional information on the youfixcars.com site so you can learn about automotive electrical systems.

If you would like to see what else is covered on the website you can visit this next link that will take you to the Homepage for YouFixCars.com.

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