How to Test a Faulty Relay


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Sep 29, 2023

How to Test a Faulty Relay

Published: April 27, 2017 Machinery & Shop, Machinery and Shop "(Required)"

Published: April 27, 2017

Machinery & Shop, Machinery and Shop

"(Required)" indicates required fields

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Many electrical components in a vehicle or machine are controlled by a relay. So if a component isn't working because electricity isn't getting to it, there is a possibility the relay may be faulty. But determining whether or not a relay is defective requires a little basic investigation. Here's how to go about it.

First, though, it's important to know how relays work.

Relays have an internal electromagnet that opens and closes a circuit to stop and start the flow of electricity. They help extend the life of switches by making it unnecessary to route the high amperage flow of electricity needed by a system directly through a switch. That can burn its contact points over time. Instead, a switch activated by a driver or machine operator simply sends a low-amperage current through a relay, which activates the electromagnet inside it. The magnet then closes a second, independent electrical circuit, allowing high amperage to flow through it directly to a component. Relays also allow for simpler routing of electrical circuits by making it unnecessary to route high amperage wires into and out of a machine cab.

To begin checking a relay, you need to know where each circuit flows into and out of it. All relays have standard numbers identifying each of the four connection pins, and those numbers will be printed or embossed into the relay body next to the pins. Here's what they signify.

Pins 85 and 86 are on the control side of the relay. Power from the switch flows into the relay at pin 85 and out to a chassis ground from pin 86, making a complete circuit that activates the relay's electromagnet. The magnet closes the second independent circuit within the relay that runs between pin 87 and 30.

Pin 87 is where high-amperage current from the battery enters the relay. Pin 30 is where that current flow from the battery goes out to the component when the electromagnet is energized and the circuit closed.


Rear-view mirrors. They’re a standard accessory on virtually every car and truck since about the time of the Model A…

The only tool required to check a relay is a multimeter. With the relay removed from the fuse box, the multimeter set to measure DC voltage and the switch in the cab activated, first check to see if there are 12 volts at the 85 position in the fuse box where the relay plugs in (or wherever the relay is located). If there isn't, check to see if the appropriate fuse has blown. If the fuse is intact, ensure the switch is allowing current to flow to the relay.

Once you can confirm there is voltage at the 85 position, set the multimeter to the continuity mode and check to see if the 86 slot has a good ground connection.

If voltage is able to flow through that side of the relay, move on to the 87 connection point and determine if battery voltage is present there. If it isn't, that also suggests a blown fuse or circuit breaker.

By once again using the continuity function of the multimeter, it's possible to ensure there is a good connection from the relay to the component. Place one lead on the 30 terminal and the other on the component positive voltage connection point. (You may need to hook the multimeter lead to an extension to reach the component, depending how far it is from the fuse box). If everything checks out properly, then the relay is probably at fault.

It's possible to do an easy double check of that, just to make sure. Simply take a cotter pin or piece of wire and connect it between the relay's 87 and 30 connection points in the fuse box. The component should begin working. Doing this is also an efficient way to make an emergency repair that will allow you to get the machine back to the yard until the relay can be replaced. Remember, a relay is not a fuse; there will still be a fuse to protect the circuit.

The term open and closed is often used to describe electrical circuits. But what does it mean, exactly? It refers to whether or not current can flow.

It's easy to understand the concept if you think of the analogy of a fence gate. When the gate is open, the fence isn't complete and the enclosure is open. When the contact points in a switch are apart, the circuit is "open," just like that gate, and current can't flow. When the contact points are together, the circuit is "closed" and electricity can flow from the battery to where it's needed.



Scott Garvey is a freelance writer and video producer. He is also the former machinery editor at Grainews.

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