Relay Purpose ... ?

The "click" isn't everything though.
In fact as far as failure modes go, the click is nothing.

Lack of a click usually indicates a failure upstream of the relay itself, the switch or wiring that tells the relay to turn on and off.

Most relay failures involve the high current contacts on the output side. Corrosion and or crud builds up, and sooner or later, the relay just fails to pass current, even tho it is switching and mechanically it is working fine.

A good example of this is the "boost relay" in my Monaco. This is a BIG relay, kinda like a Ford starter solenoid, but made for continuous duty.

In my coach, it is energized by both the boost switch (intended to connect both battery banks in an emergency)and the "BIRD" circuit (intended to connect both battery banks for charging.)

My BIRD circuit always worked. Both banks charged no problem.
But if I ever let the chassis batteries get low in storage, hitting the boost button did nothing.
I could start the genny (it's on the house batteries) but the diesel wouldn't crank when I held the boost switch down, even though I could hear the relay click.

The problem turned out to be severe gunk and corrosion inside the relay. The contacts would pass the relatively small battery charge current no problem, but too much resistance to pass enough current to crank the cummins.

I Was able to take it apart and clean it, and now I can disconnect the chassis batteries, hold the boost switch, and the cummins will crank right over and start.

hotbyte wrote:
When working correctly, you can usually hear them "click" as the contacts close - especially if you place end of a small stick to the relay, wrap your hand around the the other end and lay ear right up the end of stick in your hand. The stick will work like a stethoscope helping you hear noise internal to the relay.

If relay is disconnected and out of rig, a voltmeter can be used to check for continuity across the "power" side when 12V is applied to the "switch" side of relay.

Excellent input .... gives me much more visibilty on how the relays interface with electrical functions. Thanks!!

When working correctly, you can usually hear them "click" as the contacts close - especially if you place end of a small stick to the relay, wrap your hand around the the other end and lay ear right up the end of stick in your hand. The stick will work like a stethoscope helping you hear noise internal to the relay.

If relay is disconnected and out of rig, a voltmeter can be used to check for continuity across the "power" side when 12V is applied to the "switch" side of relay.

smkettner wrote:
Most are mechanical. DC contacts will arc some when switching under load and eventually fail to conduct as expected. This is why it needs checked when the thing no longer operates when expected.

To bad the mfg can't engineer/install a red light the size of a pin head to tell if the relay is fully operational or not.

Most are mechanical. DC contacts will arc some when switching under load and eventually fail to conduct as expected. This is why it needs checked when the thing no longer operates when expected.

FIRE UP wrote:

2ndhom wrote:
I frequently read "check your relay" in threads here on the rvnet. I've read several Google explainations and still don't understand their function. In very simple layman terms ...... could someone explain? In my electrical bay they all look the same. Are they? Thanks.

2ndhom,
Dale did a good, but technical job of explaining the function of a "relay". Well, in short, a relay does, just what the name implies. It "RELAYS" electricity from one point to another. It's nothing but a large switch. And, what you do, as Dale said, is use a small switch, to control, A LARGE SWITCH.

Many components need more power to operate efficiently or correctly. And, that means there's a need for larger wire to carry the current needed for those components. And, that also means a large type switch, to be able to handle switching such loads, on and off without damage to the switch. So, that switch, or "relay", is typically located under the hood or, any other place other than under the dash in the drivers compartment.

Then, there's a tiny switch, on or in your dash, that controls, that relay which, controls the larger current etc. to operate whatever high demand item it requested. Hope his and my explanation helps some.
Scott

Should I assume there are NO internal moving parts? All diodes/resister/capacitors?

Dale.Traveling wrote:
A relay is basically just like a switch with added stuff to turn it on or off. There are two parts to a relay, a electric magnet and the switch part.

A relay can be used to switch something that has a heavy load such as head lights. A dashboard switch heavy enough to handle the power and not melt would be pretty bulky and wouldn't fit in the end of the turn signal.

So when you turn your head lights on a small switch sends low power to the relay coil which turns into a magnet that moves the switch to send power to the head lights.

Relays come in all shapes and sizes. The shore power transfer switch uses relays to switch between the generator or shore power. The engine starter solenoid is basically a relay. Be very careful swapping relays that may look alike. You really can't be sure the relays are identical unless you cross the model numbers. A turn signal blinker looks like a relay but works very differently. Swap it with a relay and you'll have lots of problems.

Did I confuse you more?

Absolutely no confusion ... a good beginning to an understanding.

Stop by a parts store and ask for a 12 Volt Relay for a 2012 Chevy 2500 HD Diesel. They are mostly standard looking in the industry and my 2500 has one extra mounted on the firewall. They are inexpensive also/

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In laymen terms:

A relay switch is when the wife tells the child to go tell your dad to bring in the groceries. :)