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How Do I Test a Relay?


OlGeezer

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In tracking down my electrical gremlin, it would be helpful to find out if I've blown a(nother) relay. My electrical knowledge is extremely limited, so I'm a little in the dark (pun intended). I do believe that a relay is not like a typical resistor, though.

 

Any guidance would be appreciated.

 

Thanks,

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In tracking down my electrical gremlin, it would be helpful to find out if I've blown a(nother) relay. My electrical knowledge is extremely limited, so I'm a little in the dark (pun intended). I do believe that a relay is not like a typical resistor, though.

 

Any guidance would be appreciated.

 

Thanks,

 

Bill, a resistor is basically just that (it resists current flow).. Maybe it would be easier to understand if I just compared a common resistor to the filament in a light bulb.. In a lot of ways a resistor is just like a filament in a light bulb only you can’t see the light coming out.. Your kitchen toaster has a series of resistors in it (look in there as it heats & you will see the resistors heating in action)..

 

No on the other hand a RELAY is just a switch.. Think of a common relay (there are variations on that though) as just a heavy duty switch with a little man sitting inside the relay case,, that little man has a wire wound around his neck,, there are two terminals coming out of the relay that hook to those wires around that little mans neck.. There are also 2 or more wires exiting that relay that are hooked to the switch that the little man operates..

 

OK how does it work? Simple if you look at it in non electrical terms.. When you put power & ground to those 2 little wires that are wound around that little guys neck he just flips that switch on.. When you remove the power & ground to that little guys neck he relaxes & lets the switch turn off (very simple explanation here)..

 

Now to test that relay (they are not all the same or operate exactly the same way but most common little square Bosh types do)..

-Look on the side of the relay to find the little man (that would be the 2 numbered terminals with a simple looking coil running between them) one of those terminals usually goes to a power & the other to ground (using an ohmmeter you should show a resistance (not infinite and not 0 ohms).. When you put power & ground to those terminals it should click.. Now find the numbered terminals that correspond to the internal switch (will look like switch contacts on the case side) as a rule one is common (goes both ways) with one of the other two being normally closed & the remaining one being normally open).. When the coil wires are powered up the normally closed will become open & the normally open will become closed.. About the only way you can damage those little relays is to try to force too many amps through it’s switching contacts or wire the coil side backwards & fry the clamping diode (most have resistors so that usually isn’t a problem).. Probably the most common way to fry a relay is to use it to power a wire that is shorted to ground (as mentioned above that will force it to pass too many amps)

 

My question to you is: how in the heck are you burning up those relays?

 

Twisty

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My question to you is: how in the heck are you burning up those relays?

 

 

Perhaps it not the relay but rather the connection(s) to the relay?

 

Visually inspect the connections for any sign of heat damage (discoloration).

 

Also make sure all of the socket (connector) connections are tight.

 

All assume that the relay connections are close to 1/4" male blades.

If true insert a male 1/4" connector into all the socket connections to check for loose connections.

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...with a little man sitting inside the relay case...
lmao.gif

 

 

But credit where credit is due - Overall that's all one of the best layman's explanations of it all I've seen. thumbsup.gif

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My question to you is: how in the heck are you burning up those relays?

 

Twisty

 

Of course, that's the $64 question. Without doing any investigation, I have two theories at this point:

1. My fog light switch has gone bad. It seems like most of my recent problems are related to the fog lights. The first couple of times, when I turned off the ignition switch, the fog lights would stay on. I would pull the light relay, put it back in and the lights would go out. The third time I turned them on, they immediately went out. I pulled the light relay again and it appeared that the base was melted. Of course, it was late at night and I was tired. It no longer appears to be melted, but it does look distressed. Before the trip, I replaced the right bulb. Upon my departure, they both worked.

2. I have my SmartTire device and my V1 connected to my lighting system on the parking light circuit. I did this for convenience; it was the easiest place to make a connetion. The SmartTire device doesn't draw very much (60 mA), and I didn't thnk that the V1 was a problem. At full alarm, it draws a little less than 1/2 amp.

 

Thanks for the explaination.

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Thanks. This is helpful, but I still don't know how to test a relay. I guess I'm more "electrically challenged" than I thought.

 

I pulled one of the xx-412 relays. It's got four blades and they are numbered. There's even a handy diagram on the side. Based on the helpful information already provided, this is what appears to be the correct procedure. Please correct any errors I have made:

1. I measured the resistance across blades 1 and 2. I meaured about 100 ohms.

2. I connected the relay to a 12v lantern battery to the same two blades and can hear the clicking.

 

Do I need to do anything more?

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The next step would be to measure the resistance across blades 3 and 4. It should be infinite when there is no power to blades 1 and 2, and it should be zero when you apply the 12V lantern battery to 1 and 2.

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My question to you is: how in the heck are you burning up those relays?

 

Twisty

 

Of course, that's the $64 question. Without doing any investigation, I have two theories at this point:

1. My fog light switch has gone bad. It seems like most of my recent problems are related to the fog lights. The first couple of times, when I turned off the ignition switch, the fog lights would stay on. I would pull the light relay, put it back in and the lights would go out. The third time I turned them on, they immediately went out. I pulled the light relay again and it appeared that the base was melted. Of course, it was late at night and I was tired. It no longer appears to be melted, but it does look distressed. Before the trip, I replaced the right bulb. Upon my departure, they both worked.

2. I have my SmartTire device and my V1 connected to my lighting system on the parking light circuit. I did this for convenience; it was the easiest place to make a connetion. The SmartTire device doesn't draw very much (60 mA), and I didn't thnk that the V1 was a problem. At full alarm, it draws a little less than 1/2 amp.

 

Thanks for the explaination.

 

Bill, a problem with the fog light switch shouldn’t have any effect as the power to that should go off when the ign switch is turned off..

 

There are a few possible things you could have happening.. The first is sticking contact points in the relay.. Not very probable & usually doesn’t repeat with a second relay..

 

Next would be power bleeding into the coil side circuit of your relay.. Possible but not that likely (at least from a wire short or connector problem)..

 

That leaves a sneak circuit.. This would be my best educated guess (these things are hard enough to diagnose when working on them let alone over the internet with limited information).. A sneak circuit is power from that relay or another vehicle circuit effected by that relay back powering up the control side (little man side) of your relay.. (basically power form the CONTROLLED side of the relay back feeding the CONTROL side of the relay.. We test new cars & trucks over many months for sneak circuits & try all combinations of switching scenarios & computer control to not sell a vehicle with a sneak circuit but still manage to miss a certain combination of events that sets a sneak circuit once in a while.. (not often but it does happen)

 

I do not like to power up accessory relays or other add ons from any lighting circuit or other vehicle dedicated power circuit as the possibility always exists for future problems..

 

Do either of those accessories you have added interface with your vehicles wiring in any other location other than it’s power relay or ground circuit.. Even backfeeding through a dash light, radio, or other light or resistance can be enough to keep that relay energized..

 

My suggestion would be to remove one accessory connection at a time to see if that relay then shuts off as intended..

 

If you cold tell us everywhere those add on accessories connect back into the vehicles wiring we might be able to advise further on possible problem points..

 

If I were working on a possible relay sneak circuit problem I would add probe wires to all terminals of the relay then monitor the coil side & switched side with a voltmeter as the problem shows up & see if there is any back feeding or cross talk between the controlled side & control side (little man side) of that relay..

 

The more you can explain your problem, like when it started, what was added just before it started, how often does it happen, what was done just before it started, what was operating just before it happened, etc, the better chance we have of helping you find it..

 

 

Fog Light switch = not real likely

Failed relay= not very likely if more than one relay has been tried

Shorting of power into the coil side of that relay= likely (that can be tested for with a voltmeter)

 

Wisk I could help you more but with the information posted here now it is limited..

 

Twisty

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when I turned off the ignition switch, the fog lights would stay on.

 

I had this same issue about 2 years ago. It was initially very intermittent but got worse with time.

The fix was replacement of the relay.

I suffer from CRS so I don't remember what relay confused.gif

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The more you can explain your problem, like when it started, what was added just before it started, how often does it happen, what was done just before it started, what was operating just before it happened, etc, the better chance we have of helping you find it..

 

 

I don't know how much more information I can provide than I did in my previous post.

 

The only items added are the ones mentioned, the SmartTire device and the V1.

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Thanks. I'll give this a try. One difference between the document and my relay(s) is that mine only have four blades instead of the five shown in the diagram.

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Thanks. I'll give this a try. One difference between the document and my relay(s) is that mine only have four blades instead of the five shown in the diagram.

 

Bill, yours won’t have the normally closed (87a) so just test it as shown & don’t worry about that N/C (87 a) terminal..

 

Or just do as Will suggested & hook your ohmmeter across the remaining terminals then look for continuity (0 ohms) with power across the other two terminals & look for open (infinity) with NO power across the other two terminals.. If you have continuity both with & without power to the coil terminals your relay has stuck points or something came loose inside & is shorting the points arms together..

 

Twisty

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