# How do I test Contactor for 24 v at coil ??



## saltydog0463 (May 24, 2017)

I have a Carrier AC, Model 38TRA030-310 with a single pole contactor. AC would work now and then. When not working a slap on the side of the outside unit would often get it going. I have checked on you tube and all I can find is people saying to check for 24 v at the coil, however, they never show what wires or connectors I put my meter on in order to test. I do believe the contact points on the contactor are corroded and the contactor needs to be replaced but I would also like to check the low voltage.


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## 91782 (Sep 6, 2012)

Yeah, there's high voltage - it could kill you, OK?

So, when the question is in essence "which wires do I stick my tester thingy on", I sure as hell don't want to be responsible...

"IN OVER YOUR HEAD"


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## saltydog0463 (May 24, 2017)

SmallTownGuy said:


> Yeah, there's high voltage - it could kill you, OK?
> 
> So, when the question is in essence "which wires do I stick my tester thingy on", I sure as hell don't want to be responsible...
> 
> "IN OVER YOUR HEAD"


Thank you for the info. Yes I know there are 240 V coming out of the house and yes I do know where they come into the contactor and to stay away from them. I also know where the low voltage wires are that come out of the house and into the outside unit. I also know where the disconnect is and will pull it before switching out the contactor and I will pull it if I can still check for the low voltage with it pulled. What I don't know is the answer to the question I asked. I always know what I stick my tester thingy on. That's the reason for my post. Information.


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## 91782 (Sep 6, 2012)

saltydog0463 said:


> Thank you for the info. Yes I know there are 240 V coming out of the house and yes *I do know where they come into the contactor and to stay away from them. *I also know where the low voltage wires are that come out of the house and into the outside unit.



Then, if you DO know where the high voltage "come into the contactor", deduction reasons the only other connections on the contactor would be the low voltage.



> *I also know where the disconnect is and will pull it before switching out the contactor* and I will pull it if I can still check for the low voltage with it pulled. What I don't know is the answer to the question I asked. I always know what I stick my tester thingy on. That's the reason for my post. Information.


Pulling the disconnect is good. Generally, "if" properly wired, the low voltage is supplied by the thermostat low voltage circuit, run separately from inside the structure. In other words, the low-voltage circuit operates independently of the high voltage "mains".

But nothing can be ASSumed.


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## saltydog0463 (May 24, 2017)

SmallTownGuy said:


> Then, if you DO know where the high voltage "come into the contactor", deduction reasons the only other connections on the contactor would be the low voltage.
> 
> 
> 
> ...


Thank you for the confirmation that the low voltage is independent of the high voltage. Of course I will pull the disconnect and then test the 240 v coming into the unit before I remove/test the contactor or anything else. Since there are a total of 9 wires connected to the contactor minus the 240 v that leaves 7 wires. Deductive reasoning tells me that is over 20 combinations .


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## 91782 (Sep 6, 2012)

saltydog0463 said:


> Thank you for the confirmation that the low voltage is independent of the high voltage. Of course I will pull the disconnect and then test the 240 v coming into the unit before I remove/test the contactor or anything else. *Since there are a total of 9 wires connected to the contactor minus the 240 v that leaves 7 wires. *Deductive reasoning tells me that is over 20 combinations .


Then you are not looking at the "contactor" - or you aren't able to discern the difference between connection points and the wires.

This is why I say "IN OVER YOUR HEAD".

This is not being a dick, it's being wary.


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## saltydog0463 (May 24, 2017)

SmallTownGuy said:


> Then you are not looking at the "contactor" - or you aren't able to discern the difference between connection points and the wires.
> 
> This is why I say "IN OVER YOUR HEAD".
> 
> This is not being a dick, it's being wary.


There is a total of 18 connection points on my contactor. Two screws for the 240 v in Two screws opposite those screws ( power out?) and then 14 male connections similar to the ones in your photo. Of those 14 there are 7 that have wires connected to them going to different components. I truly understand and appreciate some of the info you have given since you don't know my skill level and I do believe you can never be to careful with electricity. I have trouble shot and replaced the Dual run Cap on my AC , the Cap , the Ignitor, the Motor and the Control Board in my gas furnace. I am also an ex Air Force Aircraft Mechanic. So I do have some experience. Not being a dick, but I know I need more info before I can fix my unit. That is the purpose to my post. Hopefully someone with the proper knowledge will answer it. Thanks for your effort.


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## Tinstaafl (Jan 6, 2008)

Simple. Look up the schematic diagram for your contactor--that will show you everything you need to know.

If you lack the resources to locate that schematic and/or interpret it, either pay someone who does, or just replace the contactor and see if that fixes the problem.


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## saltydog0463 (May 24, 2017)

Thanks Tinstaafl

I'm in the process of doing that now. Schematics have never been my strong suit so it may take a bit. I'll post tomorrow or Saturday with my findings.


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## beenthere (Mar 5, 2009)

saltydog0463 said:


> That is the purpose to my post. Hopefully someone with the proper knowledge will answer it. Thanks for your effort.


I thought it was just a DIY post.


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## SouthonBeach (Oct 18, 2012)

Look for where the "thermostat" wire comes into the unit. Follow where it goes and see which 2 wires go to the contactor. 
If it's like STG's pic they should go to the whitish part of it on both sides.


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## flashheatingand (May 3, 2008)

saltydog0463 said:


> Thanks Tinstaafl
> 
> I'm in the process of doing that now. Schematics have never been my strong suit so it may take a bit. I'll post tomorrow or Saturday with my findings.


Try to learn to read schematics and wiring diagrams. It will be a major help in your work. :thumbup:


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## Ernie in Dawson (Jun 16, 2017)

SouthonBeach said:


> Look for where the "thermostat" wire comes into the unit. Follow where it goes and see which 2 wires go to the contactor.
> If it's like STG's pic they should go to the whitish part of it on both sides.


^^this, your 24 volt is usually going into the side of the contactor marked "coil". Your high voltage leads will go in and out of either end and will virtually always be a much heavier gauge wire. Contactors are pretty cheap at an electrical supply place but I question if that is the problem. They rarely fail, in my experience. 
The first thing I would do is turn off the power at both the disconnect and inside at the breaker and pull each wire individually and make sure the spade connectors are good and clean off all the connection points with some steel wool or small wire brush. Also make sure none of those pins are loose in the unit. Then after that, be sure everything is back the way it was (hence doing one at a time and also take before pic with your phone) and try the thing again. If it is still sporadic, lightly tap the top of it with the rubber end of the screwdriver and see if it kicks in and out. If it does, you probably have something internally wrong and can check voltage but probably just kill the power, take a picture of the rating plate and go to your electrical supplier and get a new one. Again when replacing, either label wires or one at a time.
Most of all, be careful.


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## 91782 (Sep 6, 2012)

SouthonBeach said:


> Look for where the "thermostat" wire comes into the unit. Follow where it goes and see which 2 wires go to the contactor.
> If it's like STG's pic they should go to the whitish part of it on both sides.


The image I posted is the specced replacement contactor for the model the OP listed.

To look at that and still not be able to tell where the 24v connection is to actuate the coil tells me the OP should leave some things to professionals. 

All this nonsense on how many wires or reading schematics is just more evidence of "IN OVER YOUR HEAD".

BTW OP, mind that the fan/run capacitor isn't charged and bites you.

:whistling


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## saltydog0463 (May 24, 2017)

SouthonBeach said:


> Look for where the "thermostat" wire comes into the unit. Follow where it goes and see which 2 wires go to the contactor.
> If it's like STG's pic they should go to the whitish part of it on both sides.


There is a White and a Red low voltage (24v) wire coming out of the house to the ac unit. The white wire goes directly to the Contactor via a brown wire. The Red wire goes directly to the compressor via a Light Blue wire. This is where I have had an issue in finding the second 24 volt connection. I think that the other 24v connection is at the CTD (?) Compressor Time Delay part number Z1704-4 made by ICM. There are three spade connections lined up vertically. From top to bottom there is a T2 then T1 and then a T3 connection. Between the T1 connection and the T3 connections it says 24v. If that is the second 24v connection does it relate to the T1 or T3? Is it safe for me to test each of them in conjunction with the Brown 24v wire on the contactor? My meter automatically sets itself to test all AC voltage from low voltage to 240+ voltage when turned on. So I shouldn't have to worry about any setting.


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## saltydog0463 (May 24, 2017)

Ernie in Dawson said:


> ^^
> The first thing I would do is turn off the power at both the disconnect and inside at the breaker and pull each wire individually and make sure the spade connectors are good and clean off all the connection points with some steel wool or small wire brush. Also make sure none of those pins are loose in the unit. Then after that, be sure everything is back the way it was (hence doing one at a time and also take before pic with your phone) and try the thing again. Most of all, be careful.


Thanks, did that with no change. I will be most careful. I have a very healthy respect for Electricity.


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## saltydog0463 (May 24, 2017)

*I'm installing a new Contactor*

I'm installing a new Contactor 



A buddy got me an oem contactor for my AC, Part # HN51KB024. I believe I know how to wire it up. However, the layout is a bit different between the new and old one. Since I am not sure of the wiring I would like to post how I think it should be wired and get your feedback. I've included photo's of each contactor. The one on the left is identical to the old one the on the right is the new one.
All of the connections are the same except the ones to the coil. The old one has a double spade connection on the bottom left and a double spade connection on the bottom right. The new contactor has a double spade connection on the bottom and a double spade connection on the top. It appears to me that each of these connections go to different sides of the coil.

On my old Contactor there is one wire (Violet) on the left side that goes to the T2 connection on the CTD (Compressor Time Delay) and two wires, one (Black) that goes to the T3 connection on the CTD and the other (Brown) that goes to the white 24v line coming in from the house.

My question is, can I just hook up the Violet wire to one set of double spade connections at the top or bottom of the new Contactor and the other two wires to the other set of double spade connectors on the top or bottom? Does it matter if the Violet wire is goes to the top set or bottom set or are the interchangeable since each just goes to the coil?


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## beenthere (Mar 5, 2009)

Why not post a pic of your actual contactor. Although both of the ones in the above pic are single pole contactors.


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## saltydog0463 (May 24, 2017)

beenthere said:


> Why not post a pic of your actual contactor. Although both of the ones in the above pic are single pole contactors.


The actual contactor is installed. The one in the photo is the same make, model, and layout. However, I pulled the one in the photo from an old unit that was being replaced and the contractor let me pull it. It didn't work either so I switched back to the original in an abundance of caution.


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## Tinstaafl (Jan 6, 2008)

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