Hot Topics: AC Requires Contactor Switch

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Original Post: AC Works Only If I push Contactor Switch

James Hofsiss - Member

I have two Goodman AC units - one for downstairs, one for upstairs with a heat pump.

The compressor for the downstairs unit will not come on, no matter how low I set the thermostat. It does come on and cool normally only if I push the pressure switch on the contactor.

Also, at the same time this unit stopped working properly, I noticed that the indoor blower fan started staying on all the time. The only way to stop it is with the manual cut-off switch.

Where should I begin trying to troubleshooting the problem with the compressor? And could the indoor fan staying on be related to the compressor problem?

Thanks in advance!

firedawgsatx - Member

It sounds like you have a low voltage control problem. The 24V required to operate the contactor come from the transformer inside. Is you thermostat battery operated? If so, I suggest changing the battery to see if that corrects the problem. If not, further troubleshooting will be required. Do you have a multimeter to test voltages?

James Hofsiss - Thread Starter

Yes, the thermostat is battery operated (4 AA batteries) and it just started showing low battery, although everything else on the display is normal.

Could it really be that simple.

Gunguy45 - Super Moderator

It could be...but those batteries may just be to hold the memory. Though normally it would only be 2 batteries or a watch cell type for pure memory backup.

firedawgsatx - Member

Low batteries will cause the thermostat to act erratically. Hopefully, it is that simple. If the thermostat can't get the 24V to the contactor it won't pull it in to start the compressor or properly control the fan.

James Hofsiss - Thread Starter

Well, that wasn't it. Put in brand new Duracells and reset the thermostat. No change.

If the contactor switch needs 24V from the Tstat to engage, and everything works fine if I manually press the switch, then am I correct in assuming that the problem is probably between the TStat and the contactor?

Where and how exactly do I test for that 24V from the thermostat?

firedawgsatx - Member

Do you have a multimeter? If so, with thermostat set to OFF you can place one lead on the R terminal and one lead on the Y terminal. You should see approximately 24V. If you don't have a meter you can remove the wires from the R, G and Y terminals and twist them together to see if the compressor starts and the blower comes on. If so, it points to a bad thermostat.

I highly recommend shutting off power to the inside unit before touching any wires at the thermostat.

Restore power after wires are twisted together.

Also, with a multimeter you can test for 24V across the two wires on the side of the contactor as shown in the attached diagram. Make sure the thermostat is set to cool and well below room temperature.

AC contactor diagram

James Hofsiss - Thread Starter

When you say "place one lead on the R terminal and one lead on the Y terminal" do you mean on the thermostat?

I did try twisting the R, G & Y wires together but the compressor still didn't come on.

firedawgsatx - Member

Yes, the leads on the thermostat. If that didn't cause your condenser to start up it looks like your problem may be a low voltage blown fuse (hopefully) or a bad transformer. Turn off power to the indoor unit and look for an automotive type fuse to see if it is blown. It could be on a circuit board or just spliced into the low voltage wiring coming out of the transformer. Can you post the model number of your inside unit?

ac compressor plug

hvac fuse

hvac circuit board

James Hofsiss - Thread Starter

You mean the furnace/fan inside?

firedawgsatx - Member

Yes, that is the one. So the unit you are having trouble with is a furnace/a/c? If so, the 24V to power thermostat will originate from the furnace control board. It should have a fuse as shown in the third photo in my previous post.

James Hofsiss - Thread Starter

No it's the AC compressor unit outside. But the indoor blower fan is inside, attached to the furnace.

firedawgsatx - Member

Ok, I understand. You will need to check the fuse on the furnace control board to see if it is blown. If it is not blown then you will need to determine if you have line voltage coming into the furnace, then check the transformer to see if you have 120V in and 24V out. If that checks out good then check the low voltage wires coming from the house into the condenser outside.

James Hofsiss - Thread Starter

Going to check it now. Could that problem effect just one thermostat/condenser of the two?

firedawgsatx - Member

It should just affect the condenser you are having trouble with. Without the model number we have to pretty much guess. The items listed in my last post should pretty well narrow down the most likely cause of your issue.

James Hofsiss - Thread Starter

By the way, it's a Goodman CAPF4860C6AA.

firedawgsatx - Member

The number you provided is for the a/c evaporator coil. The furnace model number should be inside the furnace and start with something like GMH or GMS.

James Hofsiss - Thread Starter

Yay! Cracked open the furnace and found there was burned out 3 AMP purple auto fuse on the control board. Going to get a new one and see if that fixes it.

firedawgsatx - Member

Good deal. Hopefully that will get you going. The question is what caused the fuse to blow? It could be a bare spot on one of the low voltage wires rubbing metal somewhere or outside it could be a damaged wire often caused by a weedeater or an animal chewing on them.

James Hofsiss - Thread Starter

Which ones are the "low voltage" wires?

firedawgsatx - Member

All of the thermostat wires at the furnace and the two going outside between the house and the a/c condenser. Also, the smaller wires inside the condenser control box where they connect to the contactor.

PJmax - Group Moderator

The blower fan running inside was telling you that something was wrong. On many furnaces, I didn't think Goodman did it, when the 24v fails or a safety stat opens the blower comes on.

James Hofsiss - Thread Starter

It was the fuse. Less than a dollar each at the auto parts store. Once I replaced it, put everything back together, and turned the power back on everything works great.

Tomorrow I'll take a look at all those low voltage wires and see what I can find.

Thank you all SO much for the help!

PJmax - Group Moderator

Check the wiring out good.... especially where it runs near metal or something sharp. The fuse rarely blows for no reason. You may also have an A/C contactor going bad in your condensor unit.

James Hofsiss - Thread Starter

Back Again - Same Problem

Ok folks, first thanks for the help last month. Unfortunately, I haven't completely fixed the problem. Even though I found that the fuse on the furnace control board was blown, I haven't isolated the cause of that. So as soon as I power on the AC after replacing the fuse it blows again.

So, a couple of questions:

1) What wires, components, contacts & connectors, etc. should I check with a multimeter. And what reading should I get for each of them. Is there anyplace on line that spells these out for me?

2) PJmax suggested the problem might be a bad contactor. That was my first suspicion, since the contactor switch wouldn't operate. Since the contactor has multiple posts and connectors, exactly which ones do I test and what readings do I look for?

3) My HVAC system consists of: two AC compressors outside the house and a gas furnace unit in the basement; one filter/blower in the attic; one filter/blower in the basement. Since it's getting cooler, will the furnace (heater) work independently, in spite of the issues with the AC? Or will I have to identify and repair whatever the problem is with the AC before I'm able to use the heater?

Thanks in advance!

Bob14525 - Member

Your contactor may be bad in that you have a grounded winding. Turn off the power to the outside unit. Remove the control wires (24VAC contacts, often on the sides of the contactor) from the contactor. Using an ohmmeter, check for resistance from both of the contactor terminals (ones that you just removed wires from) to ground (metal frame of the outside unit). It should measure as an open circuit (infinite resistance). If it reads any resistance, you have a grounded winding and is likely the cause of the fuse blowing, requiring replacement of the contactor.

Another possible cause of the fuse blowing is the control wires (24VAC wires) shorting to the metal case, possibly where they go into the outside unit. Another possibility is an animal chewing on the wires, causing the wires to short together.

James Hofsiss - Thread Starter

Thanks! I'll check the contactor for resistance.

Is it possible to check the control wires (I assume these are the low voltage Tstat wires?) for a short with a multimeter?

Bob14525 - Member

Yes, however you would have to disconnect both wires where they connect to the furnace. After disconnecting the control wires (running to outside unit) at the furnace, check resistance to the case of the outside unit. With the wires disconnected at both ends (furnace and contactor), check resistance between the two wires. It should read infinite resistance, if you get a reading, the wires are shorted together.

James Hofsiss - Thread Starter

OK, the multimeter doesn't move from infinite at all when I check resistance between the terminal and the case. (On both X10 and X1k) I assume that tells me the contactor is good?

James Hofsiss - Thread Starter

Ok, I'm a little unclear on exactly how to check the Tstat wires.

I disconnected both ends of both wires then touched one lead to each wire. I get infinite resistance. Is that the only check I need to do, or do I need to test for resistance between the TStat wires and the case of the outside unit? If so, do I check with the wires connected or disconnected from the contactor?

And if the contactor and TStat wires both check out ok, where do I look next?

Bob14525 - Member

The test you just did proves that the wires are not shorted to each other. The final test is to check resistance from both wires to the outside unit case (wires disconnected at both ends). This will check to see if one of the wires is shorted to the case of the outside unit. It's unlikely, but it's possible.

Did you check the resistance from either side of the contactor coil to ground yet?

firedawgsatx - Member

You also need to check the resistance across the contactor coil. With power turned off to the condenser, remove the wires from the 24V terminals. Set meter to ohms reading and place a lead on each of the terminals. You should get a reading between 9-12 ohms (although this varies by manufacturer).

James Hofsiss - Thread Starter

Yes, I checked both contacts on the contactor (where the TStat wires connect) to ground and got infinite resistance on both.

James Hofsiss - Thread Starter

OK, I checked resistance across the old contactor coil. The needle bounced all over the place and then settled about 0 ohms or a hair above. Checked resistance across a new coil and got 10 ohms. Installed the new contactor, replaced the fuse, and reconnected the thermostat wires. So far, so good.

firedawgsatx - Member

Sounds like you have found the problem was the contactor coil. Good job!!

James Hofsiss - Thread Starter

Thanks to everyone who offered advice and know-how! For a $25 contactor and a $1 box of fuses you all saved me an expensive service call and who knows how much in repairs.