Carrier Central AC Not Staying On - Possible Contactor Issue?

I have an old Carrier AC (Model 38TXA036300) that is not working properly. I first noticed a problem on Sunday. I was outside, and I heard the condenser turn off and make a clicking/popping sound. I had never noticed anything like this before. It went back on a few minutes later and seemed to be working okay. Yesterday, I noticed that the temperature climbed up to 78, and the condenser was not running. This morning, I opened up the unit to take a look. I verified 240 volts at the contactor line terminals and 24v at the side terminals. I pressed the contactor manually, and the fan immediately came on. I was nervous about doing this (I’m not very familiar with HVAC yet), so I didn’t depress it for more than a few seconds.

I ended up putting the power back on (with the side panel off) and setting the thermostat to cool. The fan came on right away. I’m not sure of the exact time, but I would say within about 40 seconds, I heard a pop and saw a small spark from the contactor (it sounded similar to what I heard on Sunday). The unit immediately shut down. I looked more closely, and it looks like there’s some black discoloration near the contacts. My understanding is that contactors don’t generally go bad, so I don’t know if this is a sign of some other issue. I wanted to check the resistance between the low voltage terminals, but I wasn’t sure how to do this properly. Would I remove all the wires from the contactor? Is it okay to remove the wires connected to the capacitor (I’ve not really worked with them before, so they make me a bit nervous). I have pretty good knowledge of electrical, but I’m very new with HVAC. I’ve been reading and am slowly learning.

I appreciate any tips or suggestions. I don’t mind calling out a pro if necessary. I just wanted to troubleshoot myself first to see if it’s a problem I can fix.

This is a picture of the contactor (not the best quality):

 

The contactor is a relay, controlling the 240VAC power to the compressor & fan. The 24VAC from the air handler/furnace is the control signal. While the contactor coil can open or short to ground, the most common reason for replacing the contactor is burned contacts. It's not uncommon to see a small arc when the contacts open, and over time, this can burn the contacts, causing them not to make good contact. Contactors are relatively inexpensive. If you have 24VAC on the coil terminals, but the contactor isn't engaged, I would suggest replacing it.

 

Thanks for your post! I actually looked at this again this morning and noticed a weird issue. I had the AC on, but the plunger on the contactor was not down. I checked the voltage, and my meter was registering 0 volts. The contactor 24v lines are spliced, so I traced them back to the entry point of the condenser and also noticed 0 volts there. I had opened my inside unit to see if anything was out of the ordinary, but I didn't see anything obvious. After powering everything back on, I heard the contactor engage, and I now had 24 volts. Within a minute, it popped up, and I again had 0 volts.

I have an Ecobee thermostat which I took off the wall to check the wires there. I checked from Rc to Y, W, and G, and there is no voltage. The only time I actually see voltage is when I check from Common to Rc (24v). I thought that there should always be 24v on Rc, but I may be incorrect on that.

One thing I'm concerned about is that I do have 2 Venstar add-a-wire kits installed. I have one for the smart thermostat to add a common and one for my humidifier. I don't know if there is possibly something there that is causing an issue.

If you have any ideas for what I could check, I can test it and report back. I am very new at this but am comfortable enough with wiring, etc.

This is a picture of my control board and thermostat wiring. It's a mess with all of the wires, so I apologize.

 

Your question regarding the Ventstar units would best be answered by an HVAC professional such as Pete (PJmax). I'm just a homeowner. You should always measure 24VAC between Rc & C. R is the output of the 24VAC transformer, while C is the common (return) from the transformer and is usually connected to the chassis of the air handler/furnace. If everything is working normally, I would expect that you would measure 24VAC between the R & Y terminals with the thermostat removed. If you're not reading 24VAC between R & Y, I would be suspicious of an open circuit in the Y circuit. Normally the Y terminal ultimately connects to the contactor coil through one or more switches (if present). The switches include a possible float switch, low pressure switch, and high pressure switch. The float switch (if present) would be at or near the air handler, while the pressure switches (if present) would be in the condenser (outdoor) unit.

 

Thanks. Does it have anything to do with this being a digital thermostat? I checked again, and I only have power at Rc to C. I don't understand why I seem to be getting intermittent 24v at the condenser when I test the incoming wires. I don't have a flow switch, but it's definitely possible that a high/low pressure switch is tripping (I would guess the low pressure if anything). Even if that were tripped though, wouldn't I still get 24v at the condenser wires? I'm confused as to why I'm getting voltage intermittently. I even ran a new wire in case there was some type of break or short.

 

Yes.... I'd agree.

Interesting wiring and colors.
Connect the red R wire directly to the yellow Y1 wire for quick testing.
Does the condenser fire now ?

Keep in mind that most thermostats have a 5 minute A/C delay and many condensers
also have up to a 5 minute delay turn on timer.

There is sticky at the top of the forum discussing checking for 24v at the condenser
to determine if pressure switches are a problem.

 

PJ, thanks. I have a question and probably a stupid one. Am I doing this at the thermostat (as in removing the yellow Y1 and then putting the Rc wire into Y1). If I then put the thermostat back on, will it have no power? Or, am I doing this at the control board (taking the condenser wire and moving it to R). If the condenser does turn on, how long should I run it for?

What I've noticed is that when I shut everything down, and then turn it back on, the condenser will run for a bit and then stop.

At the thermostat, is it weird that I'm only getting 24v from R to C but not 24v from R to Y, W, or G? I don't know if the Ecobee has anything to do with this.

 

I just wanted to post an update. At the control board, I moved the condenser from Y to R. After doing that, the condenser went on. I also noticed that I had consistent 24v to the condenser, instead of intermittent voltage. I ended up pulling all the existing wires and running a temporary thermostat, so I could at least run the AC. I have a feeling that the Venstar Add-A-Wire somehow failed, so I assume that needs to be replaced. I'm not sure if it's the diode in the wall or the unit at the control board, but they come as a kit anyway. I actually wish I could run a whole new thermostat wire, but I don't think it's doable. The thermostat is in my family room, which is an addition. It's a slab with no attic access and vaulted ceilings.

 

Great detective work! Ultimately, it seems like running an 18/6 (or 18/8) new wire would fix your problem. Did you pull the original thermostat wire out completely already? If it's too late to piggyback/fish a new wire, maybe consider a new thermostat location. Since it's an ecobee, direct access is less important.

 

Thanks! I would love to run a new 18/8. I haven't pulled the old wire out (just disconnected it from the control board). Do you think I would actually be able to pull a new one through? I kind of assumed it would be stapled as it ran through the wall. If I can't, I'll either have to relocate or replace the add-a-wire.

 

Ahhhhh... a Venstar. That explains the wiring.

I have stopped using the add-a-wire modules as I've found them unreliable.
You can't read voltage with that module as it switches and uses DC.

You can rewire your system to not need the venstar.

R ---------------> R
C ---------------> C
Y ----------------> Y with a short jumper to G.
W ---------------> W

The Y wire will power the condenser and the blower.
You will not have FAN ON at the thermostat.

Running a new cable is a good idea but usually the wire is stapled in the wall.