Diy - When Your A/C Won't Stop or Start / Contactor Repair

[A Person]

Will I run into any long term problems because I replaced a 230 with a 240?

No. The voltages are all approximate. This would correlate to a 1v difference on the secondary - which is negligible.

In North America the utility companies are required to supply a split-phase 240 volt +/-5%. i.e anywhere from 228v- 252v

However I've frequently seen sustained voltages outside that range.

[Tommy]

Thanks for the informative article. Can anyone tell me if you think the contactor is my issue? Here's my dilemma.

My heat pump (AC and heat) was working fine until I decided to install a programmable thermostat. Upon installation with what I assumed was the correct wiring of the thermostat wires, The unit does not turn on at all -- no fan, no heat, no AC. I reinstalled the old thermostat and still nothing.

When I was installing the new thermostat, I had flipped off the two circuit breakers, one for the indoor unit and another for the outdoor unit. I also flipped off the two breakers on the indoor unit itself. Everything was flipped back on when I was done.

I checked the circuit breakers for the indoor and outdoor units with a multimeter, and both are getting 120V. I can't access the breakers on the unit to test them. One of them causes an electrical hum in the unit when in the on position, the other switch does not. I tried checking the thermostat wires at the thermostat, and got no reading. But I'm not sure what to touch the multimeter to at the same time as each wire, so there's a good chance I just didn't read it properly. I didn't know about checking the transformer until I just read the related article by A Person. So the transformer and contactor have not been checked yet.

Can anyone offer any suggestions as to what my problem could be? Thanks in advance.

[Tommy]

Thanks for the informative article. Can anyone tell me if you think the contactor is my issue? Here's my dilemma.

My heat pump (AC and heat) was working fine until I decided to install a programmable thermostat. Upon installation with what I assumed was the correct wiring of the thermostat wires, The unit does not turn on at all -- no fan, no heat, no AC. I reinstalled the old thermostat and still nothing.

When I was installing the new thermostat, I had flipped off the two circuit breakers, one for the indoor unit and another for the outdoor unit. I also flipped off the two breakers on the indoor unit itself. Everything was flipped back on when I was done.

I checked the circuit breakers for the indoor and outdoor units with a multimeter, and both are getting 120V. I can't access the breakers on the unit to test them. One of them causes an electrical hum in the unit when in the on position, the other switch does not. I tried checking the thermostat wires at the thermostat, and got no reading. But I'm not sure what to touch the multimeter to at the same time as each wire, so there's a good chance I just didn't read it properly. I didn't know about checking the transformer until I just read the related article by A Person. So the transformer and contactor have not been checked yet.

Can anyone offer any suggestions as to what my problem could be? Thanks in advance.

[A Person]

Your problem appears to be with the low voltage circuit. You should see 24V between the red wire(s) and yellow and white at the thermostat. Shorting the red wire to the white at the thermostat should start the heating and to the yellow should start the cooling

If you have no volts at the thermostat then you need to see if you have volts at the transformer and if there is a fuse somewhere.

[Tommy]

Thanks for the help. I was able to trace the problem to the transformer with your advice and the help of my neighbor who is an electrical engineer. Well, actually my neighbor diagnosed it with my clues. The new transformer is on order. My neighbor said we have to hope the new thermostat I had been trying to install is not somehow the culprit...this would be the case if I put back my old thermostat with the new transformer and the system works, only to have the transformer go when trying new thermostat again. He said he doubts that would happen, but I am still keeping my fingers crossed. Thanks again.

[A Person]

The thermostat can't cause a problem with the transformer. It's just a switch that opens and closes - just as a light switch can't blow the light bulb.

I suspect you accidentally shorted the leads when you replaced the contactor. It's easy to do although most class 2 transformers have sufficient inherent impedence for it not to be an issue unless it's shorted for a long time.

After you've disconnected the old transformer and before you wire in the new one I would measure the dc resistance of the low voltage circuit from the leads that will be conncted to the new transformer. You should see either mega-ohms (open circuit) if the thermostat is off, or something around 20 ohms if the thermostat is on (that's the DC resistance of the contactor coil)

If you see anything less than 1 ohm and you have a short. If it's more than that and less than 10 ohms you have a bad contactor.

EDIT
Thinking about it - here's what to do while you're waiting for the transformer. Disconnect the old transformer, connect the wires together and then go to the thermostat, disconnect the red wire and measure the resistance between it and the yellow wire and then with the white wire (if your wires aren't that color then go by the R, W & Y labels on the thermostat). That takes the thermostat out of the picture. You should see a DC resistance of between 10 and 30 ohms.

[Tommy]

Thanks for the follow up advice. I actually didn't get around to checking this page until now, after I installed the new transformer today. Everything works fine. What I did check was the voltage of poles on the low voltage end of the transformer after I hooked up the 240V but before hooking up the 24V wires. That read properly, so I hooked up the thermostat wires and then tested my old thermostat first. The heater kicked on, so I installed the new thermostat again and it works fine.

I really appreciate your assistance. I am so grateful for generous people like you guys. Thank you.

[A Person]

No problem, glad to be able to help

[ranwest]

Hi, I have a little different problem, but you seem to be very knowledgeable and don't mind helping others, so here goes...the fan on my ac/heating unit won't stop blowing. The A/C and heat work well, but when they cut off the fan just keeps blowing. I replaced the thermostat about a week ago and seemed to fix the problem, but last night while using the heating it happened again, heating cut off, fan keep blowing. Any ideas or information you can give will be greatly appreciated.
Thanks in advanve.

[A Person]

On house furnaces the fan can be controlled in two ways. Your thermostat can have a switch for continuous fan, or the furnace can be set for continuous fan, but in addition the furnace fan is controlled by the furnace through a temperature sensor/switch called the 'Furnace Fan Limit switch'



You'll notice a button on the limit switch - that puts the fan into either controlled or continuous mode

The way the limit switch works is that the thermostat will turn the furnace on, when it comes up to temperature the fan will come on (on my furnace that takes about 15 seconds). When the thermostat switches off the furnace goes off - but the fan will continue running until the furnace is cool. If the fan fails to come on the furnace will overheat and the limit switch should shut down the furnace to prevent your house from burning down

The temperatures are around 100F - fan off, 140 F - fan on, 170F overheat shutdown

It's quite possible you have a faulty limit switch - which if it fails the other way (i.e. doesn't shut the furnace down if the fan stops) can be very dangerous. Fortunately they usually fail safe by keeping the fan on.

Testing and replacing the limit switch is straight forward. I'm assuming you have an older furnace in which case there should be four wires - two which control the furnace and two high voltage (110v) which control the fan. Turn off the breaker, unplug the wires and check the continuity. If the furnace is cold and the fan contacts are closed then that's your problem.

You need to replace it with the right part. It's very important that the replacement matches the old one - the same type, same setpoints and the the length of the sensing element must be the same.

A common cause of failure is not changing your filter. This reduces airflow and the furnace can overheat which shuts down the furnace. If this happens too many times the bi-metal in the switch gets a permanent set and doesn't retract fully. It's also very bad for the fan motor

[Paul]

Hi Liv. I enjoyed your 6/19/07 story mainly because I'm faced with the same thing. The contactor in my Trane Weathertron Heat Pump needs to be replaced. The heat pump was installed new in 1985 and looks just like the one in your picture. My problem started when I couldn't find any source on the internet as to what this part number is. I guess it's too old. I went to the link you provided but couldn't tell which contactor was applicable. I called the company but they weren't any help.

I don't understand enough about electricity to be able to figure out which of these White-Rodger contactors I need. I have no trouble taking it appart and replacing it with a new one but I'm concerned if I order the wrong unit it may do damage to something in the heat pump.

The unit is a Trane Weathertron Heat Pump model BWD748A100A0 if that helps. Could you give me the correct White-Rodgers contactor part number to order if you know.

Thanks Paul

[A Person]

Contactors are pretty generic.

You need to know if you need a single or double pole - but while you shouldn't use a single pole to replace a double pole, you CAN use a double pole to replace a single pole.

You then need to know the voltage on the activation side. In most cases it's 24-30V but it can be 110-120V or 220-240V

The exact voltage doesn't matter. 24 or 30v are close enough.

Amps - it will be rated for 25, 30 or 40amps. You can use a 40A one in a 25 amp unit, but not the other way round

If you take the old one out it will likely have the part number and voltages on it. The part number is not particularly useful as there are many part numbers that can work

I think it very likely a Double pole, 24V, 30 amp since that's the most common.

If you're worried, remove the old one and take it to a place that sells electrical parts and get them to find a similar one. You'll pay a bit more than buying it online but it's still way less that paying The Guy.

[Liv]

My bets is the one linked to on the first page of this post will work, just fine for you...

[whuggs1]

I have a similar problem with my outside AC unit, We just had it serviced and the next day had a big storm came and lost power. When the power came back on the AC would not start, after a few good wacks it started. I replaced the contactor and start cap and it still won’t startup most of the time, I would say 70% chance it won’t start. I can get it to start sometimes but after it runs all day it won’t start back up at night.
To get the unit to start all I have to do is go outside to the unit and pull one of the small black wires off the side of the contactor and it starts, then I put it back on and it will run until what the TA is set to.

I’m now guessing bad TA or transformer ?

The 2 small black wires on the side of the contactor should be 24V when the AC should be running correct ?

[A Person]

To get the unit to start all I have to do is go outside to the unit and pull one of the small black wires off the side of the contactor and it starts, then I put it back on and it will run until what the TA is set to.

That doesn't make sense

The 'small black wires' are the low voltage activation. Removing the wire should switch off the contactor not turn it on.

The voltage should be 24v when the thermostat is on. If you have that then the transformer and thermostat are both OK

[handy jam]

Just want to say thanks for this thread and all those who have contributed. With what I learnt here, I've ordered a couple of those Honeywell 24 / 30A contactors and as soon as they arrive, I'm gonna install them. It sucks having nothing but hot air blowing around and no, I'm not one of those lucky enough to live in Washington D.C. But as Yosemite once said on bugs bunny after he landed down in hell - that he thought he was in Dallas, today it was 108 F and still 99 F @ 12:00 pm. YEEE HAAW! Wish me luck.

[dm034]

Thanks for the useful information throughout this post. On the comment below: when you replace a 1 pole with a 2 pole, do you need to do anything different in terms of installation?

Contactors are pretty generic.

You need to know if you need a single or double pole - but while you shouldn't use a single pole to replace a double pole, you CAN use a double pole to replace a single pole.

You then need to know the voltage on the activation side. In most cases it's 24-30V but it can be 110-120V or 220-240V

The exact voltage doesn't matter. 24 or 30v are close enough.

Amps - it will be rated for 25, 30 or 40amps. You can use a 40A one in a 25 amp unit, but not the other way round

If you take the old one out it will likely have the part number and voltages on it. The part number is not particularly useful as there are many part numbers that can work

I think it very likely a Double pole, 24V, 30 amp since that's the most common.

If you're worried, remove the old one and take it to a place that sells electrical parts and get them to find a similar one. You'll pay a bit more than buying it online but it's still way less that paying The Guy.

[Liv]

I"m a bit tired here, and I'll defer to AP when he comes a long, but from memory... you simple use only one side of the contactor.

[A Person]

A 240V circuit has two hot wires (black/black or black/red) and these must both be connected to a double pole contactor. A 120V circuit only needs a single pole for the hot (black) wire. If you replace a single pole with a double pole you can either just use one side for the hot (black) wire as Liv suggested, or you can connect both sides with the neutral (white) and hot (black). Either works and is safe.

[dm034]

Thanks for the quick reply. I actually have a picture. This is a single pole, right? My goal is to just get another single pole and wire it up exactly as it is here. Just hoping the configuration of the new contactor is the same so that nothing is even potentially confusing with regard to where the wires go.

For background, I have VERY loud buzzing coming from the unit (though the unit does come on/off and does produce cold air), so my best guess is the contactor, which I'll be replacing this weekend. If there are other possible causes of such buzzing, I'd be curious to hear your thoughts.

Thanks again for your responses.

[A Person]

yes that is a single pole contactor with the white supply being directly connected with the meta bar to the two red wires.

You have a 25 amp single pole 24V actuated contactor p/n CONT1P025024V. That's an obsolete part and you can replace this with a two pole contactor e.g. p/n CONT2P025024VS and screw the wires in the same position i.e. the new contactor will have terminals labeled L1, L2, T1, T2, just connect the wires to the same labeled terminal . The contactor will switch both the black live wire and the white neutral

[Liv]

For background, I have VERY loud buzzing coming from the unit (though the unit does come on/off and does produce cold air), so my best guess is the contactor, which I'll be replacing this weekend. If there are other possible causes of such buzzing, I'd be curious to hear your thoughts.

I'm not so sure on the buzzing though. Normal with a contactor, a hard strike with your fist to the casing directly above the contactor will engage the unit. You also should be able to manually (carefully with an insulated tool, and gloves) push down the copper contacts to test and see if it's the contactor.

[dm034]

Thanks AP. I'm assuming when you said "yes that is a single pole contactor with the white supply being directly connected with the meta bar to the two red wires" you meant the black (live) wire, not the neutral.

Thanks, also, Liv. I can check with gloves and an insulated tool to see if it's the contactor that's causing the noise, but what would that do that would be different from just turning the unit on (it is working normally -- just with the dreadful loud buzzing that makes my neighbors [I'm in a rowhouse] cringe)?

[A Person]

The supply line is coming from the right (bottom in your photo) it has a ground, black and white lead, which suggests it's 120V, so the black is live and the white is neutral. It might be a 240v supply, although by current code that should be a 4 wire cable, white (neutral) black (live) red (live) and ground.

The white supply goes to L1 which is connected by a solid metal strip to T1 and the black load wires. I'm assuming one to the compressor, one to the fan, I can't tell which is which - but it maybe one goes to the starting capacitor, which is the aluminum can.
The black supply goes to L2 which is connected by the contactor to T2 and the red load wires

The buzzing could be the contactor, but it could also be the compressor or fan. What Liv is suggesting is that you remove the contactor cover (secured with the Torx screw) and push down on the contactor with an insulated screwdriver. If your contactor is making poor or intermittent contact, pushing down can temorarily fix it. This is moderately dangerous since you're poking conductive metal screwdrivers in live circuits. If the cat decides to join in, or the neighbour's kids throw a ball at you, you may generate a lot of sparks.

Here's what I would do.

With the power off unplug both red wires from the T2, which disconnects the fan and compressor. Use insulated tools, because although the power is off the capacitor can store a charge for some time. Switch the power on. If the buzzing continues it's the contactor.

If not, the problem is with the fan or compressor, this is not a cheap fix and maybe new AC time.

[A Person]

Thinking about it some more, that might not properly test for the buzz. If there is no current passing through it the contactor may not buzz.

Anyway, by carefully listening you should be able to detect the source. But be careful sticking your ear anywhere near live terminals.

shocking.jpg (8.69 KiB) Viewed 1976 times