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contactor not coming in on 38en carrier outside a/c unit. Transformer…

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contactor not coming in on...
contactor not coming in on 38en carrier outside a/c unit. Transformer 115v to 24 volt fine. voltage 29.9volts good at red wire to ground in thermostat...but no voltage to coil on contactor at ofm and compressor contactor. Pressing contactor will run unit.
Submitted: 8 years ago.Category: HVAC
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Answered in 8 minutes by:
6/27/2010
HVAC Technician: Mr. Jeff Pearson, Mechanical Engineering Technologist replied 8 years ago
Mr. Jeff Pearson
Mr. Jeff Pearson, Mechanical Engineering Technologist
Category: HVAC
Satisfied Customers: 155
Experience: 25 years as Refrigeration Contractor,
Verified

Hi, I would be happy to help a customer that has and uses a voltage meter.

 

Did you open any access panels on the furnace,? like to change an air filter?,

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HVAC Technician: Mr. Jeff Pearson, Mechanical Engineering Technologist replied 8 years ago

 

Then there must be a open section of wire between the stat and the outdoor unit, or the stat is bad.

 

Did some wires get damaged outside? Follow the wires from the condenser to the furnace inside, and check for broken wires.

 

Remove the low voltage wires from the main contactor coil. With everything else set to run check for voltage of 24 volts. Now switch your meter to OHMS and place your leads on each side of the contactor coil. What is the reading of resistance through the coil?

 

What is the measured voltage between the two wires going to the contactor coil when they are not attached to the coil?

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Customer reply replied 8 years ago
zero volts when not conectted to coil. 18.1 ohms on coil.
HVAC Technician: Mr. Jeff Pearson, Mechanical Engineering Technologist replied 8 years ago
Did you just check between the two wires or did you check each one to ground?
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Customer reply replied 8 years ago
btwn two wires not to ground.
HVAC Technician: Mr. Jeff Pearson, Mechanical Engineering Technologist replied 8 years ago
Check both to ground please. Do you have a furnace inside the house?
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Customer reply replied 8 years ago
yes furnace inside. 1.6 volts to ground on each wire.
HVAC Technician: Mr. Jeff Pearson, Mechanical Engineering Technologist replied 8 years ago
Please go to the furnace and pop off the cover plate so you can have access to the control circuit board.
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Customer reply replied 8 years ago
ok...what now?
HVAC Technician: Mr. Jeff Pearson, Mechanical Engineering Technologist replied 8 years ago
Remove the two wires that go out to the condenser. Should be one on the yellow terminal and one on the c terminal. But they are usually red and wht in color
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HVAC Technician: Mr. Jeff Pearson, Mechanical Engineering Technologist replied 8 years ago

Did you locate the two wires? Good, now twist them together with a wire nut.

 

Now go back outside to the condenser. The two wires you just checked for voltage 10 minutes ago, check for resistance, using the Ohms settings. check ohms between two wires.?????

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Customer reply replied 8 years ago

18.1 ohms

HVAC Technician: Mr. Jeff Pearson, Mechanical Engineering Technologist replied 8 years ago

18 ohms is too much resistance to have in those wires. How long are the wires? That set of wires needs to be replaced. There should be a max of 3 ohms in the two wires.

 

Get yourself some 18 gage solid core wire an replace the two wires that go from the furnace to the contactor.

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Customer reply replied 8 years ago
why are wires going from furnace to contactor for A/C?
Customer reply replied 8 years ago

I have 18 guage..but not solid strand to replace wires with. I have already run wires to unit and having difficulty with hook ups. Y or yellow terminal and BC terminal to contactor from furnace???? right?

HVAC Technician: Mr. Jeff Pearson, Mechanical Engineering Technologist replied 8 years ago

Because the control board in the furnace tells the ac to turn on and off, and the furnace blows the air around in the house

 

 

 

While you have the furnace open, do you see the safety door switch that the cover presses in when the door in put on correctly?

 

Put your thermostat back together and set it so its trying to get the a/c going. Now go to the furnace and get ready to read the voltage between terminal "Y" and "C".

 

You will need to hold in the safety switch I mentioned earlier. And allow five minutes for the time delay,

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HVAC Technician: Mr. Jeff Pearson, Mechanical Engineering Technologist replied 8 years ago

Yes from furnace "Y" terminal and "C" Terminal to the contactor.

 

 

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Customer reply replied 8 years ago
ok
HVAC Technician: Mr. Jeff Pearson, Mechanical Engineering Technologist replied 8 years ago
let me know when done
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Customer reply replied 8 years ago
zero volts between y terminal and BC on furnace. Transformer and controls outside of furnace. panel does not need to be opened. I know about safety switch from opening panel before. time delay not issue.....was running already.
HVAC Technician: Mr. Jeff Pearson, Mechanical Engineering Technologist replied 8 years ago

And from "R" to ground???volts???

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Customer reply replied 8 years ago
was 30 volts yesterday at r and G. Now zero. I think I blew transformer trying to run new wires. but still was not pulling in coil on contactor yesterday before that happened today. transformer blue and yellow 24 to 30 volts and R and G 30 volts...yesterday.
Customer reply replied 8 years ago
I connected wire to R and C to run to contactor. Smelled burned wire covering a bit.
Customer reply replied 8 years ago
what do you think? Blown transformer? Thermostat damage now maybe? tired and hot?
HVAC Technician: Mr. Jeff Pearson, Mechanical Engineering Technologist replied 8 years ago

 

 

0.00751 Ohms per foot of 18 gage wire. Even if your run was 100 feet from furnace to outdoor condenser, that would be 200 x 0.0075 = 1.5 ohms.

 

The wiring was your problem, now you just need to replace the burned transformer and run the new wires, this time unplug furnace when working on the wires.

 

Sory its taking so long, if I could I would drive over to your place and fix it all up for you.

 

When you hooked the R" and "C" to the contactor, did you use the new wires?



Edited by Mr. Jeff Pearson on 6/27/2010 at 7:17 PM EST
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HVAC Technician: Mr. Jeff Pearson, Mechanical Engineering Technologist replied 8 years ago

 

It is the "Y" and "C" that are supposed to go to the contactor.

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HVAC Technician: Mr. Jeff Pearson, Mechanical Engineering Technologist replied 8 years ago
I will be back in60 minutes
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Customer reply replied 8 years ago
Not what the problem is. I bought a new transformer and ran the 18 gauge wires to contactor. Still no voltage...with wires off the contactor or not. I check voltage from 24 volt side of contactor and 24 volts there. 24 Volts at wires on control panel. Wire nutted both sets of wires directly together...still no voltage. There must be a short in relay or other wire somewhere else. I give up. I also replaced thermostat prior to this. Set in cool mode at 65 degrees and 90 degrees in house. air handler fan running...not shutting off..... Relay bad....but should not keep a/c from coming on as it should.
HVAC Technician: Mr. Jeff Pearson, Mechanical Engineering Technologist replied 8 years ago

I am sorry we have not made progress fixing your ac .

 

I don't understand, you say there is power at the control board, at the "Y" and "C" terminals, but you connect the new lines to them and there is no power at the other end?

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Customer reply replied 8 years ago
exactly, short in the other wires. Can HVAC repair today.
HVAC Technician: Mr. Jeff Pearson, Mechanical Engineering Technologist replied 8 years ago
could you please let me know what they find?
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Customer reply replied 8 years ago
Yes, I know the relay is bad. I think some of the connections are bad. Obviously there is a short somewhere.
HVAC Technician: Mr. Jeff Pearson, Mechanical Engineering Technologist replied 8 years ago
Often the wires running to the outdoor unit get damaged by lawn mowers, weed wackers , etc. Could this be the case?
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