contactor not working

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  #1  
Old 05-03-09, 09:46 PM
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contactor not working

My contactor does not engage when the thermostat trips but the blower does. I've traced the 24 Vac to the unit, but it does not make it to the contactor. Could the cut off switch need to be replaced? I've pressed the reset and that has no effect. I can manually push the contactor in and the fan and compressor start, but they stop when I release.

Any thoughts?
 
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Old 05-04-09, 07:08 AM
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if you don't have 24V with a cooling call on the 2 wires from the air handler out to the condenser....you need to check the same 2 color wires within the air handler for a true 24V.if your getting the supply fan in the auto/cool mode on the stat then your away from the stat with the search at least.the fan is running but your not gettig 24V out to the contactor for the condenser fan and compressor....something is open on the run out either 1 or both are broken/open.if you have a digital meter or anything that can read continuity..shut the furnace/air handler 115v toggle OFF....disconnect the 2 /24v wires out at the condenser tie them together.now inside the air handler finfd those samr wires that are headig out to the condenser and disconnect them(note locations where you removed them from)...that the digital meter on OHMS or audio signal and put the test lead on each wire that goes out to the condenser...NOTE this is 24V wiring you are testing and nothig else.with the meter i either setting you should get an audible beep or 0.00 reading,if you get neither the wires going out are broken some place.....lets hear back if you have the tester.just so you know the 24V that operates the fan and condenser thru the STAT are 99% of the time from a transformer located within the furnace/air handler!
 
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Old 05-04-09, 09:54 AM
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Thanks for the reply.

I've verified that the 24V signal is getting to the outside unit, so there are no broken wires between the blower and condenser. There are two LV wires coming into the condenser that I measured with the meter - a yellow one and a black one. The yellow wire goes to a cut off switch that is connected to the cooling lines. A blue line comes off the cutoff switch and goes back to a relay/switch. The black wire is split with one leg going to the contactor and the other going to another post on the relay/switch with the blue wire. The relay/switch with the black and blue wires also has a yellow wire going to the contactor. When testing the relay with the meter, there's continuity between the posts with the yellow and black wires, but open circuits for all other combinations. So the yellow and black wires going into the contactor are at the same potential. I've pushed the button on the cutoff switch inside the unit to get it to reset, but that doesn't make any difference.

Thoughts?
 
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Old 05-04-09, 11:42 AM
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so the black/yellow coming into the condenser you are reading cooling signal of 24Vs! the black being split to the contactor and the relay is the common wire(unscrew the wire nut and put one meter lead to the black wires and screw the wirenut back on.this will give you an easy way to trace the hot /yellow 24V going into the LP(on copper line) and the relay to see where it stops or why it isn't closing the contactor.basically just like 115V in a socket you have the meter into the neutral and tracing down the hot thru the controls.#1 first check the yellow into the LP and the blue going out...you should have 24Vs #2 the lp controls the relay coil so continue to the relay from the LP for 24V/IN and #3 the relay to the other side of the contactor coil if you read 24Vs there opposite the C black the coil on the contactor is NG...same with the relay if you have 24Vs to the coil and it didn't switch(CLOSE sending 24Vs to the contactor)it is NG.it is either the the LP is holding the 24V from the relay,or the relay is holding the 24V from the contactor...or the coil is getting 24Vs but not closing.you can start from the contactor and trace it backwards also from there thru the relay and then to the LP.if you have a sensor(with 2 color wires) on each copper pipe just off the compressor the smaller pipe is your high pressure control and the one your on is the low pressure control both will control the relay which intern controls the contactor for the compressor as a safty trip...might just have the LP depends on the brands there.i'll check back for your progress take your time.if your up to it shut the compressor line power OFF and take a jumper/piece of thin wire and tie it to the incoming yellow from the house and touch the opposite side of the compressor so you are force feeding 24Vs to the contactor with that black a constant looking for the jumped 24Vs the contactor should close(compressor/fan won't run line power is OFF)the 24VS as mentioned is from the stat with powwer supplied by the TR in the air handler! that relay???? might be a anti-recycle timer if it is 2" square with just terminal on it,come back with some numbers off it if you can.
 
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Old 05-04-09, 01:12 PM
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Thanks for the info once again.

Is the LP easy or difficult to replace (if it turns out to be the problem)?
 
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Old 05-04-09, 02:43 PM
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if the 24Vs is not coming out of the LP it is only 2 things with it...the unit is flat of gas LP is working or the unit is charged and the LP is electrically shot(should be closed with freon in the system.to fix it you have to have a tech recover/pumpdown the freon and flip a new LP onto the suction line.if money is tight you can pickup a LP and just add it to the port the tech puts his guage on during a service check out.
 
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Old 05-05-09, 08:49 AM
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Ran the yellow 24V line to the contactor and the contactor engaged, so the contactor is good. Then ran the 24V line to the relay (replaced the line from the cut off valves) - the relay worked and the contactor engaged. That narrows the problem to the cut off valves - I suspect that it's time to call in a tech. Thanks for your help.
 
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Old 05-05-09, 12:32 PM
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thats great.....even though you need to get somebody in it being narrowed down to the LP saves you money.to verify if the units out of gas or the LP switch is shot (stuck in the open position)uncap the shradder where a tech puts the suction guage hose and press i the pin.....should sound louder then a tire on your car at 100 plus psi in the off non-runnig position.if you get that big hiss the switch should be closed.to change that it is a maximum of 10 minutes to do...with no recovery of the system charge required.
 
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