# No power (24v) to compressor contactor



## mghaffer (Sep 21, 2013)

Hi all,
Hopefully someone can assist me. My A/C quit working two nights ago. I am by no a professional with HVAC, however, i have done some research into troubleshooting for the system to find out what is going on. First day i thought that it was just a capacitor burnt out, so i replaced it. The compressor turned on for a few seconds and cut out. I did this twice, just to make sure that i didnt have a bad capacitor that i replaced it with. Same thing, turned on and right back off. I have since borrowed a multimeter and started trying out some troubleshooting steps. I also replaced the contactor with a new one seeing as I missed no 24v from a1 to a2 the first time, figured i already had one might as well replace it with a new one. I metered the new contactor switch, from the breaker i am getting 252v at L1 to L2, L1 to T1 was almost 0, L2 to T2 was 252v, and a1 to a2 is 0. I then jumped the system with the test button on the contactor, the system runs 4.0 when i over ride the 24v pull. I checked the thermostat next, btw there is power on the thermostat. I pulled the face and metered it, Honeywell 6000, the only time that i get power is from R to any other wire and it is 27v. I tried to bypass the thermostat by connecting the power, fan, and compressor lines together to simulate system asking for cooling on, nothing. In the crawl space with the furnace the fan is running, i do have a low flow of ambient air coming in at all times from the vents to the house. Tried to meter a few things up by the furnace but not too sure what all I would be looking at, although the fan is running. I tried to push the system into heat mode as well and after about 10 minutes it was still not pushing out hot air and didnt look like the burner was on, so i turned the system back off. The best that i can tell is that i am getting power to the thermostat, and that it is trying to send it out, i can hear it clicking to try and select, but that somewhere between there and the compressor it is dropping the power.
Does anyone know what i could be looking at here? Or know anything else that i can try to narrow it down a bit more? tx in advance


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## firedawgsatx (Jul 15, 2013)

Please post the model number of your furnace and type of outside unit (heat pump or straight a/c). I would suggest you locate the control board inside the furnace and do the same voltage tests you did at the thermostat on the corresponding wire terminals on the control board. In other words test from "R" to "G" to see if you have around 24V, from "R" to "Y". and etc. It sounds like you have a problem between your control board at the furnace and your 24V going to the contactor. The 24V to the condenser contactor come from the "Y" terminal of the furnace control board and the "C" terminal which is the common wire from the 24V transformer at the furnace. Those tests will get you started in the right direction. You can also try to place a "jumper" wire between the "R" terminal and "Y" terminal to see if the compressor starts up outside. Once you post your furnace model number it will give us something to help be more precise.


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## mghaffer (Sep 21, 2013)

furnace model GB5BM-024K-A, compressor is MSH4BD024K i think it is a nordyne (maytag?). I did the voltage test once before but couldnt remember all them, so i ran it again, will attach a few pics from the furnace area. Furnace had c, r, g, y/y2, o, w1. c gave 26 to all but w1, all others gave 26 to w1 and 0 to all others. I also jumped y to r, pulled y off the board and directly connected it, while loosening r it did spark a little and i could hear clicking from the right side (assuming the furnace itself as the air handler is directly to the right of the board and assort lines). the thermostat was on entire time so its delay should not have impacted, i left r/y jumped for about 7 or 8 minutes with no effect. will post some of the pics as soon as i get them loaded up.


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## gregzoll (Dec 25, 2006)

Check that fuse at the bottom. Also check for any nicked or cut line at the outdoor compressor. If you have a tone tester, you can check for continuity from the thermostat to the furnace on all wires, and from the furnace to the outdoor compressor for continuity on those low voltage wires.

If you get continuity, you can rule out a cut wire somewhere. If you do not get continuity, you then have a problem with the Low Voltage wiring, and need to pull a new run to replace the section with a bad pair or bad wire in it, unless there is a spare available in that wire run.


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## mghaffer (Sep 21, 2013)

had to reload pics, forgot to add that in with them on the edit, fuse shows no signso f burning or damage. I didnt see any obviously damaged wires anywhere that i have been yet, i have been keeping an eye out for that. and how would i run the continuity test on the individual wires? i only know how to check the ohms on a full circuit not individual lines and i dont know which one to expect what with if i were to just try them like i did the voltage check. ill run it like the voltage check for now but will retest if i am doing it wrong. and i think that there are some spare wires in there from outside to furnace, not sure from thermostat to furnace.


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## mghaffer (Sep 21, 2013)

checked ohms from thermostat, y-r 474 ohms, y-c 474 ohms, y-w1 486 ohms, r-w1 15, r-c 1.4, c-w 15, all others 1 to 1.4 kohms, from furnace y-c 473, y-r 473, y-w 1.4kohms, y-o 486 ohms, r-c 1, r-o 14, c-o 14, w-g 2kohms, all others 1 to 1.4 kohms. On the thermostat i have y2 and w2, they are not there on furnace, but i do have o on furnace and not thermostat. it looks like y1/y2 combine to hit furnace as y. went ahead and checked ohms on contactor while i was at it. a1-a2 17.2, L1-T1 .1 i think it was, L2-T2 and L1-L2 both OL, dont know if supposed to read that way or not but figured was already metering.


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## gregzoll (Dec 25, 2006)

Get something like this







http://www.amazon.com/gp/product/B000LYTTRK/ref=oh_details_o09_s00_i00?ie=UTF8&psc=1 This particular one runs around $33, but worth it, when trying to chase down open wires on a circuit. You keep the Black lead connected to one wire, then connect the Red lead to each wire that you are toning out, to find the bad wire.

As for reading Ohms, that really is not going to tell you anything, unless you are looking for poor insulation, that could be causing current bleed between wires.

As long as you have 240vAC at the Outside unit, 120vAC at the indoor unit, you then have to make sure that when the contactor closes on the outdoor unit, that the motor kicks on.

Really the only things that would cause the outdoor unit to not work, is a bad disconnect outside, a bad breaker inside, a blown circuit board in the furnace or air handler inside, a bad contactor, a bad capacitor, cut control wiring from the furnace to the outdoor unit, bad thermostat, or bad wiring from the thermostat to the furnace.

First thing to check is all wiring visually, then check for the voltages (bad disconnect, or it was reversed when put back in after maintenance, or someone stole the pull for the copper, or bad breaker in the panel inside), then check capacitor on the outdoor unit, to make sure it is not bulging, then check if the contactor is moving, when there is a call for cool.


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## mghaffer (Sep 21, 2013)

Ill check the items listed tomorrow and then try looking for the continuity tester. Will let you know how it goes. Tx for the help so far


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## beenthere (Oct 11, 2008)

Your heat pump probably has a low pressure switch. Your system may be low on charge, and the safety is opening up and breaking the 24 volts to the compressor, to prevent damage to it.


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## mghaffer (Sep 21, 2013)

checked a few more things, pulled the 5 amp fuse and metered it just to be sure, it does check good (the one on air handler), so went to outside unit to try and meter it as it was the only remaining part that i didnt do last night. Metered again from a1 to a2 on contactor to be sure, 0. so followed the lines back to the board, metered at board and 0. checked the low voltage lines coming into the board, from power (r) to common i am getting 26 volts. So i tried to do a defrost diagnostic test per the instructions on the back of the panel. Set the system into heat, 5 minutes later i can hear it click out on the unit outside (did the same when i turned in on to cool earlier this morning). jumped the system t2 to dft in the test section and then test from test section to common on the low voltage side coming in. within a few seconds i got a click but nothing happened. I am right in presuming that my board is bad?


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## gregzoll (Dec 25, 2006)

Possible. You are doing everything correct, in troubleshooting this problem. Here is a pdf of the replacement parts, and the parts #s. http://www.foxmacon.com/craig/MAYTAG/SPLIT HP/MSH4BD.pdf I am surprised that the Service manual is not coming up online, when I try to search for it. Any other unit out there, you can find the manuals for them, no problem.


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## beenthere (Oct 11, 2008)

mghaffer said:


> checked a few more things, pulled the 5 amp fuse and metered it just to be sure, it does check good (the one on air handler), so went to outside unit to try and meter it as it was the only remaining part that i didnt do last night. Metered again from a1 to a2 on contactor to be sure, 0. so followed the lines back to the board, metered at board and 0. checked the low voltage lines coming into the board, from power (r) to common i am getting 26 volts. So i tried to do a defrost diagnostic test per the instructions on the back of the panel. Set the system into heat, 5 minutes later i can hear it click out on the unit outside (did the same when i turned in on to cool earlier this morning). jumped the system t2 to dft in the test section and then test from test section to common on the low voltage side coming in. within a few seconds i got a click but nothing happened. I am right in presuming that my board is bad?


No, more like its low on charge.


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## mghaffer (Sep 21, 2013)

Pulled the board, accidently jumped t2 from test section to t1 and compressor kicked on. After pulling board found 3 dead tree frogs behind board, one very black burnt one. The board on back side shows obvious signs of damage, will attach picture. Cant install one today tho as all the stores that carry are closed so will update after I get a new board. Tx again so far for help.


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## gregzoll (Dec 25, 2006)

That could have been a problem. Cooked Kermits. Before you install the new Circuit Board, place something like this http://www.lowes.com/pd_96175-1410-...&currentURL=?Ntt=weather+stripping&facetInfo= (split in two sections, so that you have just a single bubble around the edge), to keep them from getting back behind the board again. You can also use Lexan over the top of the board, as a box, that would also keep the critters from crawling on top of it, or behind it. Just have enough space, that the board will breathe.

You would have to cut the Lexan in sections, to form the box cover, to go over the board, but that is just a suggestion, if you have a really bad issue with Kermit wanting to get in there for spiders, etc..


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## Doc Holliday (Mar 12, 2011)

I want pics of frogs...


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## mghaffer (Sep 21, 2013)

So I oredered the new board and replaced it. No good, so bypassed the low and high pressure sensors, no good. I quit and called repair tech. He came out this afternoon. Turns out it was the board, he bypassed the board and she started right up. Turns out I got a bad board from the store so going to trade it back in to fix it. Tx for help again. 
I will def look into sealing it off, tx greg.
Doc wish I could show u a severly charred kermit, but the severly hungry fire ants from hell ate all three of them the first 24 hours I had tosses them on ground.


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## Doc Holliday (Mar 12, 2011)

Your board wasn't bad, you just talked too much.


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