# AC unit not working - no 24 volts to contactor



## chowtrix (Apr 18, 2017)

Hello all,

I'm in a dire situation while struggling to keep cool in the Texas heat (luckily it's not summer yet).

Our outside unit stopped working all together 2 weeks ago - compressor and fan would not turn on when calling for cooling. At first I thought it was the contactor; however, after replacing the item, it would still not energize after calling for cooling. When I press down on the contactor, the compressor & fan would start right up on the unit (although it produces a TINY spark when pushing down, ultimately going away).

I then measured the wires coming out of our house, and noticed I was not getting 24 volts to the unit. I am however getting 240v AC to the other side of the contactor.

This lead me to looking at my control board on my air handler within the attic, but I'm not even sure where to start. I checked the condensation pan, but it is completely dry. Could it be possible that the transformer is not sending 24v to the unit? I checked the fuse on the control board, but it is completely fine. Would I need to pull out all the connections on the transformer to check for continuity / ohms on my multimeter? I noticed that it has two yellow wires, a black & white one, along with the purple wires on the bottom. I'm not even sure what they go to, or how to test this, to be fairly honest. What else could be the problem? The thermostat does work and has new batteries. 

Please bare with me, I have never troubleshooted on a control board or had this issue before. I am eager to fix this issue and save on costs - sleeping in 83+ degree weather is getting a bit too much for us. :help:

Thank you!!

Thermostat wiring
https://s16.postimg.org/7rtq0ommd/17976771_10154265086906574_910361750_o.jpg
https://s14.postimg.org/9qqfofp41/20170412_204823.jpg

Control Board
https://s2.postimg.org/fi9didxl5/20170412_201120.jpg


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## kok328 (Apr 19, 2017)

Check to see if the thermostat is working and also check the 5 amp fuse on the circuit board. 
Also check the output on the transformer.


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## chowtrix (Apr 19, 2017)

Thermostat is working w/ fresh batteries (no common). The fuse looks good. How exactly would I need to check the output on the transformer?

Here's the schematics -
https://s15.postimg.org/q8s2zqzm1/20170418_210013.jpg


Here's the condensation pan with a fairly new float switch - it's dry.. and dirty.
https://s17.postimg.org/u58b2esj3/20170418_205521.jpg
https://s15.postimg.org/9nfv6cj4b/20170418_205555.jpg



Here's the first panel to the blower/furnace (left to the panel w/ control board)
https://s12.postimg.org/5ko22rxm5/20170418_205750.jpg
https://s15.postimg.org/6lp2we5zf/20170418_205815.jpg
https://s15.postimg.org/toflvk7gr/20170418_205820.jpg

Here's the second panel with the control board -
https://s15.postimg.org/cwuact77v/20170418_205803.jpg
https://s2.postimg.org/fi9didxl5/20170412_201120.jpg


Unfortunately the blue wire was not marked with "C"; however, I measured R to "C" and received ~27.5 volts, same with all the others EXCEPT for R to G. R to G = 0VAC. Is that normal?
https://s15.postimg.org/lo5wktfwr/20170418_210117.jpg
https://s15.postimg.org/d76e9wb7v/20170418_210203.jpg
https://s15.postimg.org/5fpobc72j/20170418_210205.jpg

The fuse looks good.
https://s15.postimg.org/mmho9d3uz/20170418_205916.jpg


Here's the (transformer) with 2 purple wires under, and 2 yellow wires on the top with 1 black and 1 white wire on the other side. Wouldn't I need to test this out?
https://s15.postimg.org/ev0yaszpn/20170418_205943.jpg
https://s15.postimg.org/qqn8flp6z/20170418_210453.jpg
https://s9.postimg.org/4fvqllim7/Scr...418-214515.png
https://s9.postimg.org/skwg3b2wv/Scr...418-214615.png


The blower switch is taped with the buttons in fairly decent.
https://s15.postimg.org/rhfyldrkb/20170418_210547.jpg
https://s15.postimg.org/czirde097/20170418_210551.jpg


Then the tstat calls for cooling, it will stall for about 4-5 minutes before it turns the fan/blower on, which still blows hot air and not engaging the outside unit.


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## homeremodeler (Apr 21, 2017)

If the transformer exterior is bulged or shows what appear to be burn marks, don't test the transformer. Instead, replace it.

The voltage being supplied to the primary should be labeled both on the transformer and the schematic.
The voltage being generated by the secondary should be labeled in the same fashion as the primary.


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## kok328 (Apr 21, 2017)

set the thermostat to A/C
check voltage between chassis ground and R terminal, should have 24-27 volts.
now with the system still calling for A/C, check voltage between chassis ground and Y terminal, should have 24-27 volts; if not, then the thermostat, thermostat wires, relay solenoid is bad or if applicable a fuse in the compressor disconnect is bad.


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