No 24 volts at Control board fuse holder

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Old 08-27-18, 05:55 PM
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No 24 volts at Control board fuse holder

Hi , I am not a hvac person but knows a little about electricity , I was helping a friend out this morning troubleshoot his ac not coming on. I went to check the unit up in the attic and found a blown fuse , replaced the fuse and control led light came on but still not getting 24v to terminals on the control board , thermostat or contactor out backyard. So I decided to check primary to xfmr and read 120v , secondary read 28v but when I put my meter leads across fuse holder without the fuse I was only getting 15v. Could you please tell me where to check next or does he have a bad control board? This all happened when he was trying to install a new Nest thermostat 3rd Gen.
 
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Old 08-27-18, 05:58 PM
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Welcome to the forums.

28vAC across the secondary is good.
Measuring voltage across an open fuseholder is pointless.
You need to locate the R and C connections at the unit. Should be 24vAC there.
Is this a gas furnace or just an air handler ?
 
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Old 08-28-18, 02:02 AM
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15v between R and C and itís a gas furnace.
 
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Old 08-28-18, 02:58 AM
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If you have 28vAC at the transformer but only 15vAC at R and C....... you've probably have a damaged safety/limit switch. I'll need the full model number of the furnace. It's posted inside on the side wall near the burner.
 
 

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