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Discussion Starter · #1 · (Edited)
I noticed the fan was turning itself on and off every few seconds. Before I could turn off the system, it turned itself off for me (thermostat control display went dark and system was off, no breakers tripped). I did a search and saw mention of the capacitor being a culprit for the fan off/on issue. I replaced it with a new one but before powering everything back on, I checked around for anything else that might be visibly damaged, etc and noticed the 5 Amp fuse on the controller board was blown. From what I could read on it, 5A 36V, so I purchased replacement fuse.

Thermostat is set to off position, fan is set to 'auto/off' position. I turned on the main breaker and then went over to the HVAC and turned on both breakers together and the fuse blew immediately. I then tried the reverse, turning on the HVAC breakers first and then the main, same thing.

Now I'm trying to figure out what might be causing the fuse to blow (blower motor bad?) and am here looking for some guidance for this HVAC newbie armed with a multi-meter... :biggrin2:

Brand: York (heat pump/electric)
Model: F+RP036H06A circa 1999

Thank you,
WD
 

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That’s a 24V circuit.
Your short will be in the control circuit. Not in the high voltage circuit. The capacitor doesn’t have any bearing on the control fuse blowing.
 

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Discussion Starter · #3 ·
Hey RN. Yes. I replaced the capacitor because (aside from being 20 years old) what I was finding related to the motor starting and stopping every few seconds was a bad capacitor. It was while i was checking the rest of the boards, wires, etc, I noticed the blown fuse.

Ok, so now that it's narrowed down a bit. How do I go about finding the cause of the short/location so I might solve the issue? Thanks!
 

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Use a meter with power off and check resistance between transformer output (leading to R thermostat wire) and ground.

Either you have a short to ground in the air handler, outdoor unit or thermostat/wiring or you have a shorted component.

You can disconnect the tstat and low voltage outdoor unit wires, see if it blows to determine if the short is inside the air handler or not. But I don't like this testing method - waste of fuses and hard on the transformer.
 

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Discussion Starter · #7 ·
I get 0 when R (Red wire) and ground are tested for resistance. So something is shorted. What should I disconnect to try and isolate the issue? Ty.
 

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Since it blew the fuse when you had all loads turned off at the thermostat, you need to disconnect the red wire from the thermostat end and at the air handler end and check for continuity from the red wire to all the others, and also to ground, to see if a short exists.

If no short exists in the thermostat wire, check the red wire going to the outdoor unit for a ground condition. The defrost board on the outdoor unit may be the culprit.

Red to ground is a valid test procedure since the 24 vac transformer has a jumper to ground on the C side of the transformer.
 

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Discussion Starter · #9 ·
ok. So it appears that the outside unit, or wire to it, is causing the short as the red wire (R) that leads to the outside unit shows near 0 resistance while the other wire going to the thermostat isn't. I have to grab another pack of fuses tomorrow to test if everything seems ok when that outside leading red wire is disconnected. Anything else I should try/do before I test it that way? Thank you.
 

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Save the fuses and use your multimeter. The ohms reading is all you need to find the short. That red wire will have a value well above zero when you unlug whatever it is that's shorted.

I'd start by checking the coil on the compressor contactor. Remove either one of the wires from the coil and see if the resistance of the R line is no longer zero.

We need the make and model of that unit to go any futher with any degree of certainty. A picture of the circuit diagram that is on one of the covers somewhere would be good too.
 

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It could also be a short in the control cable going to the unit since it's outside and the squirrels can get to it. Inspect it closely.
 

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Discussion Starter · #12 · (Edited)
roger. I'll get model # in am, it is also a YORK unit. I'll also unplug the wire to track down the short further, scan the wire itself and report back. Thank you for your help tracing this.
 

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Discussion Starter · #13 ·
Status update...

I disconnected the red wire leading to the heat pump. I checked the wiring and it looks completely intact. I tried to disconnect the connector to the board outside but it seemed to be pretty tight on there and I didn't want to damage the board (if it is still working).

So with that said, I left the outside unit disconnected as well as the red wire from the inside control board. Reconnected everything else and replaced the fuse. Powered it up and it didn't blow the fuse. The fan runs ok. So this brings me to the question: since the outside heat pump is disconnected, would it be bad to turn on the heat setting? Do I set it to the normal setting or use the AUX/Emergency heat setting?

I've attached the outside unit's model # and 2 control board pics, 1 with board # and one with the wire circled that appeared to be really tight on the board and ties in to the wire from inside unit. Thank you!
 

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Unplug the compressor wire from the board (see picture below) and see if the short disappears from the red circuit. If so, it will pretty well mean that the compressor contactor has failed and the operator coil on it is shorted.
 

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Discussion Starter · #15 ·
The wire still comes up as shorted. So likely the board so I then pulled the board off and attached a pic of the area (blue resistor upper left corner) that looks like it blew (i couldn' tell before as that area was shadowed). But the compressor etc could still be an issue no? Compressor capacitor? Have to test those.

Thanks!
 

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The resistor you referred to is a 2000 ohm one so check it with your meter. It looks OK to me but pictures can foolers. It wouldn't be possible to fry that value of a resistor with 24 volts dead shorted through it so I doubt it's the problem component.

The compressor and the capacitor are not the problem either, those are line voltage components. Your problem is on the class 2 low voltage control system.

You can't condemn the board until you unplug all the loads it feeds to get them out of the list of possibilities.

The reversing valve needs to be unplugged and also the line going down that energizes the heaters. Any of the low voltage control wires could be nicked so check for that as well.

With everything unplugged, the R terminal shouldn't show zero ohms to the C terminal for sure. That would indicate a failed board component. That isn't ordinarily fixable unless you can troubleshoot down to the discrete level and replace circuit board components using special soldering equipment and more than a little technique.

If the board is proven to be bad, you will still need to be sure nothing else failed to cause it to fry lest you risk frying the replacement as well. Check the resistance value of the compressor contactor coil and the reversing valve. They will likely be somewhere around 30 ~ 40 ohms.

When you get the new board installed and all wired up, check the red line once more to be sure the zero ohms reading is no longer there before running it for a test.
 

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Discussion Starter · #18 ·
Been swamped with work. Hoping to further test the outside components in the next day or 2 and report back. ty. Definitely helping having heat again, even if it is the AUX/Emergency heat.
 

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Discussion Starter · #20 ·
Apologies. Crazy over here, much like everywhere with this covid-19 stuff. Hoping to get further testing done later this week/weekend and report back. Hope everyone is doing ok, stay safe and healthy. Thank you.
 
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