Carrier gas heating furnace: no 24Vac

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Old 01-07-18, 08:36 PM
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Carrier gas heating furnace: no 24Vac

Hi All,
Suddenly our heating unit gave up, I see it has the hk42fz013 control board. 110 VAC is going from the panel interlock switch to the "A" relay top terminal, i believe this is the common terminal. But nothing comes out - when we turn on the fan from the thermostat, I do not hear the relay click. Hence, no 24V AC coming from the transformer. The LED is dead off - no error codes.
I have powered the transformer directly with 110 and I see its output has ~ 24 V AC.
Also, if I connect a laptop DC power across the Relay A, I hear the relay click and I measure 0 ohm between COM and EAC-1, but still open between COM and PR-1. I would think PR-1 is the input to the transformer.
I am completely confused, as I do not have any schematics of this board. I see the relay A coil is connected to the micro controller via the darlington - but what is preventing this control signal.
 
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Old 01-07-18, 09:25 PM
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Welcome to the forums.

Those two big relays.... A and M are for the blower motor. The other two smaller relays are for the draft inducer and igniter. There are many variations of that old board. I can only troubleshoot them by sight. Some of those boards supply 120v to the transformer and some don't. If the transformer primary is connected to the board.... that's where you need to start checking.

The 120v connected to the relay goes thru the relay and is soldered to the board. It only switches the blower.... the power to the board should still be there. Check the foil from below the relay to the PR-1 terminal.

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Old 01-07-18, 10:12 PM
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Board Photo

Thanks for your reply.
This is how I found out the A relay popped out of the board when I opened it. Now I see what you mean, I can see the 110V is coming to the COM QC on the A Relay from the interlock switch. So the COM terminal should be directly connected to PR-1 terminal by a Copper on the board?
Is it possible to find a schematic of this board?
Thank you once again.
 
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Old 01-07-18, 10:21 PM
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A Relay

Forgot to mention, that I did solder the relay back on the board. Now I realize I should have ohmed out the connection first. I could energize the coil and see the COM Quick Connect - short to the EAC-1 pin. But no connection what so ever between the top COM QC and PR-1 Pin. This tells me that the relay is busted.
Just to confirm- The PR-1 and top COM QC should be short - no matter relay is energized or not?
 
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Old 01-07-18, 10:36 PM
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The PR-1 and top COM QC should be short - no matter relay is energized or not?
Yes.

It's not COM...... it's L1 (AC 120v hot).

I didn't see you had replied. I added a picture to my first post.
No.... no schematics available.
 
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Old 01-08-18, 10:59 AM
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Getting the relay tonight

I am getting the relay from amazon tonight - I will put solder it on and update.
Thank you
 
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