Temporary Fix for A/C Blower and a Few Related Questions

[Donald100]

Below is a scenario of problems/tests recently conducted in an attempt to get my A/C Blower working - I eventually found something that worked, although I still need to get a final question nailed down before I send off for an expensive part. I started a new thread here, showing start to finish, as I only had one reply initially. Hope this might be useful to some else and catch someone's attention for
my final question. Thanks.


The blower on our central heat/air conditioner has quit working. I've confirmed the following:
+ does not work in either heat or cool mode
+ does not work with the thermostat set to FAN ON
+ does not work if I manually spin it while having FAN ON
+ the outside condenser unit works normally during the above

It seems that the blower is not getting power (no hum). Will try testing voltages if I can manage to read the wiring diagram (small print, hard to read). Any ideas? Any common problems with controller boards, overrides that might cause this? Can blower start capacitor be ruled out?
Thanks.

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Reply from Grady:

Most likely the capacitor can be ruled out since spinning the fan by hand does not help. You could have a bad board or fan motor. Tip: some equipment has a door switch which must be closed (opens when door is removed or opened) for any power to go to the blower section. This is almost always true with a gas furnace.

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Grady

Thanks for the reply. I've done a little more testing since my first post:

+ 120V at access panel switch (the switch you mentioned); I did make sure it was closed in my earlier tests
+ no (or low) voltage at controller board pin for power to fan
+ jumpered 120V to fan, fan seems to run fine (no problem with capacitor)
+ checked water pan switch – ok
+ check 5A fuse - ok
+ swapped out my current programmable thermostat with an older White-Rodgers mercury switch type - no change

What to do now?

+ beginning to suspect the control board on the furnace/blower (HW ST9120C, electronic fan timer). Is there a simple way to confirm bad? Do control boards fail frequently?
+ other ideas?

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More Testing

+ Bypassed level switch – no change
+ Checked 24VAC transformer output – 29V (ok)
+ Checked G (Cooling On) – 28V (ok)
+ Checked Cool Out Terminal (power to blower) – 0.1 to 3.2V, waited >2min (should be 120V?)
+ Checked unused Continuous terminal (thought I might use this if it works) – initially 120V, then 0.1)
+ Checked another unused terminal (EAC – electronic filter?) – 120V after “2 clicks” (probably timer relays).

+ Plugged Cool wire to blower on EAC – wife is happy now (me too)!

Final Questions if someone can help:

1. Still need to fix this, so my basic question is does the above prove conclusively that the HW ST9120 board is bad (I see these things for about $160 on the internet). Want to be sure before I buy. The associated heater has a HW Smart gas valve, so presume I need to stay with this board.
2. The heating unit on also had a problem late in the spring (ignitor would not fire), but I never got a chance to investigate much. Any ideas if this could be related to the board also? Any simple checks to make?

[Maturin]

This is Maturin. I posted with what sounds like the same problem. While I'm waiting for some clarification, too, I wanted to find out about your quick fix.

Did you disconnect your COOL wire from the terminal and then connect it to the empty EAC terminal? On my schematic, it looks like my empty EAC terminal is on the other side of the transformer, but maybe not. I don't read these things very well.

When you did this, does the whole system operate properly? Maturin

[Donald100]

Maturin

That's exactly what I did (I did check voltage levels before connecting). I have cycled it on by lowering the thermostat, waiting a bit and raising it. It even seems like the delays might still be working (and/or some are built into the condensing unit). The one thing I don't know is if there is an amp capacity limit on this particular connection.

[Maturin]

I've got 7 wires coming out of my fan motor (8 including the ground). 2 brown ones going to the capacitor. The others to terminals on the board.
Black to "COOL-H"
Blue to "HEAT-H"
Red to "PARK"
Yellow to another terminal marked "PARK"
and White to a terminal marked "CIR-H" in a different place on the board.
How did you connect your jumper wire...from what to what?

Thanks

[Donald100]

My blower motor had the following:
+ 2 brown leads to the capacitor (same as yours)
+ 4 other connectors: Blue to Cool power pin on ctl board
White to Neutral
Red to Heat
the remaining one is not used (for
another speed)
I connected the EAC terminal on the ctl board using the existing Blue wire already connected to the"Cool" connector on the blower motor.

[Grady]

Not being totally familiar with that board I can't say for sure if it has anything to do with the ignition or not, but probably. One thing you can do is to unplug the ignitor from the gas valve & check for continuity. An open reading would indicate a broken ignitor.
From all of the testing you have done, it certainly sounds like the board is bad. Just make sure you have 120 going to the board as well as 24 if there is a separate transformer.