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Fixerguy123 07-09-2012 11:03 PM

A/C Issue - No ground at PC board terminal
Lennox G20Q3/4E-100-3 furnace.

No ground at Compressor ground terminal on PC board inside furnace lower section.

24V found at Y wire / positive to compressor terminal with meter grounded to chassis when T-Stat in in "cool" mode.

Jumper from Chassis (internal transformer) ground to compressor ground wire makes compressor function normally.

No issue with heat side. All wires from T-Stat test as normal.

Can I assume that the PC board in the furnace needs to be replaced?

scottmcd9999 07-10-2012 05:19 AM

With the tstat calling for cooling, if you jump from the Transformer COMMON to the COMMON wire routed to the outdoor unit, the system comes on? If so, then the board is probably bad.

Before doing that, I'd jump Red + Yellow at the tstat, just to be sure you're checking in the right place. If you jump those two and the system comes one, then the tstat is bad. If not, and if you still must jump the Common wires, then I'd have to say the board is bad.

Marty S. 07-10-2012 06:17 AM

A break in the common wire going to the AC is more likely than the control board being bad.

Fixerguy123 07-10-2012 08:31 AM

I meant Common
Sorry...more experience in Auto wiring than HVAC. I forgot we were talking about AC voltage. I was using DC terminology.

I verified proper function of the T-Stat by checking continuity across appropriate terminals on the back of the stat during their various functions.

Re-verified at the terminal strip end of the wire before proceeding.

Yes, It is the common to the compressor that I jumped to the chassis.

I haven't pulled the board yet for inspection of the traces and can't seem to find a schematic online. Is the "C" terminal only a common for the compressor?

All other functions of the board seem normal. If only this one connection is bad, do I have to replace the board or can I just make the chassis contact more permanent? The Transformer common output is already grounded to the chassis.

This is a friend of mine's furnace. She doesn't really have any money for this repair (which is why I got the call in the first place) If I can avoid buying the board it would help her out.

Fixerguy123 07-10-2012 08:33 AM

Nope..not it

Originally Posted by Marty S. (Post 962125)
A break in the common wire going to the AC is more likely than the control board being bad.

I eliminated that posssibility first.

scottmcd9999 07-10-2012 09:20 AM

Common from the board is just a straight circuit from the transformer common wire to the C terminal on the board, so you could essentially connect the Y terminal directly from the tstat to one of the control wires going to the outdoor unit, and connect the other wire from the outdoor unit directly to the C wire on the transformer. On most of those boards, the Y terminal is just a dummy, and the C terminal is a straight circuit., so bypassing it wouldn't really hurt anything if it works. I'd be more concerned about why the board doesn't work. It's rare that only the C circuit of that board would go bad, so you'd have to assume that something else is going on ...

The C terminal on the board would provide common voltage to the outdoor unit, and it could also provide common voltage to other items (humidifier, sometimes the tstat, etc etc)

Fixerguy123 07-10-2012 09:26 AM


The compressor common is the only wire hooked up to "C" on the BCC.

I'll just disconnect it and sheet metal screw it to the chassis.

That'll make her happy.

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