My furnace fan was running in AC mode for a long time but it wasn't getting cooler.
I checked the air from a register, sure enough, not cool.
I checked the outside condenser unit and it's not running.

I checked for 220V at the disconnect on the outside unit. 220VAC present.
I pushed down on the contactor and the unit started.

No 24 v at the contactor. Sure enough, or it would work.

I went inside and took the cover off the furnace, an older Day and Night unit with a pcb in the fan compartment with two relays on it.

I had trouble with this a number of years ago, the CFR relay had green flames coming from the contacts. I replaced the relay on the pcb at that time. All was fine for years and years.

I notice when I push the fan door interlock some green flames from the HFR relay, which I didn't replace. It's soldered on the board.

I have determined that at no time does 24 v get to the outside unit, or it would be on. There is 24 v in the furnace so the transformer is OK.

What have I forgotten? What should I check? I am going to get the HFR relay tomorrow and change it, but that might not be the problem.

Thanks in advance for your help.

At least check for 24 V between the common and Y terminals inside to rule out wiring.

I took the pcb out and cleaned the contacts on the relay, just as a test since I couldn't get one now.

Now, I put the unit on cool, the fan does not start, and the outside condenser still doesn't start.

Using the thermostat I put the fan on "on" and it starts. But it won't start when the stat calls for cool.

Could I have mixed up a stat wire? I don't think so.
There is (and was) no wire on the terminal labeled GH. All the rest are used on the furnace end.

Quote:

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Originally Posted by the OP

I put the fan on "on" and it starts

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Define "it". If "it" is the inside fan, then that's what it's supposed to do. If "it" is the outside unit, then you may have mixed up a wire.

Because:

In heat, the furnace controls the inside fan and in cool the thermostat controls the inside fan via the (G) terminal.

If it set the stat fan control from auto to on, the inside fan starts. If I set the stat to cool, the stat clicks, it indicates cool, but nothing happens at all.

This is different from before I took the board out, then as you recall if I set the stat to cool the inside air handler started, but the condenser never did.

With the thermostat OFF (Fan OFF), Mode OFF connect Y & R at the furnace. The outdoor unit should come on. Check for 24 VAC at the contactor coil outside.

And
Connect R and G at the furnace to see if the indoor fan comes on.

If you connect Y,R and G together you should get cooling forever.

When you lay with wires, you have to make sure power is off when you make the connections and create the compressor off delay of about 2 minutes to avoid the AC unit starting with high head pressure.

I wish I knew what had changed, though.

Well, last night I thought to look at the wires on the back of the stat. I loosened and tightened the screws. That seemed to help the furnace fan to run in cool.

Then this morning I tried cooling again. The outside unit tried to start several times but didn't. There was a short cycle timer in series with the contactor, a function that is now provided by my gee whiz Honeywell stat.

So I bypassed it and the unit runs normally.

Combination of bad relay (maybe), bad wiring/loose wire on the stat and failed short cycle timer.