I have a 1988 Rheem RGDA-125-ER furnace. I noticed over the summer that the A/C, when switched to cool, would come on and then immediately switch off if the set temp was higher than the room temp. If I programmed the set temp far below room temp and then switched on, it would run normally and I could then set desired temp and have no problems (would cycle on/off normally). I figured it was the thermostat and would replace later since I could still get it to function. Heat works normally.
Recently I noticed at the furnace that the yellow terminal has nothing connected to it and now I'm wondering if I have a wiring problem.
Current wiring: red from thermostat to furnace red, white from thermostat to furnace white, green from thermostat to furnace green, blue from thermostat to white A/C and blue ("O" or "C") from furnace to red A/C (wire is blue from furnace terminal but marking on furnace terminal is either "O" or "C" - can't really tell).
Your input is really appreciated.
Thanks,
Mike

Usually there is a 24 volt AC transformer in the furnace with the secondary winding connected to a red wire running to the thermostat and a blue wire, common, to the gas valve, A/C relay, and fan relay. From the thermostat there will be white wire to the gas valve, yellow to the A/C, and green to the fan. The thermostat is wired to switch the power from the red to the white, yellow, and green as needed with the blue completing the circuit. Most thermostats and furnaces have the contacts labeled R, B or C, W, Y, and G for the corresponding wire colors. Many have RH and RC to allow separate transformers for heating and cooling. With just one red wire, jumper them. It may be wired to have the A/C control wires return to the furnace and its controls and then a second wire goes to the A/C unit. Internal wiring may replace the green wire if the thermostat does not give you the option of fan only or continuous fan. Digital or programmable thermostats may need the blue wire connected to them.

When the thermostat calls for cooling, it switches power from the red wire to the green and yellow wires. The green runs to the furnace where it activates the relay for the blower. You must have a wire running from the Y terminal of the thermostat out to the coil of the compressor and a second wire returning to the common side of the transformer. Usually the yellow wire from the thermostat is connected to a wire running out to the relay at the Y terminal of the furnace. It does not have to be. If you have a red wire running from the C terminal of the furnace, and a blue wire connected to a white wire from the Y terminal running out to the A/C as you say, it should work. I don't quite understand the A/C coming on momentarily when you turn it to cooling if the temperature is below the set temperature. If you can easily connect the blue and white wires to the Y terminal of the furnace, you might and see if it still does it.


To do simple checks like this you do need some tools. A test light, a meter, or a voltage detector might be the best place to start with. I came across the niftiest gadget for trouble shooting, a voltage detector. They work through the insulation of wires. There are several brands. I have a GB Instruments GVD-505A, less than $15 at Home Depot. Touch it to a hot wire, and the end glows red. Find the doodad that lights it on one side, and not the other, and you have the culprit. You do not have to open up housings and expose electrical contacts. You are looking at where your hand is, not where the meter is. Most people are capable of doing repairs and will get it going and not get hurt if they use a little sense. The voltage detector makes it even easier.

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Originally Posted by WiredMike

I have a 1988 Rheem RGDA-125-ER furnace. I noticed over the summer that the A/C, when switched to cool, would come on and then immediately switch off if the set temp was higher than the room temp. If I programmed the set temp far below room temp and then switched on, it would run normally and I could then set desired temp and have no problems (would cycle on/off normally). I figured it was the thermostat and would replace later since I could still get it to function. Heat works normally.
Recently I noticed at the furnace that the yellow terminal has nothing connected to it and now I'm wondering if I have a wiring problem.
Current wiring: red from thermostat to furnace red, white from thermostat to furnace white, green from thermostat to furnace green, blue from thermostat to white A/C and blue ("O" or "C") from furnace to red A/C (wire is blue from furnace terminal but marking on furnace terminal is either "O" or "C" - can't really tell).
Your input is really appreciated.
Thanks,
Mike

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Thanks labman.
I connected the blue and the white to the Y terminal of the furnace but got the same results (condensor and fan turn on for 1 sec and then shut off).
Does it matter if [blue/white to Y terminal//red to C] or [blue/red to Y terminal//white to C]. Or is this just completing a circuit?
I have the same thermostat upstairs and downtairs so I checked the "good" one to see what would happen when switched to cool and nothing did happen (didn't here a signal click). On the questionable one, did the same thing and I could hear the signal click from the thermostat. Funny thing is, the "good" one gives signal click when heat is switched on (even though set point is below room temp) but does not turn heat on (does not give click when switched to cool). The questionable one, gives signal click on cool (maybe reason flash on/off?) but does not give signal click for heat (heat works fine). But like I said above, if I set it cool enough it will continue to run when switched to cool.
I tested with thermostats connected and disconnected from wall (didn't change the results).
All seems goofy to me.
Could a thermostat function like this if it got shorted or maybe something shorted on furnace end?
I'll go and get that tool you described to test but wanted to see if you had any other thoughts.
Thanks again,
Mike

Wire color doesn't make any difference to the furnace and A/C. It is just a convenience so we can tell what the wire is connected to at the other end. Even more convenient if everybody uses the same colors. May even be code now. I may have figured out how some things work, but I don't claim to know code.

I don't know what is going on with your thermostats either. Digital and programmable thermostats can do more weird things than a simple mechanical one.