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Nordyne MGHA-090ABPQ-01 T-stat A/C on/off

Posted: Sat Jul 23, 2011 11:37 am
by Schoolie
I have a Nordyne model MGHA-090ABPQ-01 furnace with A/C. The control box on the front of the furnace has a Heat-Off-Cool switch and a Fan On-Auto switch. The furnace also has a System On-Off-Fan switch.
Terminal connections on the control box are C, Y, R, and W (no G)
Connections on the new digital t-stat are W, RC-RH(jumpered), Y and G (no C)
The A/C unit outside is connected to the C and Y on the furnace control box. T-stat W, RH/RC and Y connected to furnace W, R and Y.
Problem is that the only way to get cooling is to put the furnace control switch in COOL position which manually turns on the blower fan and the A/C unit. House gets cool, but is not controlled by the t-stat. Cooling stays on all the time until you manually put the furnace switch to OFF. Connecting the t-stat Y to the furnace Y, and putting the Heat-Off-Cool switch on the furnace in OFF does not work. t-stat calls for Cooling, but furnace does not react or come on.
Seems like if there is a Y connection on the control box, t-stat should control cooling cycle, but not sure with this older furnace model. Is there possibly a relay not working in the control box that I need to replace? Or, is there a conversion kit that I need to buy to make the cooling t-stat controlled? Or do I need to invent something myself with a 24v relay to jump around the COOL switch, or better, put in series with the COOL switch wires, and use the t-stat to control the relay? If so, not sure which wires on the t-stat I hook to the relay coil to energize it. Any suggestions?
Thanks! -- Schoolie

Re: Nordyne MGHA-090ABPQ-01 T-stat A/C on/off

Posted: Mon Jul 25, 2011 8:21 pm
by Bill
First off lets go over how a thermostat wires up. For a heat(gas) and cool thermostat you have 4 or 5 wires that need to hook up. R, Y, G, W, and sometimes depending on the thermostat C. The R terminal is for power to your thermostat. The Y terminal is to control your A/C. The G terminal is for the furnace blower. The W terminal is to control the heat. All thermostat's work off of 24 volts. (unless you bought a line voltage stat) So with that being said and what i read on your post you only have W,Y,R, and C, for low volt connections. I would like to know how your old thermostat is hooked up. The wires coming from your a/c and go to your Y, and C, is right so that is not the problem. Next question for you does your heat work right? I guess once you can get some more info i will be able to help more.

Re: Nordyne MGHA-090ABPQ-01 T-stat A/C on/off

Posted: Tue Jul 26, 2011 11:05 am
by Schoolie
Hi Bill,
This is my daughter's house, so I have to travel back there to do further investigation.
I am going to go back, trace as many of the wires down as I can, and get as many voltage measurements as I can in all modes.
She purchased the home during cold weather and so this is the first summer there, and now dealing with cooling the house.
To answer your questions:
The furnace does work in Heat mode with the new or old t-stat. The old t-stat was a mercury bulb type. The t-stat R and W are connected to the furnace R and W and, with the furnace control box switch in the Heat position, and the pilot light lit, everything works just fine. Stat calls for heat, furnace fires up, blower comes on, heats the house, flames goes out and blower stays on for a minute or so to cool the plenum then shuts off. All OK.
In the Cool position, the blower fan comes on, the A/C compressor outside comes on, the house cools just fine. Just never goes off unless you turn it to OFF position.
The Y on the old mercury t-stat was originally not connected by the previous owner.
My daughter connected the t-stat Y to the furnace Y, and of course it did not work.
Thinking it was the old stat, she purchased the new programmable one and hooked it up, R to R, W to W and Y to Y. Tested the Heating cycle and everything works fine. But still no cooling control. In Cool mode on the t-stat and temp set to below room temp, the t-stat does click and call for cooling, but nothing happens at the furnace. Time to call Dad.
The Heat-Off-Cool control box switch on the furnace is a 2-pole-double-throw rocker switch. It makes one connection on one pole in the Heat position. The Heat position of the other pole is unconnected.
It makes two connections in the Cool position. I have not located the wiring diagram anywhere on the furnace, yet, but I am presumming the two Cool connections are, one turning on the Fan full time, and the other pole energizing the Y connection to energize the A/C compressor relay.

Without a wiring diagram, I do not know if the Cool switch is energizing a Fan Relay to turn on the fan, or sending 120v directly through the switch to the fan motor. If there is a fan relay, as I suspect, then I could make my own G connection for the furnace (your thoughts on that?)
But still need a control relay to control sending 24v to the A/C unit to turn it on/off. (A/C unit does not have its own 24v transformer)
My thought was to break the two connections on the cool side of the 2-pole switch and run them through a 2-pole-SingleThrow relay, with a 24v coil, and control the relay with the Y connection off the t-stat. T-Stat in Cool mode, furnace switch in Cool position, 24v out of the t-stat Y wire when calling for cool to energize the 2-pole relay, which will then energize the blower fan and the A/C unit. Round about way to do it, but simple and it should work. But just wondering if there was a more appropriate way.
One final question: Do you have, or can you get, a wiring diagram for this Nordyne furnace that I can purchase? Thank you for your time and thoughts on all this.
- Gary

Re: Nordyne MGHA-090ABPQ-01 T-stat A/C on/off

Posted: Tue Jul 26, 2011 11:23 am
by Schoolie
One other thing I just thought of: When the funace control switch is in Cool position, I get 27v across the C and Y terminals. But that is to be expected, since the A/C compressor does come on with the furnace switch in Cool position.
A question you may have next, and I don't yet have an answer to, is am I getting 24v on the t-stat Y terminal when the stat is in Cool Mode and calling for cooling? I am presuming I am getting 24v at the t-stat Y terminal when calling for cooling, since the heating all works OK.
But, as I mentioned in my previous reply, I am going back to do further voltage measurements, and tracing of wires.
- Gary

Re: Nordyne MGHA-090ABPQ-01 T-stat A/C on/off

Posted: Wed Jul 27, 2011 1:57 am
by Schoolie
Lo and Behold, I found the wiring diagram. And solved the problem.
By the way, the top of the wiring diagram says, "A/C Blower Relay Control Model 901994. For use with a 2 wire thermostat" which doesn't make sense to me since I still need 3 wires to control both the Heating and Cooling. Not sure what they are thinking.

Looking at the wiring diagram, I discovered the two load sides of the 24v transformer is connected to the furnace W (for 24v out) and Y (for 24v return) terminals on the furnace terminal block, and not the R and C terminals as I understand is normal (correct me if I am wrong on this).

Looking further at the diagram, I also discovered that 24v on the furnace R terminal will activate the Heat Cycle relay, and 24v on the C terminal will activate the Fan Relay, as well as the A/C unit (still connected to the Y and C terminals). All three relays return back to the Y terminal to return back to the transformer (24v return side).

If I understand correctly how a thermostat works, you need 24v going into the t-stat R line to be able to get 24v out of the W line in Heat Mode, and out the Y (and G?) lines in Cool Mode.
So, I reversed the R and W lines from the T-Stat to the furnace at the furnace terminal block.
I connected the t-stat R to the furnace W to get 24v going into the t-stat R line correctly. And connected the t-stat W to the furnace R.
With the furnace HEAT-OFF-COOL switch in the OFF position (for everything), I put the t-stat in Heat Mode and turned the temp up to call for heat. The furnace heat control relay kicked on and the furnace cycled through the heating cycle OK.
I then connected the t-stat Y line to the furnace C terminal. I put the t-stat in Cool Mode, turned the temperature down until it called for Cooling. The Fan Relay kicked on, and the blower fan, and the A/C unit came on. I let it run for about 5 minutes, got nice cold air, and then turned the temperature up until the stat stopped calling for Cooling. The blower fan and A/C unit cycled off.

Amazing what you can figure out when you have a wiring diagram to look at.
I would still like an thoughts or suggestions you may have on what I have done, here.
Is it correct? Have I created a potentially unsafe condition in some way?
Everything seems to be working OK.
- Gary

Re: Nordyne MGHA-090ABPQ-01 T-stat A/C on/off

Posted: Wed Jul 27, 2011 9:25 pm
by Bill
Hi Schoolie
Well let me start off with , YOU DA MAN! Good job on the wiring job. I am pretty sure that the way you have it working is not going to cause any problems. I have a big book of furnace wiring guides and i was looking at your furnace and they way you have it wired and the way the guide shows is different. So i am glad that you got it to run.