Ducane MPGA-05083 blower not running

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Old 12-02-15, 09:48 PM
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Ducane MPGA-05083 blower not running

I am trying to help a friend with her furnace. It's a 1997 model Ducane MPGA08053, natural gas, down flow unit. Everything works fine up to the point where the blower should start circulating warm air. The fan does not come on, and shortly thereafter, the furnace overheats and the temperature limit shuts everything down. However, I can switch the fan from "AUTO" to "ON" at the thermostat and the fan will run normally. Switch it back to "AUTO" and the fan shuts off. For now, I am having her run the fan on all the time so she will at least have heat. I have been unable to find any wiring diagram or other service information on this model. I'm having to figure it out as I go. It uses the Honeywell Smart Valve. The control board does not have a diagnostic light function included, so no help there. So, what controls the blower? Is there a fan limit switch somewhere, or does the dreaded Smart Valve have anything to do with signaling the control board that it needs to think about starting the blower? I've seen a lot of troublesome Smart Valves in the past, but I can't see where it would have anything to do with whether the fan runs or not. I don't see anything that I recognize as a limit switch. Without a wiring diagram, I'm kind of flying blind. Any advice would be appreciated. Thanks in advance.
 
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Old 12-02-15, 11:06 PM
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Welcome to the forums.

Unfortunately that furnace uses a blower fan that is controlled by a timed function off the 80M27 control board. It looks like you'll be replacing the control board... or having it done.
 
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Old 12-02-15, 11:21 PM
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Rats!

Ok, Pete, thanks for the reply, and the info. I had just about decided that was going to be the case. I have no problem replacing the board, if I can find one. Being an electronics tech, I may try to analyze the board and see if I can repair it. It may be nothing more than a bad relay or relay driver transistor on the board. I didn't want to dig into it before I knew for sure it was the problem. Now that I know the board does the control function for the blower, I can do a few more checks and see if it's doing it's job. For future reference, do you know where I can get a diagram of this thing? I have found absolutely nothing on the internet about it. I can make one with some tedious wire-tracing, but I'd rather just buy one if I could only find it. Thanks again.

Dave
 
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Old 12-02-15, 11:58 PM
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The boards are proprietary and NO schematics are ever released on them. The problem is going to be in the timer section. Not a relay problem. The area I circled is part of the timer. The jumper pins select blower shut off time.

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Old 12-03-15, 12:37 AM
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Oh, I didn't word that very good. I meant a wiring diagram of the furnace wiring, not the control board. No, that data is never made available. But for the rest of the furnace, I've usually found a wiring diagram on the inside of the front cover of most furnaces, but not this time. Of course, the owner has no idea where any docs on it are, or if she ever had any.

I can already make some assumptions just from the picture you sent. I suspect one of those 2 large on-board relays will be what operates the fan, and it's possible it's coil is open, or the contacts are bad. If it's actually in the electronics of the board, that might be a little tougher to figure out, but if the board is bad, I don't have anything to lose by trying.

So, if I've got this right, the control board sends a call to the valve for heat, then starts a timing period before starting the blower, and just assumes the exchanger will be hot enough by then? Then there must also be a timer function that keeps the blower running for a pre-set time after telling the valve to shut down? Oh, I see now...you said the jumper sets the SHUTOFF time, so the ON delay must be hard wired into the board. Got it.

Again, I appreciate your input. It's given me enough info to do some more testing now. Thanks.

Dave
 

Last edited by Good Samaritan; 12-03-15 at 12:39 AM. Reason: add clarificaton
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Old 12-03-15, 12:49 AM
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You have it correct.
I'm looking for a better manual. The wiring diagram will also be on the back of the blower door.

Ducane MPGA manual.pdf
 
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Old 12-05-15, 09:07 AM
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Could it be the smart valve?

I finally found the wiring diagram in the furnace, on the back of one of the other front covers. It's basically the same as in the link you sent me. Can you tell me what I should see on the blue wire (EFT Output) from the Smart Valve? Should that be 24 volts when there is a call for heat, or when the burner is on, or what and when? Does "EFT" mean Electronic Fan Timer perhaps? If so, I'm thinking maybe I'm missing that signal from the smart valve, rather than a bad board. The relay itself on the control board seems to be good, since the switch on the t-stat appears to just bypass the timer on the board. Of course, I realize it still could be a bad timer circuit on the board. Just trying to eliminate as many things as possible via testing before I dig into it. I know those smart valves have a history of being troublesome.
 
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Old 12-05-15, 11:11 PM
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I don't know. I'm not fully up on the smart valve system. It looks like there should be 24vac on the EFT line as soon as the burner lights.

Some good info on the smart valve.... although not a direct answer to the question.
Honeywell/combustion/universal_smart_valve/install/69_1270.pdf
 
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Old 12-06-15, 05:21 PM
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That's the way I take it. I put my voltmeter on the EFT lead and cycled the furnace with the fan switch in AUTO postion. The pilot lit, the burner lit, but nothing ever showed up on the EFT lead, and finally the high limit shut the burner down. So, it looks like a bad smart valve to me. I seems a valid test would be to jumper 24vac to the EFT lead going into the control board and see if the fan kicks on after the delay. It's looking more and more like a fault in the smart valve. I understand the smart valves are notorious for cold solder joints on the board, as well as the little spring fingers that connect power to the solenoid coils losing spring tension or breaking off. If jumpering 24vac to the EFT lead causes the fan to run, then I'm going to disassemble the valve and see if I can find why there's no 24vac on that lead. It would seem that it would just mirror the 24vac going to the coil on the valve, in which case about all it could be is a poor solder joint, broken wire, bad pin on the connector, etc. I'll post the results and final resolution when I know what it is. Thanks for your help.

Dave
 
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Old 12-06-15, 06:15 PM
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It seems a valid test would be to jumper 24vac to the EFT lead going into the control board and see if the fan kicks on after the delay.
I agree with that diagnosis.
 
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Old 12-11-15, 06:16 PM
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Got it!

Resolution:

I got it fixed today. It wasn't a bad board, nor was it a bad smart valve. What I finally found was a bad connection on the EFT Output wire in the square 4-pin connector that plugs into the valve. At some point, it had come loose and when the plug was pushed into the socket, the pin pushed out the back of the plastic plug and was not contacting the mating pin in the socket. Once reinserted into the plastic housing and carefully plugged back in, everything worked just fine. It has apparently been that way a long time as the owner stated that this fan problem has been intermittently showing up from time to time over the past few years, before going completely dead a couple of weeks ago. It's probably been a fault in progess ever since the last time that plug was inserted, perhaps since manufacture, who knows. Thanks, Pete, for your help. It turned out to be a "free" repair, the best kind.

Dave
 
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Old 01-05-16, 09:23 PM
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Ducane gas furnace won't heat, blower runs continuously until power reset

More problems with this Ducane model MPGA-05083 with Honeywell smart valve. Furnace works correctly most of the time, but intermittently homeowner finds temp in house well below thermostat setting, blower fan running and blowing cold air. Turn thermostat or power off for few seconds and back on, furnace cycles properly, brings house temp back up to setting, and works properly for hours or days, then fails again. Once failed, blower will run continuously until thermostat is turned off or power is cycled by turning breaker off and back on.

Analysis: Smart valve should not be sending EFT signal to control board to cause fan to run if it has not proved the flame, of if flame fails during operation. Does this in itself indicate a failed smart valve? The control board can not be the problem because the furnace would go ahead and run through the heat cycle even if the blower fan were stuck on. I know this to be the case because I have seen her run the fan in the ON positon on the thermostat and have observed the furnace going through full heat cycles repeatedly while the blower never shut off. I have not been able to catch it entering the failure mode, so I don't know what happens leading up to when it fails. Have only been able to observe it after it has failed.

Question: Is my analysis correct? Won't the EFT signal be lost if the flame is lost before the heat cycle is complete? Also, what failure mode would cause the EFT signal to be supplied to the control board even though flame had failed? What checks can I make to pin this down, or prove the smart valve is bad before ordering a replacement? It's too expensive to use trial and error.

Thanks in advance.

David
 
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Old 01-05-16, 09:32 PM
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Without going back and re-reading the thread..... it sounds like the furnace is going into high heat limit. That mode shuts the burner down and keeps the blower running.

How's the filter ? Are all the registers open and returns not blocked ?
It boils down to not enough airflow thru the furnace.
 
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Old 01-15-16, 06:41 AM
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Thanks for the reply. I really should have posted a new thread I suppose, since the original problem was solved. But, having said that, to address the current issue, I have come to the same conclusion as you. I have changed the filter. It was dirty, enough that I suspected it to be the problem. It worked for 2 days, then failed again. I have measured the temperature at the intake to the return air, 70 degrees at the time, while the output at a register about halfway between the one closest to the furnace and the one farthest was 110 degrees. That 40-degree differential seems high to me, which would indicate insufficient air flow through the heat exchanger. That only leaves the air conditioner evap coils in the path, and the original installer left no way to get to them. It's a downflow furnace and is going to require disassembling the furnace to gain access. Or cutting into the box. Neither one a good option. I was hoping for a different opinion, one that would be easier to check. It's a very intermittent problem, and I have never been able to catch it failing, only after the fact. This model has no indicator light of any kind on either the smart valve or the control board, so it is giving me no help at all. What is a normal temp differential between return air and register? Is that a valid test?

Thanks.

David
 
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