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Old 10-30-2010, 12:11 AM   #1
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gmp075-4 won't start in heat mode but start in ac mode


Just replaced circuit board on gmp075-4 last summer and everything works fine with ac. But now thermostat in heat mode but furnace is not doing anything. The LED light just stay on and not giving any code. Pretty sure all the wires are placed in the right place since I took it out one by one. Also check the schematics and they are all in correct place. Do you think it's the grounding issue? I need some input please.

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Old 10-30-2010, 07:18 AM   #2
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gmp075-4 won't start in heat mode but start in ac mode


Simple test to check the board:

1) Remove and label the tstat wires at the board
2) Put jumper wire between R and W terminals and turn on power/press door switch in. If it works then you have crossed up your original tstat wires and need to trace them out to the tstat.

Grounding issues usually blow the board, works with AC so I doubt that is the issue.

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Last edited by yuri; 10-30-2010 at 07:20 AM.
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Old 10-30-2010, 08:20 AM   #3
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gmp075-4 won't start in heat mode but start in ac mode


I already did that on the board - put the jumper on R & W but nothing happens. It's not even attempting to start. Recheck all wire connections against schematics and they are all fine. The LED light just remains lit up, no flashing and not giving me any code. The T-stat is already ruled out. Hoping I have another new board to work with. Are there manual reset on this furnace that I should be aware of? If so where are they located?
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Old 10-30-2010, 08:36 AM   #4
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gmp075-4 won't start in heat mode but start in ac mode


They would give you an error code so I doubt they are open. Remove and reconnect all the molex plastic white wire connectors to the board. While you have them off take a magnifying glass and see if any of the female terminals inside them got pushed backwards out a bit. Can happen and make a open connection. Push it back in if necessary.
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Old 10-30-2010, 08:56 AM   #5
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gmp075-4 won't start in heat mode but start in ac mode


Have that white plastic harness connector checked and everything's fine and still no code. Thinking it may be bad board in the heating side. So there's no manual reset somewhere? The thing is the old board was thrown out already and that one works fine with the heat mode. Do you think I should get another board?
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Old 10-30-2010, 09:07 AM   #6
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gmp075-4 won't start in heat mode but start in ac mode


Hoping to find another way to check before spending more money on the board.

Last edited by archerie; 10-30-2010 at 09:13 AM.
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Old 10-30-2010, 09:14 AM   #7
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gmp075-4 won't start in heat mode but start in ac mode


Yep, buy it from a "reputable" source. There is a flame rollout switch above the burners or on the side of them and a limit switch above the burners and a spill switch on the box where the vent pipe attaches but they will all give a code if open. Static electricity can zap those boards and you have to be very careful not to bend/twist them or rough handle them. Check for continuity from the C terminal to ground/copper water line. If the board is mounted on a bracket on the fan add extra screws to it and make sure it is not loose. Common issue with losing ground that way. Check for continuity from the furnace bare metal to a copper water line with a cheater wire if necessary.
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Old 10-30-2010, 09:21 AM   #8
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gmp075-4 won't start in heat mode but start in ac mode


Thanks a lot for your info and all the help. Will definitely buy a new one. Thanks again.
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Old 10-30-2010, 04:17 PM   #9
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gmp075-4 won't start in heat mode but start in ac mode


Just to make sure that all the wirings are correct - the yellow wire from the
t-stat has no terminal at the control board and I hook it up to the red wire that goes down to the condenser contactor with 24V coil. And of course the other wire (white) is on the other side of the contactor and hook up to the common side of the 24V (secondary) of the transformer. Like what I've said earlier, the AC works fine except in the heating mode it doesn't do anything.
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Old 10-30-2010, 04:29 PM   #10
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gmp075-4 won't start in heat mode but start in ac mode


Correct. Should work that way.
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Old 10-31-2010, 06:17 PM   #11
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gmp075-4 won't start in heat mode but start in ac mode


Thanks a lot again yuri and definitely will buy another board from a reliable source and be extra careful with the board.
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Old 11-04-2010, 12:39 AM   #12
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gmp075-4 won't start in heat mode but start in ac mode


I'm having the SAME exact problem with the same exact unit. Did you ever figure out the resolution?

Kruso



Well after coming to the ends of the internet and trying every combination here is what I've found.

Checked the igniter for any cracks, none present.
Verified the pressure switch hose integrity, very good.
Connected the R + W wires manually at the T-Stat, nothing.
Sucked on the pressure switch hose while the R + W wires were connected at T-Stat and VOILA! took two seconds and lit up like the forth of July!

I'm very much so a novice at this so someone please have mercy and correct my thinking here. After analyzing the way the furnace is setup I noticed the pressure switch was connected to a larger housing toward the top of the furnace with a secondary blower fan, via a vac hose, that appears to have an airbox connected to it where all the ducts begin, which eventually end up in each room of the house.

So I initially thought that in the event of clogged vents (???) if enough air pressure builds up in that "air box", this "secondary blower" kicks in and vents the air into the atmosphere via a duct going directly to the roof.... kind of like a wastegate on a turbocharged engine. That theory worked fine and well except for the pressure switch is activated with VACUUM (negative pressure), which now leads me to believe that the "secondary blower" being turned on creating a vacuum is a prerequisite to the furnace working. SO either I'm back at square one with the board being shot or "secondary blower" being fried.

More searching lead me to this link which gives a very comprehensive troubleshooting guide. Turns out the "secondary blower" is actually called a Flue Venter Atleast I'm feeling a little bit more confident, just need to figure out why the Flue Venter is not working.

ps... sorry for the long post...

Last edited by Kruso; 11-04-2010 at 03:32 AM. Reason: Found the problem
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Old 11-04-2010, 07:04 AM   #13
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gmp075-4 won't start in heat mode but start in ac mode


if you are saying your ps isn't closing. Disconnect the rubber hose where it connects to the vent motor . Use very small drill bit ,etc, & clean port
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Old 11-04-2010, 11:42 AM   #14
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gmp075-4 won't start in heat mode but start in ac mode


Quote:
Originally Posted by kenmac View Post
if you are saying your ps isn't closing. Disconnect the rubber hose where it connects to the vent motor . Use very small drill bit ,etc, & clean port
The draft motor isn't working at all, so there is no vacuum for the PS. It's not a clogged port/ pipe issue, I even removed the draft motor and tested it seperately and that works fine. The LED light is solid red, could this be a limit sensor issue?


Edit: Just checked the continuity between all the reset-able sensor and they all (3) seem to be working right. I live in TX so we hardly use the heater, I always suspected that the A/C repair man I called out a year ago had something to do with it. I noticed that the gas valve was shut when I initially looked at the furnace so I simply just turned it on to no avail. Is there a safety trip switch on the board that disables the furnace if there's no ignition (due to no gas in this situation) in X amount of times?

Last edited by Kruso; 11-04-2010 at 01:29 PM. Reason: New finding
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Old 11-04-2010, 04:28 PM   #15
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gmp075-4 won't start in heat mode but start in ac mode


If you have 24 v at white wire to c at the control board on a call for heat & the vent motor isnt's comming on. it's either the board or vent motor. You can also check for 110v where the vent motor connects to the board. If you have 110v at the vent motor connections on call for heat & vent motor isn't comming on. bad vent motor

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