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Old 09-12-2009, 09:11 AM   #1
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Furance & AC Issue


Having a problem with my AC/Furnace. AC was working fine and was turned on for about 3 weeks. Thursday night it cooled off and we opened up until Saturday morning. On Saturday morning the AC would not turn on, neither would the furnace side. Furnace is a Janitrol GMP100-4, 14 yrs old.

Checked all breakers and they were ok. Thermostat was calling for heating/cooling. Checked circuit board and 3 amp fuse was blown. Replaced fuse and it popped again. Called local HVAC guy since it was Saturday and hot and he came over and stated that the transformer was shot and replaced it. He also checked and recrimped connections for the thermostat wires. Afterwards, he checked the AC and furnace the furnace appeared to begin starting up, the element would light but NO burner activity. This would just continuous cycle like this. The outside AC unit would not kick on but the inside blower would start.

He thought there was a short somewhere. Told me that problem could be 1 of 2 items, 1) bad thermostat since it was only sending 20V to unit instead of 26V or 2) thermostat wiring short between furnace and outside AC unit since he saw some gnawing in the garage from apparent mice activity. He advised me to check those two items to save some money. I was not real certain that he was on the right track or just confused.

Today, I replaced the thermostat and ran a thermostat cable from the furnace down my hallway and through my garage doors right to the AC unit to test his theory, same issue. I see voltage of about 20 volts at the end of the wires. When I connect the wires, there shows 0 volts at the contactor connections. I also replaced the control board, no change.

I did some measurements:

25 V or so from R to C
25 V or so from Y to C in cool mode
25 V or so from W to C in heat mode


During heat cycle, red light on board stays on solid, unit attempts to start, igniter glows but no burner action. This happens 3 times then unit goes quiet. Gas valves and control appears ON. I never get 24 Volts at the Gas valve. The transformer output jumps around and does not stay a solid voltage.

During cool cycle, exterior unit does not kick on.

Could the transformer be bad that he installed? Could he have wired the transformer wrong to cause both of these problem? If so, how do I correct without bringing him back?

Any suggestions?

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Old 09-12-2009, 09:31 AM   #2
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Furance & AC Issue


Transformers are rated in VA, voltamps,like horsepower for the layman. You probably need a 40VA and if he put in a smaller one it may not have enough power to open the valve and that is why you get a voltage drop. Did he put the exact same type as original? get it from Goodman?.

The circuit board needs proper grounding and that is done by the mounting plate to the furnace fan or housing. Make ABSOLUTELY sure it is tight and put new larger screws in if necessary.

On a lot of the newer units the secondary side of the transformer is grounded and I would use the OEM Goodman transformer so the wiring is correct. Not sure if mixing the 2 wires up would cause the problems but it is possible.

On a secondary note, that furnace should have it's heat exchanger CAREFULLY checked for safety. There has been LOTS of problems with cracks and popped compression rings in it.


Last edited by yuri; 09-12-2009 at 09:40 AM.
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Old 09-12-2009, 10:10 AM   #3
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Furance & AC Issue


Thanks Yuri,

I will check the transformer rating and the board grounding. Do you think that this could also cause the AC to never receive the 24V to activate the conductor?
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Old 09-12-2009, 10:45 AM   #4
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Furance & AC Issue


Possible, I worked on lots of those units and they get very weird when not grounded properly. Hope you used #18 wire to the outdoor unit. If not, then do so. NO smaller telephone wire allowed!
It is possible the new board is damaged from poor grounding. We had lots of them blow from that reason, Sorry.
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Old 09-12-2009, 07:32 PM   #5
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Furance & AC Issue


Hey Yuri,

The transformer is a 40 VA but is not an original Goodman unit. I checked the voltage coming out of the transformer and it never gets close to 24V, extremely low voltage. The circuit board was attached using 4 plastic connectors that was supplied with the furnace.
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Old 09-12-2009, 08:22 PM   #6
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Furance & AC Issue


Check the power going to the transformer, should be at least 110 volts. You need a good 24 volt transformer. The mounting bracket that the board attaches to needs to be secure to the fan or furnace. The secondary side of the transformer is grounded on some units. Get the proper Goodman transformer and follow the wiring diagram carefully and make sure the mounting bracket is secure.
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Old 09-12-2009, 10:30 PM   #7
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Furance & AC Issue


Did you compare the voltage reading while the secondary leads were connected to the board versus un-connected? If the reading is always low, and the incoming voltage is right, then I would suspect the transformer. If the voltage becomes quite low while connected to the board, disconnect the t-stat wires from the board. Does the voltage change?

Also, I would make sure that the secondary leads were not crossed. Something made the fuse blow, I haven't seen too many situations in which the transformer was the culprit. But, there have been many ocasions in which the t-stat wiring was faulty.

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