Troubleshoot Goodman GMP075-3 gas furnace

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Old 12-17-07, 01:02 PM
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Troubleshoot Goodman GMP075-3 gas furnace

My heating system consists of a gas furnace and an electric heat pump. The heat pump is functioning properly, but the furnace itself will not kick in when needed. It is a Goodman GMP075-3 furnace with an automatic ignition system. My thermostat is a RiteTemp 8085C, which should be compatible with a dual fuel, heat pump w/aux heat system. Once the temperature difference requires the aux heat to kick it, the heat pump turns off, as well as the small fan, and the furnace does nothing. The fan and main blower do not kick in, the automatic ignition system does not glow, and the furnace does not run. I hear a faint click occasionally as well as what sounds like a faint electrical hum from what appears to be a main control box next to the furnace, but nothing ever happens inside the furnace itself. Does this sound like the problem lies within the furnace itself or possibly with the thermostat? Are there any simple tests or overrides I can use to isolate the problem to either the thermostat or the furnace? Assuming it is the furnace, are the any basic tests I can run myself before having to contact an HVAC service to repair my system? Thank you.
 
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Old 12-17-07, 07:11 PM
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Fossil Fuel Kit

I'm not familiar with Goodman's fossil fuel kits but something you can try is to set the thermostat for "emergency heat".
 
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Old 12-18-07, 10:42 AM
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OK, I continued to work with the heater last night, and was able to confirm that both the heat pump and heater are working. Once I joined the W2 and RH wires manually, the system finally kicked in, but not on the first try. I touched the two wires together a couple times and it finally worked. Then it continued working with the thermostat attached until later that night, and it went back to the same problem - heat pump will not work because it's too cold outside (30F) and the gas furnace fails to kick in. At this point it wouldn't even kick in when connecting the wires manually. I tried again this morning and found that when it failed, the blower kicked in, and the light blinked once, signifying a gas or ignition problem. However, if I cut electricity for a few minutes and allow the system to reset, the light turns solid signifying OK and the heater will then run properly. Does anyone have any ideas as to why the system only works part of the time? It seems like there is some type of intermittent problem with either a relay or the ignition system. Any possible tests I can run?
 
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Old 12-18-07, 04:42 PM
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Locate and clean the burner flame sensing rod. Use steel wool, not sandpaper.

This link will help you indentify what you're looking for:

http://arnoldservice.com/furnace_flame_sensors.htm

Scroll down the page some. Let us know how you make out.
 
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Old 12-20-07, 10:11 AM
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The small fan starts, the ignitor turns on and glows, the gas ignites, and then the gas turns off after a second or so. It sounds to me like your analysis that the flame sensor is failing is spot on. Only, after cleaning the sensor and trying again, it still fails. The system did run successfully once while testing it, but then went back to the same problem. Is there some way of bypassing the sensor? I imagine there may not be due to safety hazards, but if you know of any means for testing only since the gas does ingnite, please let me know. Any other suggestions would be appreciated as well. Thanks
 
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Old 12-20-07, 01:27 PM
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Make sure the stat is still calling for heat when this happens. I have run into strange scenarios where there is like this overlap where no sooner does the furnace start up and then due to sensitivity of the stat (1 degree temp swing and/or anticipator not set high enough, or wind currents in house, or ?), just enough warmth comes from somewhere to make the stat turn off. Make sure THAT is not happening.

You can discount the above easy enough simply by turning up the stat a number of degrees.

Then also make sure that you do not have a flame rollout sensor (often a dics with 2 wires on it out in front OF the burner compartment), that this is not tripping out. Easy test with voltmeter should always show full continuity whether furnace is off or on.

Also make sure that if you have a similar looking disc sensor in your ventor fan housing, that it is not tripping out. (Same test as jsut above).

Also make sure pressure switch is not shutting down also.

Also make sure that your flame sensor is mounted in correct position and if it can swivel a tad by it's mounting screw, make sure it swivels a tad closer toward the burner flame.

You may also have to inspect that any burner tube slits in the area of the flame sensor are not clogged up with carbon or whatever.

......

This should be a good start for you.
 
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Old 12-21-07, 09:51 PM
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Goody for Goodman!

Goodman furnaces are a pain in the neck on the safety circuit!

You will see rollout sensors all around the combustion chamber, three I believe, wired in series with a red or violet colored wire. If you temporarily jumped out all the rollout sensors and the sensor at the fluepipe, which are all wired together with one end wired to the valve, I believe you may find your problem. Usually it is only a loose connection on the sensor, or a bad sensor, but the problem may be intermittent.

If it works by jumping out the safety circuit you need to do a little detective work. Shut down the furnace. I believe the sensors are the resetable type, so push the little red button on them a few times to see if it helps any while checking out continuity on a meter. Make sure you hook up everything again when you finish!

I have found this to be a common problem on Goodmans and also the door panel switch.

Also I believe that this furnace has two flame sensors. One millivolt sensor (looks like a rod and is usually on the left side and has a white wire) for your pilot sensor, and then of course the hard surface ignitor which doubles as the main burner sensor once the burners are lit.

BTW, I have also run across a faulty HSI that functioned ok to lite the burner, glowed nicely and looked good , but failed in its function as a sensor . Upon very close examination I was able to find a hairline crack on the ignitor.

Please keep us informed.

Good luck,

Charlie
 

Last edited by boilersrus; 12-21-07 at 09:58 PM. Reason: forgot to mention something
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Old 12-22-07, 11:48 AM
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A fairly common problem with these Goodman furnaces is that the sampling port for the pressure switch gets plugged up. That can cause erratic operation of the furnace, since anytime the pressure switch opens, the furnace shuts off.


From the pressure switch you will find a piece of rubber tubing running to a fitting on the inducer motor housing. You can pull that hose off at the inducer housing. Using quite a small drill bit, you can insert the bit down into the fitting to the level that it's down to where the fan will be inside the housing. Move the drill bit up and down a bit to break off the corrosion that can plug up that port.

Reinstall the rubber hose and give it a try.
 
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Old 12-27-07, 10:03 PM
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GmP075-3 Heating Problem

Unit would turn on for about 10 seconds and the flames would cut off. Trouble code (4 blinks), Limit Switch Problems.
After checking the circuit I found no problems. The problem was unit overheating causing the circuit to trigger. I checked the output vents and noticed low output out of all vents.
Exposed the A coil and found the A coil clogged. After cleaning the A coil the unit works like new.
 
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Old 12-27-07, 10:24 PM
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Originally Posted by Fixitoday View Post
Unit would turn on for about 10 seconds and the flames would cut off. Trouble code (4 blinks), Limit Switch Problems.
After checking the circuit I found no problems. The problem was unit overheating causing the circuit to trigger. I checked the output vents and noticed low output out of all vents.
Exposed the A coil and found the A coil clogged. After cleaning the A coil the unit works like new.

God diagnosis Fixit.


With what was the coil plugged?
 
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Old 12-27-07, 10:27 PM
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That was a GOOD diagnosis, rather than one involving the deity.
 
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Old 11-29-08, 08:40 AM
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Wink Same problem - where is A coil located

Originally Posted by Fixitoday View Post
Unit would turn on for about 10 seconds and the flames would cut off. Trouble code (4 blinks), Limit Switch Problems.
After checking the circuit I found no problems. The problem was unit overheating causing the circuit to trigger. I checked the output vents and noticed low output out of all vents.
Exposed the A coil and found the A coil clogged. After cleaning the A coil the unit works like new.

I am having the same problem, BUT don't know where the A coil is located. Can you please tell me??
 
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Old 11-29-08, 03:47 PM
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'A' Coil

The 'A' coil is the air conditioning evaporator coil. On an upflow furnace it is mounted in the plenum on the top of the furnace.
 
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