Goodman GMP only lights sometimes


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Old 03-17-19, 08:21 AM
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Goodman GMP only lights sometimes

Older GMP furnace sometimes will call for heat but the igniter stays on for a few seconds then shuts down. Repeat 3 times, no light. Sometimes if I turn the power off or cycle the thermostat it will run fine for the rest of the day. The one flash LED lights on the board.

i have cleaned the flame sensor and even replaced it, symptom is still intermittent.

HVAC Pro site suggests itís cracked heat exchanger but not willing to accept that yet (as Spring approaches).

im going to put an endoscope in the heat exchanger but not until the outside temp is good one day.

Any troubleshooting tips.

thanks.
 
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Old 03-17-19, 09:20 AM
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GMP has long history of heat exchanger issues but would not be related to your issue. I would check for voltage at the gas valve.
 
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Old 03-17-19, 09:50 AM
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Thanks. Any idea what shouldread? Take reading I series I Suppose? If not to spec would mean bad circuit board or?
 
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Old 03-17-19, 10:16 AM
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Before you go any further check the heat exchanger. If it’s cracked your furnace is dead and not worth fixing.
Pull the blower and check the crimp rings from the bottom.
There’s a high chance yours is cracked. Most GMPs are long dead.
 
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Old 03-17-19, 12:00 PM
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Thanks. I have an endoscope, will that work?
 
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Old 03-17-19, 01:00 PM
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Pull the blower first. The crimp rings pop off the bottom of the heat exchanger and usually end up laying on top of the blower housing.
 
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Old 03-18-19, 07:52 AM
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Temp as down 5 degrees below thermostat setting this AM so it must have been on a bit last nite but not operating correctly. Itís in the 40ís here.

i disconnected the wires to the gas valve and inserted a voltage meter and pre ignition showed 21-22 volts AC (should be 24 plus?). During ignition jumped up bit to 25.5 volts AC with igniter on.

Voltage seems low.

I reattached the valve wires and of course the furnace came on as it should first time and has functioned well for the past hour.

I noticed my rollof switch is corroded but function and has continuity. My replace it.

Ill probably put some deoxit on the valve contacts, maybe just a poor connection.

ideas?
 
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Old 03-18-19, 08:01 AM
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Wires need to be connected for testing.
Like I said previously you need to check the heat exchanger prior to anything else. If it’s cracked the furnace is dead.
 
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Old 03-18-19, 06:40 PM
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Been working 100% all day. Dunno.

From looking at the flame the heat exchanger is functioning as intended. Satisfied for now that is isnít cracked.
 
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Old 03-18-19, 06:48 PM
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The look of the flame isn’t enough to fully verify the integrity of the heat exchanger.
 
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Old 03-18-19, 07:19 PM
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i disconnected the wires to the gas valve and inserted a voltage meter and pre ignition showed 21-22 volts AC (should be 24 plus?). During ignition jumped up bit to 25.5 volts AC with igniter on.
There should be 0v on the gas valve until it's time to open. The igniter preheats for approx 30 seconds and then the gas valve is powered with +/- 24vAC. If the burner lights..... the igniter shuts off and the gas valve stays powered. If the burner doesn't light within 7 seconds..... the gas valve and the igniter are powered down for a restart.

Corroded terminals on the gas valve are a common problem.
 
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Old 03-19-19, 06:25 AM
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Thanks. Thought it odd that I was reading voltage at rest. Using basic multimeter set at Voltage AC.

Been working fine for last 24 hrs. At some point (once Spring kicks in) I will inspect the heat exchanger for cracks.
 
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Old 03-19-19, 06:43 AM
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What error is it failing on?
 
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Old 03-19-19, 03:39 PM
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1 Led, working flawlessly since I disconnected/ reconnected the gas valve electrical. Also tapped the rollout switch.

Or or or the other I suppose.
 
 

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