Goodman GMT090-4 won't ignite


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Old 01-16-12, 08:30 AM
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Goodman GMT090-4 won't ignite

Trying to fix a 10yo GMT090-4 gas furnace here. It's been failing to ignite intermittently for a couple of months, but a power cycle or three "fixed" it until last week. Now it seldom ignites, and when it does it won't stay on for very long. Good thing we are having a really mild winter here.

The unit is blinking once which indicates ignition failure. ID blower sounds kinda tired but still activates the pressure switch every time. Water heater works as usual so I'm assuming gas supply is OK.

What I'm finding though when taking ACV on the gas valve is that it's showing nothing when the valve is ON. But when I turn the valve OFF I get a 24V reading on it every time after the blower and HSI come on. Turn the valve back on and the reading goes back to zero...

This is not normal is it? Would this seem like a bad gas valve?
 
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Old 01-16-12, 10:25 AM
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With the gas valve turned to the on position, what should happen is...


The hot surface ignitor should heat up for about 30 seconds.

Measuring across the two connections of the gas valve, the 24 VAC voltage should be switched on.

The main burner gas should turn on and light off the HSI and the HSI should switch off a few seconds after the main burner gas turn on.

The main burner gas should stay on until the thermostat is satisfied.


If the gas valve is open, you have AC voltage to the gas valve, so you may not be measuring the voltage properly.
 
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Old 01-16-12, 11:14 AM
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The valve does get 24 VAC on it after the HSI heats up. But only if the little blue switch on it is in the OFF position.

If it's ON, there is no voltage, almost like the valve has a short in it, and the valve doesn't open so the burner doesn't ignite. But I still hear the same click that I hear when the voltage does come on.

I'm measuring the AC voltage between the purple/red wire on the valve and the ground (the metallic body of the unit).

Then after a few seconds the HSI is turned off, and the blower also goes off after the unit retries to ignite two more times. The red indicator light then starts blinking.

Edit - Just measured again, and with the valve ON the voltage is not exactly zero, it's actually about 1.5 VAC. With the valve OFF the voltage is 24 VAC. Valve doesn't open, so no ignition.
 

Last edited by donaspen; 01-16-12 at 11:49 AM. Reason: additional info
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Old 01-16-12, 12:09 PM
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What is the make and model number of the gas valve?
 
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Old 01-16-12, 12:26 PM
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B12826-14, probably Goodman? The original one that came with the unit.
 
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Old 01-16-12, 12:54 PM
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I presume your gas valve is like this:

White Rodgers Gas Valve 36F22-202 Goodman B12826-14 on eBay!


That gas valve has an electrical switch to shut off the gas. Most gas valves have a mechanical valve to do that job.

It sounds like the gas valve has a bad connection that is grounding the 24 VAC power that should turn on the gas valve. When it's grounded out, there is then no voltage to open the gas valve.

By switching the gas valve to the off position, you are disconnecting that defective ground, which allows the 24 VAC to be measured. So I'm agreeing with your earlier conclusion on that point.

The short answer is---- it shouldn't do that. Replace the gas valve. I recommend against DIY replacement of the gas valve, because it needs to be properly adjusted to match the gas valve to your particular furnace.
 
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Old 01-16-12, 05:18 PM
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Understandable.

Your help is very much appreciated.
 
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Old 01-28-12, 07:08 AM
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OK so I got myself a new gas valve, and I am seeing the exact same behavior voltage-wise on the new valve...

24 VAC when the switch on the valve is OFF, 1.5 VAC when it's ON.

I am totally lost.

Haven't done any piping work.

Any ideas?
 
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Old 01-28-12, 09:51 AM
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What happens if you jumper 24 VAC to the gas valve from the "R" terminal?
 
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Old 01-29-12, 07:46 AM
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Originally Posted by SeattlePioneer
What happens if you jumper 24 VAC to the gas valve from the "R" terminal?
Jumpering the limit switch to the gas valve (when the HSI is lit) results in ignition. But only until the jumper wire is disconnected.

Same happens when I jumper the pressure switch to the gas valve.

But simply shorting both roll out switches doesn't do it. And neither does shorting the pressure switch.
 
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Old 01-29-12, 09:17 AM
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It appears that you have a defective circuit board.

Suppose you have a relay that is making a poor connection to switch on the gas valve. When the gas valve is switched to the off position, there is zero load on the relay and you get 24 VAC when you measure it.

Switch it to the on position and the relay isn't capable of supplying the needed current. Voltage falls to 1.5 VAC.

If you eliminate the circuit board and apply the 24 VAC to the gas valve directly, it operates normally.


So this would account for the symptoms.

Check the gas valve which should have the current rating listed on the gas valve ----could be 50 MA or so.

Then measure the AC current the gas valve is drawing when the circuit board is trying to switch it on ---- probably minimal AC current flowing I would suppose.
 
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Old 01-29-12, 09:30 AM
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Sorry my earlier diagnosis wasn't adequately thorough.

Unfortunately, this episode indicates why multiple methods should be used to double check a suspected problem.

Diagnosing a bad gas valve was reasonable under the circumstances, but the additional test of jumpering 24 VAC directly to the gas valve would have disclosed that the gas valve was in fact OK. That would require that another part be the problem.

The circuit board is then the suspect, and you can check and see if the circuit board is providing adequate current and voltage to operate the gas valve. You already know that the voltage is inadequate.
 
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Old 01-29-12, 03:07 PM
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Well my cheap little multimeter doesn't do AC amperage. But assuming it's the relay on the board that's bad, what are my options for fixing this?

The board is like $125, but the service manual says it's not field serviceable and wants me to replace the unit itself...
 
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Old 01-29-12, 03:27 PM
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I would replace the board.

As a bonus, you renew many other parts which could be the source of a problem in the future.
 
 

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