Goodman furnace no voltage to gas valve

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  #1  
Old 10-13-19, 03:58 PM
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Goodman furnace no voltage to gas valve


I have a 20 yr old Goodman furnace that gets to ignition stage but does not open the gas valve. Everything has proper voltage except for the purple wire running from the board wire cluster through all the rollout sensors and up the gas valve. When I pull the wire cluster (white plastic hub) from the board and jump the red 24v power and the terminals for the gas valve run only, I get a solid 24v-27v reading through all sensors up to the GV, so I know all rollouts and connections are good. If I add in jumping the red and yellow limit switch circuits, I still have 24v to the GV. However, as soon as I add in the final jump for the blue ground wire, all voltage drops from the GV circuit - voltage is still 24v for the red and yellow limit switch runs.

This is frustrating because I know what the problem is, but donít know why or how to fix it. Any ideas? My thought is that with the potential for 24v to the GV circuit there, itís not the board and must be a grounding issue. Thanks!

Update: after troubleshooting some more it seems like the igniter is not actually finishing its cycle and the system is being shut down by a switch. There is no code, but everything drops out after a quick igniter run and goes into lockdown right away. Tested both limit switches and pressure switch, and all have continuity.
 

Last edited by Bean616; 10-13-19 at 06:30 PM. Reason: Update
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Old 10-15-19, 05:55 PM
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Anyone have any ideas? Replaced the HSI today thinking it was faulty, but same problem. System goes into lockdown after 10 seconds of the igniter glowing - no voltage to gas valve.
 
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Old 10-15-19, 09:56 PM
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Welcome to the forums.

A model number of the furnace would be terribly helpful.

As far as I know...... the gas valve power does not go thru the safety limit loop. The limit loop should be 24v leaving the control board and returning back as 24v. If the 24v does not return..... the board will not allow the ignition process to advance.
 
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Old 10-15-19, 10:33 PM
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20 year old goodman needs a thorough inspection before spending money on more parts.
 
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Old 10-16-19, 05:05 AM
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Thanks for your responses. The furnace is a GMP series. The gas valve loop leaves a molex cluster via purple wire with 24v and runs in line through 4 rollout switches then directly to the gas valve. Leaving the valve is a ground that runs back to a piggybacked ground on the transformer than takes off to a chassis ground. For this model, if any of the rollout switches are tripped or faulty, power will to the gas valve will be cut at that point.

The problem is that I never get 24v on that circuit during HSI glow, and the whole system shuts down before HSI even finishes glowing. When I pull the molex plug and jump between the 24v supply from the transformer and the purple GV line only, I get constant 24v at the GV - so I know the rollout switches are good and the connection through the molex is sound. But, if I add a jump to the ground through the molex, it immediately cuts 24v to the GV line.

Thoughts: Adding the ground kills the constant 24v to the GV intentionally and it should only open again on signal from the board during HSI warm up - so the problem is the board.



 
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Old 10-16-19, 12:55 PM
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24v will only be applied to the gas valve after the igniter warmup period, your problem is elsewhere.

you should see if the pressure switch is opening during the cycle.

meter required.
 
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Old 10-16-19, 05:26 PM
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The pressure switch checks out, and I believe the HSI wonít kick on at all if the pressure switch isnít satisfied.
 
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Old 10-16-19, 06:23 PM
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the pressure switch can open up again during the cycle and cut the igniter and gas valve off.
 
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Old 10-16-19, 07:17 PM
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Ok, bypassed the pressure sensor to test with a jumper after the draft inducer started and got the same result.
 
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Old 10-16-19, 09:24 PM
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You should never bypass the pressure switch with the gas on! if the switch is opening, there's probably reason and you can get flame rollout or even an explosion.

you're lucky nothing bad happened.

You'll have to see if there's anything else in the circuit shutting it down. could end up being a bad connection or the board itself - you have to test with a meter.

I can't help further without seeing a schematic.

To be honest i wouldn't bother trying to fix this furnace. In all likelyhood, the heat exchanger has failed by now. GMPs are apparently known for really bad heat exchangers that have the crimp rings pop off.

20 years ago goodman was known to be a really bad make - they took amana's design in the mid 2000s and got more decent.

https://www.youtube.com/watch?v=84JDkuYNeBs
 
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Old 10-17-19, 10:53 PM
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I bypassed the switch as a controlled test to see if the sequence advanced to firing the burners which would confirm if the pressure switch was the problem. Since the system cuts out prior to the gas valve opening, there is no chance of explosion or rollout, and if the gas had kicked on I would know that the PS was the culprit and shut the machine down until a replacement arrived or the vacuum leak corrected.

Since Iíve already triple checked every switch, and I donít see any logical wiring issue, Iím assuming itís a bad board and have a new furnace held at the supply house.

My main reason for posting here was to see if anyone had experienced something similar and had an solution or if there were any unknowns or tricks I might be overlooking. No reason to go any deeper or spend $100 on a replacement board when I have a brand new unit ready for less than $500. Weíll add this to the unsolved mysteries pile.

 
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Old 10-18-19, 11:54 AM
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GMP is a group.... not a model number.
 
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Old 10-18-19, 01:06 PM
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If the pressure switch opening was the culprit, it could have fired without a proper draft.

My main reason for posting here was to see if anyone had experienced something similar and had an solution or if there were any unknowns or tricks I might be overlooking. No reason to go any deeper or spend $100 on a replacement board when I have a brand new unit ready for less than $500. We’ll add this to the unsolved mysteries pile.
There are no tricks.

You have to use a meter to troubleshoot and make sure the board is getting the necessary inputs consistently before condemning it.

Troubleshoot and you won't need any tricks.

You would be better off with a new furnace anyhow at this point - preferably one that's 90%+ efficient if you live in a climate that gets real winters and properly sized/setup.
 
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