Hello, I have a Rheem forced air furnace, gas, (mod# RDG-10EQGR) and main burner won’t light
The prepurge vent motor starts and after 34seconds the hot surface ignitor glows red, 34 seconds later
I hear the relay click in the ignition module for the burner valve, but valve doesn’t open and no gas goes to
Main burner , after 4seconds I hear another 2 clicks the ignitor shuts down ,and restarts cycle again

I have replaced the Robertshaw HS 780 ignition module with a used one , and still don’t get any
Voltage at the gas valve or the module terminals when the valve should be energized (24v)

MY QUESTION: I noticed next to the ignition module is a 24V transformer and I’m not sure what it’s for.
I ran a continuity check on the coils and it had very little resistance , but I was unable to get any voltage
Readings from any of the 4 wires coming from it (2 yellow on one side 2 black on the other side) at any
Stage of the ignition cycle. ….. Is this the power source for the main valve? And if so how can I confirm
It’s working? I’m trying to avoid buying a new Ignition module unless its needed ($120)

Thanks in advance for any help….. IT’S COLD IN MINNESOTA

I suppose everything is on one of those difficult to trouble shoot circuit boards. If that transformer was supplying the 24 volts to the thermostat, I wouldn't think things would progress as far as they are if it isn't working. I can't see a separate transformer for the gas valve. Sometimes they use a second for the A/C for reasons I don't know. Can you trace the black wires to where they come from? Usually they are the primaries.

If you never get power to the gas valve, I am guessing the problem is on the board. I am afraid the choices are cough up big dollars to buy a new board that may not fix the problem, or pay a technician $100 labor to replace the board when he can't find any other problem. Replacing the board is easy if you can remove about 10 wires and plugs and put them back where they were. Hint, the red wire goes on R, the white on W, etc.

Is there another transformer that seems to be supplying power to the board and thermostat?

I keep hoping somebody that know as much about HVAC as Tom does about plumbing shows up here and lets me quit fighting stuff like this. So far, it is me and a few others doing our best.

First I do not suggest that you play around with a gas furnace for obvious reasons but you might check the end switch on the draft inducer motor to make sure it is making contact when the inducer is blowing. Also there might be a problem with the gas valve especially if it is a White Rodgers Gemeni style. There was a recal so to speak for that valve but usually it stuck in th on position. Good luck

I have a similar problem w/Carrier gas furnace. The problem is the relay(?) on top of the valve. I have found the two pins on the solenoids, there are two of them, no longer make a good contact. Two screws hold this on the valve. Remove the screws and slide this pt up and down and see it that works. This may not be your problem but it sounds just like the trouble I am having. Technician came ($100) and told me that he thinks I need a new valve assembly. Afterwards I stumbled across the bad connection. I need to purchase this little part, but I can't find a source without buying the whole unit.

Due to safety regulations gas valve parts cannot be purchased. No manufacturer will sell any parts that have to do with the flow of gas through a gas valve. They do not want to be sued.If you have a defective valve just replace it and you will also have to set your pilot flame and the burner pressure as per the furnace mfg instructions. If you do not set the main pressure you could over fire or under fire your furnace. Not a good thing.

It is looking more and more like this may in fact be a problem with the coil. By messing with the contact pins as I mentioned earlier, I got the furnace to work for about 9 months initially. Then it would work for sometimes days, weeks or only a few minutes. This is electronic ignition, will I still need to adjust the gas valve as you stated when I replace the valve? Thanks for your help.

Yes you will need to adjust the valve. All valves need to be adjusted for proper operation. Manufacturers set the valves to a generic setting for both the pilot and main gas pressure which are never exactly correct. You will need a pressure gauge and adapter for the valve to do it correctly. While the generic setting might work possible damage or safety issues can arrise. If you read the manufactures instillation instructions with the new valve it will tell you you have to adjust it for proper operation.