Hello, hello.
Well. Damn. This service call is killing me. I cannot figure it out for the life of me.
A goodman modulating furnace. GMVM960603BXBA
Originally, the problem was that the gas valve's pressure was wonky. The ignition happened and then the pressure would gradually reduce until finally winking out.
The inlet pressure was 7.8 w.c. I changed the gas valve and it worked fine ... for about five minutes and then ceased to open completely. No weird modulation, it just simply refused to open with a brand new gas valve.
The pressure is still 7.8. When the HSI ignites, 26 volt gets delivered by brown wire (terminal 7) to the gas valve. The low fire pressure switch closes fine. (It's in series with the neutral wire of the gas valve. Terminal 8) The grounding seems fine. I get continuity to ground on neutral transformer of 120 and 24 side, as well as all other neutrals. Just to improve the grounding, I've ran some bared wire around a rigid gas pipe and attached it to the chassis of the furnace. That didnt help. Grounding the gas valve's neutral wire to the chassis, or the ground wire of the supply voltage didnt help either. Running 24 volt directly from the transformer to the gas valve failed to open it, or do anything what so ever. Changed the gas valve 'again', along with the control board, and all pressure switches for some weird reason. No difference. Gas pressure is still 7.8. The Water Heater is working just fine, the pressure drops to 7.0 when the water heater kicks in. Used an independent transformer that ran 27 volt to the gas valve direct from an independent 120 volt line that's not being used by anything else. No difference. The tubing between the Gas Valge's pressure switch connection and the back of the High Fire pressure switch is clean from any debris, as are obviously all other tubings. Tried running the furnace with exhaust pipe disconnected. No difference. Set the system to stick to low fire only, and high fire only just to see if this changed anything. No difference. Certain it would be no different, I used the gas piping that lead to the water heater to connect to the furnace gas valve instead. It shouldnt have helped and it didnt. Just because I had another one with me, I changed the gas valve the third time. Just figuring all these experiments managed to somehow burn something out. No ... difference. Got an enercare friend of mine to swing by and do a pressure check at the meter side, he's still reading 7.8ish and his manometer is not digital.
The only electrical part that I havent changed is something that helps the ventor motor modulate. It's the only remaining aspect I havent explored. And I gather that the gas valve modulation is happening due to ventor motor modulation. But if it was wonky, then it might be that the gas valve would act strange, not completely and totally refuse to open. I feel like I'm missing something. Has anyone encountered something similar?
