[gsmiley]

I have a Goodman GMPN120-5, service guy replaced PCB in Aug due to A/C failure, root cause unkknown or not documented.
Last week I turned on heat for first time this season, now nothing runs. Service guy returned, located blown 3A PCB fuse, replaced, still blows. Replaced fuse with 5A for testing (see sequence below).
Main fan and combustion blower both work normally. Pressure sw works OK testing suck/blow on tube. All flame rollover sw's in NC position. 24V normal.
Disconnected all tstat wires, removed stack overht sw from circuit, jumpered R+W at PCB to get 'always ON' demand for heat.
Sequence :
1. Switch on 120V power to unit
2. combustion blower starts immediately, runs normally
3. after several secs HSI relay clicks, after several more secs HSI glows brightly
4. several more secs, relay chatters rapidly (I assume it's the gas valve relay), followed in ~0.5 sec by fuse blows
(If I turn OFF 120V pwr quickly enough during chatter, I can save the fuse and repeat sequence 1-4.)
Removed leads from gas valve, checked terminals at gas valve with ohmmeter, neither grounded.
Leave gas valve leads disconnected, repeat steps 1-4, same result.
Low V connections to outdoor unit are also disconnected for testing.
Controller board was replaced second time by service tech.

Another bad controller board?

Thanks in advance for any suggestions.

[Kender]

There is a limit switch that sets by the inducer motor. It sticks inside by the heat exchanger. Pull that and make sure that is not touching the heat exchanger inside and grounding out

[NorthernHeat]

Gas valves have a bridge rectifier in them and are difficult to test simply by a continuity check. You have to apply 24 volts to it directly and measure the amps, actual rating is on the side of the valve. No its not uncommon to get a bad Goodman board. I thinks you have a shorted low voltage wire to ground though. Check the top of the burner box for any wires that have gotten hot or melted, usually limit/rollout wires.

[gsmiley]

Thanks for the replies, gentlemen.
If limit sw was bad or grounded wouldn't fuse would blow immediately as it's in line with fuse, and unit works fine during startup until GV relay energizes. Also removing limit sw wires properly prevents HSI from energizing, and never gets to GV relay cycle.
I have also tested with wire from PCB to (gas valve and rollover switches in series) disconnected, same symptom.
Bad PC board, maybe short somewhere or bad GV relay? Relays are not socketed so I can't easily remove and test separately.
Fortunately it's only cool in Cincinnati, not freezing...

[gsmiley]

PS, any suggestions where I might get a schematic for the controller board? I've looked all over the Internet and can't find one.

[gsmiley]

PPS I just fouind out that the service tech changed the GV thinking it was bad, but works same.

[juggallojed]

Check the circuit from the control board to the gas control, look for a grounded wire in that circuit.

My best guess without being there sorry

[darylej]

[QUOTE=gsmiley]Thanks for the replies, gentlemen.
If limit sw was bad or grounded wouldn't fuse would blow immediately


I have this exact problem- the 3A fuse blows as soon as I turn on the heater. This is a new unit(2 yrs). The A/C works fine all summer and when I turn on the heat it blows the 3A controller board fuse. Last year I just replaced the fuse and it worked. This year it has blown 3 fuses in a row as soon as the heat is turned on...

Any idea what to do next ?

[labman]

Follow the suggestions in this thread, checking all the wiring. Since it blows immediately, it is not the gas valve that isn't energized immediately. Try it with the thermostat in the of position. Try it with the red wire to the thermostat disconnected.