24V short? - http://www.hvacmechanic.com/ Forums
Original message
| gsmiley
 |  "24V short?"
, posted Sat 28 Oct 00:12  
Goodman GMPN120-5, service guy replaced PCB in Aug for A/C failure, root cause not documented.
Last week tried to turn on heat, 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 flue blower both work normally when tested separately on line AC. Pressure sw works OK testing suck/blow on tube.
Disconnected low V connections to outdoor unit.
Disconnected all tstat wires, removed stack overht sw from circuit, connected R+W at PCB to get 'always ON' demand for heat.
Disconnected internal low V wires sequentially and probed for short with ohmmeter, none found.
Sequence where I'm stuck:
1. Flip 120V sw to ON
2. flue blower starts immediately, runs normally (main blower is not connected so is not running, but get same result even if it's running)
3. after several secs HSI relay clicks, after several more secs HSI glows brightly (HSI is new by service guy, not sure if it's OEM part or not )
4. several more secs, very rapid relay clicking heard (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 rapid clicking, 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.
I am an instrument engineer but not HVAC tech.
Service guy and I are both stumped.
Thanks in advance for any suggestions.
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Replies:
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| nitronorm
| "Re(1):24V short?" , posted Sat 28 Oct 12:48
frist start here remove all the t-stat wires from board jump r and w see if unit run. if not it in the stat wire. if fuse blows when starting see if the wires on the harness are all cover, bear wires are not showing. then see if you have two wire touching. if all this is ok look at the back of the board to if you see there an burned spots. the board is under warrenty for one year. have the tech put a new one in.
sheet metal local #104
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| bubbad
| "Re(1):24V short?" , posted Sat 28 Oct 08:40
Well, that seems strange. You may have an internal short in the circuit board at the gas valve relay. Fast clicking is usually signs of a voltage drop, or some sort of electrical problem. Does your tech carry PCB's with him? I think I would have swapped it by now if its blowing the fuse with no wires attached to it. Hard to say without seeing it. I carry a 3 amp mini circuit breaker that I plug into the fuse holder for occasions like this.
bubbad
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| | gsmiley
| "Re(2):24V short?" , posted Sat 28 Oct 08:53
Thannks for quick reply, bubbad!
He replaced PCB once already, I suppose it's possible new PCB is bad...?
Good idea about mini cb.
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| | bubbad
| "Re(3):24V short?" , posted Sat 28 Oct 12:15
The only other thing I can think of is to do a thorough inspection of the wiring harness from the board to the gas valve - maybe there is something going on there.
bubbad
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