Carrier 9200 blowing 3 amp fuse within 20 seconds of inducer blower starting

mike953

Carrier 9200 blowing 3 amp fuse within 20 seconds of inducer blower starting. Does not blow fuse without the white wire connected. Not a wiring short, I have the thermostat wired directly to the terminals in the furnace. Not the thermostat, I have used the old one and a new one. I'm not sure where to go from here. Any help would be appreciated, it's getting cold in here.

STEVEusaPA

It's a short, maybe the board, maybe the wiring.
Your diagnostics are incorrect.
Also "Does not blow fuse without the white wire connected." (which white wire) contradicts "It's not a wiring short".

mike953

I have tried to eliminate any possibility of a wiring short by wiring the thermostat direct to the terminals on the board with new wiring. I have inspected every inch of wiring past the board. The white wire I am referring to is the heating wire from the thermostat. I believe that I've eliminated everything but the board itself, but I'm looking for a second (or third) opinion. Any way to test the board?

DZoro

Does the igniter start to glow? Try unplugging it.

GW

Get a Popper fuse. Mans best friend when diagnosing pesky issues

JUGHNE

Is 20 seconds long enough for the pressure switch to close and then send power to the board to start the igniter?

If the 20 seconds is the pre purge time then it checks the pressure switches and blows fuse, I would disconnect the pressure switch wiring and see if it holds.
If so check switch wiring for shorts, if OK then maybe the board??
It also would be good to check the limit circuits for shorts to ground. Back to the board??

mike953

Tried it again and did not see a glow from the ignitor

SuperTech

The fact that it blows the fuse after 20 seconds makes me think it could be something with the pressure switch or a component that's in series with the pressure switch since after 10-15 seconds after the inducer starts the pressure switch closing is next in line sequence of operation.

You are referring to the 3 or 5 amp fuse in the control board? Not the fuse or circuit breaker for the wiring in your house?

DZoro

Agree with JUGHNE, unhook p switch, and igniter,,,, Is the main blower running at this time?

SuperTech

If it's blowing the low voltage fuse then it wouldn't be a hot surface igniter since only low voltage components will blow the fuse on the board.

You can eliminate the board by checking if any of the components that are powered by the board are shorted to ground. I've also tested parts in the past by sending 120 or 24 volts directly to the component. On the average gas furnace there's not much to check as far as electrical loads. The inducer, igniter and blower motor are line voltage and everything 24 volts are relay coils and contacts

SuperTech

My mistake. It's obviously the fuse on the board if it's three amp.....

mike953

Sorry I haven't been getting back to everyone. Been trying what you've all suggested and when the pressure switch is disconnected the fuse holds.

Jamie Hall

Not likely to be the pressure switch or its wiring itself, but something perhaps in the wiring in whatever the pressure switch allows to energize.

Big Ed_4

The W is the 24V back to enigize the board .Thermostat wire we know is good . Fuse would just sizzle at start....

The inducer starts within an second or two the pressure switch pull in ... I have to say the pressure switch and wiring good ...

It's an hot surface ? which is 110V starts though an relay in board , But it's not glowing ... After 20seconds you say ? the board fuse pops ... Close to when the gas valve should pull in ...

Hmm ...Bad igniter and an short in gas valve ? Odds against ... But to eliminate , disconnect the gas valve ... Throw in another 3amp fuse ( I too use the Little Popper , and carry one in my truck) , If the fuse blows again with in 20 seconds...change the board ..... This is from the hip

mike953

disconnected the gas valve and reconnected p-switch. fuse blew just like b-4. was not sure which wire was the supply on the p-switch so I connected one wire at a time. fuse did not blow with either wire, so i'm guessing no short to ground. From what i see the p-switch wire comes from the board and returns to the board. anything left but a bad board?

mike953

Thanks to all of you who helped me with this. your time and effort are much appreciated

unclejohn

Whats the full model number I look up on hvacpartners

mike953

furnace model 58MXA080. board # HK42FZ011

JUGHNE

Sometimes it is easy to pull the board just to peek at the back side (you may not have to unwire it completely). If any burn marks then it is an easy call.

Years ago some used an insert limit that was a few inches long,
One side of that would touch the cabinet and short out the 24 VAC. That was pretty old school device and I haven't seen one for a long time........worth a look. You usually can check the limit circuit with 2 (maybe red) wires at the board.

mike953

no burn marks. no discolored components. no burned electrical smell. of course with chips involved all of the old tests don't mean what they used to.

JUGHNE

If you unplug the gas valve does the fuse still pop?

unclejohn

The pressure switch comes out of the 9 pin plug on terminal 3 and returns on terminal 4. Sequence says when call for heat, board checks to be sure PS contacts are open and then starts inducer motor, then 15 sec pre purge and PS contacts close. that sounds like when you get your short. You can try removing wires from PS and waiting for inducer to start and touching them together and see what happens. And also check wires at the 9 pin plug.

mike953

limit switch checks out ok

mike953

yes, fuse still pops when gas valve is disconnected

unclejohn

After the PS closes the igniter warms up so if something on the board is bad it would be the igniter relay you can see if that looks crispy.

mike953

did as you suggested, unclejohn. the fuse blew immediately when the PS wires touched. the 9 wire plug has nothing visually wrong with it

mike953

just reinspected the board. visually it is perfect. if i remember my days of working with circuits, a burned out relay stunk. no smell.

DZoro

I believe on that model all the wires come up from the blower cabinet. They go through a small hole that has a split plastic protector. The blower door pushes against this area also, check that out carefully.

unclejohn

If it's not the board it haS TO BE A SHORTED RETURN WIRE FROM THE PRESSURE switch. Sorry hit the caps lock key. Check that wire, I seem to remember there was some sort of metal clamp strain connector right at the board.