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airheatman
airheatman, HVAC Technician
Category: HVAC
Satisfied Customers: 7737
Experience:  Furnace, A/C and heat pump specialist.
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I was following a thread on here about a pressure switch

Customer Question

I was following a thread on here about a pressure switch being stuck open but that problem wasn't exactly the same as mine because the exhaust fan doesn't run at all. Any ideas why?
Submitted: 1 year ago.
Category: HVAC
Expert:  airheatman replied 1 year ago.

If exhaust fan does not come on this may be a bad exhaust fan motor or we are not getting signal to motor. Can you give me a few more details?Do you have a volt meter for troubleshooting? Model on furnace?

Expert:  airheatman replied 1 year ago.

OK I have the manual on your furnace on my desk.Tell me what the status lights on board are doing.

Customer: replied 1 year ago.
The model is a Nordyne M1MB 090A BW I believe and my meter is dead apparently
Customer: replied 1 year ago.
The light is flashing twice repeatedly and the troubleshoot guide says pressure switch open
Expert:  airheatman replied 1 year ago.

First step is to see if we are getting power to vent motor,Can you get meter to work or grab a cheap one at Walmart (about 15 bucks)

Customer: replied 1 year ago.
I got mine working. (Stupid battery) This is not my forte though so I may ask some dumb questions...
Expert:  airheatman replied 1 year ago.

No problem What I want you to do is turn power off to reset board and then back on.Then call for heat and check to see if we are getting 120v to the vent motor.

Customer: replied 1 year ago.
OK, first layman question, where do I hook up to tell that?
Expert:  airheatman replied 1 year ago.

There should be two wires going to exhaust motor.One lead on each wire.

Customer: replied 1 year ago.
Well, apparently I did something wrong! Hooked up the meter, flipped the switch, heard a pop, and now the panel is saying "L1/neutral reversed or voltage not present on L1"
Expert:  airheatman replied 1 year ago.

Can you upload a pic showing what you did?

Customer: replied 1 year ago.
Only difference was the meter was on
Expert:  airheatman replied 1 year ago.

OK you had the red lead in wrong place.That is DC.

REd lead should be in the port by V/OHM/MA

Meter probably fried.

Customer: replied 1 year ago.
Damn! Didn't notice that. I think the meter is OK, switched it and tried again but it didn't read any difference and the light is still saying the same code.
Expert:  airheatman replied 1 year ago.

You could have damaged the board by doing that, creating a dead short through the meter.This may become more expensive than in the beginning.I hate it, but since you did not ask, I figured you were familiar with your meter.Stick meter leads in an outlet(Set to 200ACV and leads in right place) and see if you get 120v

Customer: replied 1 year ago.
Sorry about that, I did as you suggested and nope, no reading on the meter
Expert:  airheatman replied 1 year ago.

I suspected. Meter is fried and board may also be shorted. Easy mistake to make but costly.

Customer: replied 1 year ago.
Ouch. Any further suggestions to try? What is the L1 it is referring to?
Expert:  airheatman replied 1 year ago.

L1 is power into board. Try flipping circuit breaker off and back on.It may have tripped breaker instead of damaging board.

Customer: replied 1 year ago.
It had tripped the breaker. Code didn't change though after I flipped it back on.
Expert:  airheatman replied 1 year ago.

Ouch may have got board when you had the probe in wrong place. I think from here we need to get another meter and start from scratch.May have to replace board now.r if you prefer, get an onsite tech. Either way,I will be around tomorrow if you need help.

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