Goodman gms80704BXBD, Pressure Switch Open
#1
02-27-17, 09:37 PM
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Goodman gms80704BXBD, Pressure Switch Open
Furnace: GMS80704BXBD, sn 0710200712, ~10 years old.
I'm getting 3 blinking lights: Pressure Switch Open. I've read a lot of troubleshooting steps online but not sure where to look next.
Background: The furnace stopped working; a tech came out and diagnosed the switch as the problem. Showing a little handiness, he suggested I replace the switch as his company would've charged ~$500.
I replaced the switch, but had still had issues. The tube was clear. I ran a paperclip into the inducer port, and then the furnace worked for a few days.
Since, it has stopped again.
I got my hands on a DMM and did a few tests at the pressure switch which seem to tell me that the switch and inducer are both working fine.
1) Switch continuity: the switch is open with power off, then closes when the inducer comes on
2) There is ~26.5 VAC from both leads of the switch to ground (should be 24-28 VAC)
3) The voltage drop across the closed switch is ~0.1 VAC (I read it should be <0.8 VAC)
What's the next troubleshooting step/steps? I appreciate the help.
I'm getting 3 blinking lights: Pressure Switch Open. I've read a lot of troubleshooting steps online but not sure where to look next.
Background: The furnace stopped working; a tech came out and diagnosed the switch as the problem. Showing a little handiness, he suggested I replace the switch as his company would've charged ~$500.
I replaced the switch, but had still had issues. The tube was clear. I ran a paperclip into the inducer port, and then the furnace worked for a few days.
Since, it has stopped again.
I got my hands on a DMM and did a few tests at the pressure switch which seem to tell me that the switch and inducer are both working fine.
1) Switch continuity: the switch is open with power off, then closes when the inducer comes on
2) There is ~26.5 VAC from both leads of the switch to ground (should be 24-28 VAC)
3) The voltage drop across the closed switch is ~0.1 VAC (I read it should be <0.8 VAC)
What's the next troubleshooting step/steps? I appreciate the help.
#2
02-27-17, 10:00 PM
Group Moderator
Welcome to the forums.
I have never had to change a pressure switch yet. They are that reliable.
When we say 24vac..... it could be 22v-28v.
With the inducer not running.... there should be 24v on only one side of the switch.
When the inducer is running there should be 24v on both sides of the switch to ground.
The drop needs to be as low as possible across the switch. 0.1v is lower than 0.8v.
I have never had to change a pressure switch yet. They are that reliable.
When we say 24vac..... it could be 22v-28v.
With the inducer not running.... there should be 24v on only one side of the switch.
When the inducer is running there should be 24v on both sides of the switch to ground.
The drop needs to be as low as possible across the switch. 0.1v is lower than 0.8v.
#3
02-27-17, 11:12 PM
Join Date: Mar 2006
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As PJ states, the pressure switch is rarely the problem. That nipple into the induced draft fan scroll IS usually the problem. Pushing a paper clip into the nipple is not the cure, you need to ream out the tube and hole with a drill bit, often a succession of bits, back to the original size. It is the rust, crap and corruption (technical terms) blocking the air pressure signal that is the problem.
#4
02-28-17, 07:22 AM
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With the inducer not running.... there should be 24v on only one side of the switch.
When the inducer is running there should be 24v on both sides of the switch to ground.
The drop needs to be as low as possible across the switch. 0.1v is lower than 0.8v.
The drop needs to be as low as possible across the switch. 0.1v is lower than 0.8v.
#5
02-28-17, 10:58 AM
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As PJ states, the pressure switch is rarely the problem. That nipple into the induced draft fan scroll IS usually the problem. Pushing a paper clip into the nipple is not the cure, you need to ream out the tube and hole with a drill bit, often a succession of bits, back to the original size. It is the rust, crap and corruption (technical terms) blocking the air pressure signal that is the problem.
#6
02-28-17, 11:02 AM
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Bad Gasket
Thanks for all your inputs. I do appreciate not having to rely on a tech for simple checks/fixes.
When I did some tests this morning, there was a slight rattle in the inducer. I pulled the inducer off to inspect for debris and check the pressure switch port. What I found was that the fiberglass gasket was falling apart. Instead of spending any more time on this old part - I bought a new one on my way to work and will simply see if that helps me out. Not the best troubleshooting technique - but the part was fairly inexpensive, and I'd rather just buy that part and know it's not my problem than go out and by a manometer.
When I did some tests this morning, there was a slight rattle in the inducer. I pulled the inducer off to inspect for debris and check the pressure switch port. What I found was that the fiberglass gasket was falling apart. Instead of spending any more time on this old part - I bought a new one on my way to work and will simply see if that helps me out. Not the best troubleshooting technique - but the part was fairly inexpensive, and I'd rather just buy that part and know it's not my problem than go out and by a manometer.
#7
02-28-17, 05:39 PM
Group Moderator
You can make a simple manometer out of plastic tubing and water.
It's what I used many moons ago before I bought one.
A bad gasket will definitely cause problems.
It's what I used many moons ago before I bought one.
A bad gasket will definitely cause problems.
#8
03-05-17, 09:18 PM
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Another good tip. Thanks for that one. Without being absolutely sure of what's going on, I've taken the horrible approach of just replacing parts. After replacing the inducer, I had working heat for a few days. Then a warm day hit with no calls for heat. The next morning - cold house. Having checked the vent, replaced the inducer and switch, I decided to replace the control board. It still didn't make sense to me that I was getting the proper electrical function of the switch but was getting the pressure switch open error code. I shouldn't need a manometer if the switch is closing and I'm getting expected voltage.
