3 lights / concord / pressure switch good /

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  #1  
Old 11-12-15, 06:40 PM
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3 lights / concord / pressure switch good /

My concord furnace
80g1uh110cp20-02
Was giving me 3 lights .. Pressure switch open

I tested and the switch is good .. Tested all other safety's .. No issues

Couldn't get the unit to move along the sequence..

Tested switch has power going to switch and back to the board.
Unit comes on inducer runs ..
Then shuts off .. 3 lights
Jumped switch just to be sure .. Same thing ..

So I replaced the board .. New board
94w83
Same thing .. So I started back at square 1 .. Tested voltage .. From board back to switch

Pressure switch gets power from piggy backing primary limit ..
So at board harness ..
24 volts Primary limit one side 24 volts / other side of limit / it piggy backs to pressure switch ..22 volts @ piggy back one leg goes back to board(feed back for primary) the other to pressure switch

At the switch its getting 21.5 volt same back to the board ..

So I think I may have a transformer going bad ??? ..

Ok.. I removed the power from pressure switch by leaving off the wire coming from the primary limit because of low voltage ..

Decided to pull power from one leg of a roll out .. It Tested 27 volts .. So I piggy backed /to pressure switch .. And now everything is working perfect ..

My question is .. Does it sound like transformer going bad ?? Cause I've already replaced the board ..

The other big question is .. Since the pressure switch is working and all other safety's are working .. Do you see a problem pulling power from one leg of a rollout / instead of primary? I've tested tripping both and are working .. I really don't see an issue or am I missing something ??

Thanks
 
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  #2  
Old 11-13-15, 10:34 AM
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Anyone ? I've been running like 12 hours like this with no issues .. I ordered a transformer in the mean time.

Doesn't make sense that I can run the unit like this if the transformer is going bad ..

Unless the original run is maxing out the transformers amperage because it's starting to fail (?) or something in the board is regulating the voltage below what's needed ?? (New board though )

All other safety's running off the board show 27-28 volts. Only 24volts @ board for primary/pressure switch run.. By them time it completes the pressure switch side back to the board it's like 21.5-22 volts .. So the board doesn't see it ..gives 3 lights code ..

Transformer with no load .. (wires off )
Shows 27-28 volts ??? Don't have an amp probe to test vs rated amperage..
 
  #3  
Old 11-13-15, 10:41 AM
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I doubt the transformer is bad. What do you mean by " piggy back "?
 
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Old 11-13-15, 03:08 PM
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By piggy backed I mean .. 2 wires tied to 1 terminal.. They are daisy chained..
 
  #5  
Old 11-13-15, 03:09 PM
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The manual for your furnace can be dowloaded at:
Technical Literature


The circuit diagram is at Figure 23 Page 25.


<<Pressure switch gets power from piggy backing primary limit ..
So at board harness ..
24 volts Primary limit one side 24 volts / other side of limit / it piggy backs to pressure switch ..22 volts @ piggy back one leg goes back to board(feed back for primary) the other to pressure switch

At the switch its getting 21.5 volt same back to the board ..
>> v


The circuit diagram shows what is being described. It's an odd wiring arrangement.

I doubt very much that you have a transformer problem.


Connect one side of you meter to furnace sheet metal ground.

Turn up the thermostat, which will turn on the inducer motor.

Measure the AC voltage on both sides of the pressure switch.

One side should have 24 VAC to it as soon as the thermostat calls for heat.

The other side should have 24 VAC when the inducer motor comes up to speed and the pressure switch closes.


According to the diagnostic code, the pressure switch isn't closing. You need to verify that that is the case.


There are many reasons why the pressure switch isn;t closing, but you need to verify that that is the issue before looking further.
 
  #6  
Old 11-13-15, 03:26 PM
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The pressure switch works as it should ..
Switch makes and breaks with inducer fan as it should ..
Voltage doesn't drop across it once it makes ..

The lead coming off the primary .. voltage to the pressure switch/ disconnected is
21.5 -22 volts

I took the pressure switch lead off at the primary to measure voltage across it .. 24 volts on 1 side it drops a little to the other like 23.5 volts on the other ..

Once I reconnect the otherside (lead going back to the board ) which is the power for the pressure switch .. It drops to like 22 - 21.5 volts ..
 
  #7  
Old 11-13-15, 03:44 PM
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The voltage across is 21.5 to 22 volts ..
Gives 3 lights .. It to low of a voltage for the board to see ..

Leaving the power lead off the pressure switch
I pulled 27 volts using a 2 way spade connector to tie on the terminal of a rollout switch .. To power one lead of the pressure switch ..
The unit is working 100% as it should
 
  #8  
Old 11-13-15, 03:51 PM
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My only concern .. Is why did they power off the primary in the first place .. Now that I'm powering it off a rollout .. I kind of was concerned if there was a safety lock out / / reason ?? Some reason they would want to drop the pressure switch / once the primary was tripped.


But after thinking about it and testing ..everything seems to function the same.. So I don't see a safety concern .. Even if the power to the pressure switch stayed up after primary trip .. I still dont see an issue .. As long as the pressure switch function properly along with the inducer
 

Last edited by Piranhaz; 11-13-15 at 04:30 PM.
  #9  
Old 11-13-15, 04:06 PM
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Response to Piggy backed ..

Two wires are tied to 1 terminal .. Using a 2 way spade connector..and proper wire
 
  #10  
Old 11-13-15, 05:01 PM
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I took the pressure switch lead off at the primary to measure voltage across it .. 24 volts on 1 side it drops a little to the other like 23.5 volts on the other ..
That is a bad pressure switch. The contacts are dirty or arced. There should be 0v of drop across the switch.

There should be almost no drop in voltage thru the circuit. Any drop can be a potential problem.
 
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Old 11-13-15, 05:27 PM
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<<I took the pressure switch lead off at the primary to measure voltage across it .. 24 volts on 1 side it drops a little to the other like 23.5 volts on the other ..

Once I reconnect the otherside (lead going back to the board ) which is the power for the pressure switch .. It drops to like 22 - 21.5 volts ..>>



Yessssss. I think I agree with PJ Max.

I think I'm interpreting your comments correctly. You have 24 VAC on one side of the pressure switch and 21-22 volts on the other side when the pressure switch is closed.

It should be 24 VAC when the pressure switch is closed.

You can test this by jumpering around the pressure switch so that 24 VAC is applied when the pressure switch is closed. If the pressure switch is causing the problem, the furnace will operate with 24 VAC applied.
 
  #12  
Old 11-13-15, 05:28 PM
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That drop is referring to the primary .. I took everything off the other side so I could get a reading across it .. I lost a .5 volts across the primary ??
I lost more voltage going to the board .. Down to 22 volts

When I reconnected the lead going back to the board .. Leaving off the pressure switch .
The voltage dropped to 22 volts .


So the voltage at the P.switch is approx 22 -21.5 volts.. The loss happens before the switch .. ( I jumpered the wires p.switch just to make sure it wasn't the switch ) still got 3 lights error code..



That only leaves .. Bad wires / bad board or transformer ??? Replaced board already .. The wires appear fine .. Nothing loose or shorted .. I guess I could test across them to make double sure .
 
  #13  
Old 11-13-15, 05:48 PM
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Sorry, I'm not finding the descriptions of your tests especially intelligible.

I'd try jumpering 24 VAC to the side of the pressure switch that goes back to the ignition control board, so that the ingition control will be seeing that 24 VAC.

IF the problem is the pressure switch, that should cause the furnace to start operating.

Normally I would expect a value of 21 volts to cause the circuit board to call that an open pressure switch, but perhaps it does do that.


It's worth checking to be sure, though.
 
  #14  
Old 11-13-15, 05:55 PM
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Yes ..you would think 21 volts would be in tolerance of 24 VAC .. But it's not .. But the bottom line here .. It's working .. The pressure switch works correctly..

I am going to get in contact with the company and discuss it with them.
Thanks for your help ..
 
  #15  
Old 11-13-15, 07:06 PM
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You state that the pressure switch is good but HOW did you determine this? A fairly common problem with (draft) pressure switches is that the connection to the induced draft blower will become internally corroded and NOT allow the pressure signal to reach the switch diaphragm. This WILL give a diagnostic light sequence of a failed pressure switch.

The cure is to make certain the passage through the fan connection nipple is clear by running a succession of small drill bits inside that nipple until the inside is definitely clear as well as carefully checking the rubber hose to the pressure switch for both obstructions and leaks.
 
  #16  
Old 11-13-15, 07:59 PM
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How did I determine the pressure switch is good ? Because when the inducer fan comes on .. I can hear the contacts inside it make .. I also tested the voltage on both leads .. The same Voltage on both sides means it's working correctly ..

If you read my post you would know that ..

Also I've determined what the problem is .. By making the unit run correctly .

My questions were about ..
Transformers and powering the pressure switch off of a different location .



Guess I should have stated somewhere .. I'm not an idiot .. I was/am school trained /certified HVAC tech .. Well 20 years ago .. And worked for the Trane company for a few years before going into the Air Force .. I only mess with this stuff when need be .. So I'm out of touch . But trouble shooting is trouble shooting .. Start at the problem and work your way back. Process of elimination..

Thanks for your help ..
 
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