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I have a 9 year old Lennox model G40UH-488-90-02 central gas furnace. Its symptom is that when the thermostat calls for heat, the burners come on briefly and then go out.
I have a 9 year old Lennox model G40UH-488-90-02 central gas furnace. Its symptom is that when the thermostat calls for heat, the burners come on briefly and then go out. The system automatically tries several times to light the burners and then gives up.Sounds simple, no?Here's the rest. I measured the AC voltage going to the flame sensor and it's 62 VAC. Then I measured the current when the flame is lit and it's 2.2 micro amps DC. According to the manual, this should be ten times more current than is needed to keep the gas valve on. Then I replaced the flame sensor with a new one and I got the same result.I checked the rollout switches and they are closed except when I put heat to them. This seems like normal behavior. Can you diagnose this problem?The model number of the Sure Light control board is 0110 P 50A65-120-03 Lennox 56L8401 Made in Mexico(###) ###-####br/> Read more: http://www.justanswer.com/hvac/47m02-.html#ixzz3rpB0EsXz
If the control board is the problem, where can I get a new one at reasonable cost?
Before I condemn a circuit board, let’s verify your ground connections at the board to furnace and furnace to ground incoming.
I will inspect the grounding this evening and get back to you tomorrow. The furnace has not been disturbed so the only reason I can imagine for the ground to go bad is corrosion. Do you want me to take any resistance or voltage measurements?
I would like you to take line voltage and low voltage readings to neutral and then to ground and compare. Take low voltage to C or common and 115v to neutral and then to ground!
I took apart the make-up box, removed the wire nuts and left the furnace wires connected to the incoming house wiring. I measured the voltage from black (hot) to white (neutral) and got 124.9 VAC. Then I measured from black (hot) to green (ground) and got the same number.I disconnected the black and white furnace wires from the house wiring and measured the incoming house voltage from black to white as 124.9 VAC and black to green as the same number. So connecting the furnace had no effect, which is a darn good thing.I measured the voltage across the secondary of the transformer as 28.3 VAC and from the transformer secondary blue wire to the flame chassis as 28.2 VAC. I inspected the green ground wire from the make-up box to the control board chassis and from there to the flame chassis and everything looks good. I tightened the #8 sheet metal screws holding those in ground wires in place just to be sure.I noticed that the inducer motor was making abnormal noises, from squeaking to rattling as if some pieces had broken off the fan blade and were rattling around inside the inducer blower compartment. On one occasion, the inducer motor would not even start turning unless I gave it a twist. So I started thinking that there was not enough inducer air and that the pressure switch was not kicking on. I disconnected the pressure switch wires and checked the pressure switch resistance, which was infinite if the blower was off and 7 ohms if the blower was on. I think that the pressure switch is OK.I can picture having to change or at least oil the inducer blower motor, though this is not related to my original problem of the flame going out too soon. Once the inducer blower gets going, it works fine but the flame still goes out. Three 8-32 screws hold the inducer motor to its fan housing. When I loosen those screws, will they fall into the fan housing never to be seen again or are they welded to the frame?The schematic wiring diagram supplied with my Lennox unit is not very helpful.
It appears it will be a new circuit board is in your future.
Also the inducer mounts will come out of tapped holes so you don’t need to worry about losing any nuts.
This may help also - http://www.cozyparts.com/Lennox-Parts/forum/Lennox_service_manual/Furnaces/G40UH_G40UHX_09-01-2006.pdf
Where can I buy a new circuit board? I have the name of some local Lennox distributors but often they won't sell to the average Joe. How much do you think the board should cost (ballpark)? Finally, do you think that oiling the inducer motor will do the trick or will it need replacing? I'd like to order everything at once.
See link - http://www.climatedoctors.com/items/item.aspx?itemid=1785842
As far as the inducer motor sir, I try to refrain from hypothesizing. I have oiled them and had them work just fine and totally wasted my time on some.
Motor - http://www.cozyparts.com/Lennox-Parts/ProductInfo.aspx?productid=24W95
What if I replace the circuit board and I still have trouble? In your experience, what are the chances that the circuit board will do the trick? Please note that the original circuit board is no longer made but must be replaced with a model 83M00.I called some Lennox places and as expected they won't sell me the part. Online is the way to go. I like all things mechanical, electrical, and chemical. Did you ever stop to think how amazing it is that a flame rectifies AC to DC?
I was intrigued by how a simple thermocouple generated magnetism.
Then the flame sensor with ac to dc rectification
Ultraviolet cadmium cells that "see" flame
I have units that when they fail, they call or email me. Not the company, not the maintenance man. The unit itself will fault and call and tell me what it faulted on.Then I call its owner and tell them.
As far as if the circuit board is not the fix. We have a board we have inputs. When an input doesn’t "fit" it creates the lockout function. Now, we have 2 ways to cause this lockout negative input, or internal failure. By checking all the limits, sensor, ground, pressure switch etc we eliminated inputs.
Now we move on to internal failure. If the new board also failed it just means we did not identify the negative input yet. And given the process so far, I am confident the inputs are reporting positively.
The worst case scenario has come true. I purchased and very carefully installed a new circuit board in my Lennox G40UH-488-90-02 furnace but I have the same symptom as before. The old board was a Lennox 56L8401 and the replacement board is an 83M0001. I made careful note of where each wire went on the old board and made sure to connect the wires to the right place on the new board, according to the instructions that were enclosed with the new board. You will recall that I previously replaced the flame sensor. With the new sensor and board, the flame still lights up briefly then goes out, presumably because the controller board "thinks" that the flame is not lit. I measured the voltage from the flame sensor to ground as 69.6 VAC with no flame and 56.0 VAC with the flame lit. The current through the flame sensor is 2; .7 micro amps, which should be plenty to keep the flame lit.I noticed that with the blower metal door removed from the cabinet, the flame sometimes stay lit long enough for the blower to start to come on. The unit quickly shuts down as the blower gets up to speed. I believe that this may be normal behavior as the blower is supposed to run only when the door is in place. Does that sound right?
Did you have this to install?
Yes, these directions came with the circuit board and I had also downloaded them from the Lennox website (or maybe you referred me to them online). Even though I had to mount that metal plate behind the new board and install the circuit breaker in series with the transformer secondary, the installation was not really very difficult. I oiled the inducer motor and now it behaves fine.
What do the diagnostic lights say?
With the AC power turned on and no call for heat, the two lights both flash slowly. The new board has a red light on the left and a green one on the right, slightly different from the old board but probably of no consequence. When a call for heat is initiated, both lights flash rapidly. The inducer motor starts with the call for heat. The igniter comes on, and then the gas valve opens. The burners light up but then quit after maybe 20 seconds. It seems like the time the burners are lit shortens with subsequent attempts to light them.Maybe the short red hose leading to it gets blocked by debris in the inducer fan. At first there is 27.6 VAC across the pressure switch but when the inducer motor gets up to speed, the voltage drops to zero, indicating that the switch is closing. When the flame goes out, the voltage across the pressure switch briefly rises to 27.6 VAC but then falls to zero again.
As a test remove the 2 wires from the pressure switch. Start furnace after the exhaust fan starts touch the 2 wires together and hold together. They must maintain metal to metal contact. Tell me if operation still faults.
I ran the test you suggested. I removed both wires from the pressure switch, set the thermostat to call for heat, waited until the inducer motor got going, and then shorted the two pressure switch wires together by plugging them onto a thin 1/4" x 1" copper strip. This time the flame stayed on and then the main blower came on. I tried it again and got the same result. The pressure switch is highly suspect. I will take it apart when I get a chance. You will recall that I tested the pressure switch earlier just by measuring its resistance with the inducer motor on and off. Although it seemed fine, it apparently opens the circuit momentarily, maybe due to dirty contacts.
That is why I hem and haw and try to check everything before buying parts.
You found it. I highly doubt a tech would have caught such an intermittent fault.
We had concluded that the pressure switch was bad. What if there is not enough inducer air to keep the pressure switch on reliably? Maybe the pressure switch was actually OK. Therefore I took the inducer fan housing off the furnace was shocked to see that the squirrel cage blower was half rusted out. You might recall my observation of November 12, reproduced immediately below."I noticed that the inducer motor was making abnormal noises, from squeaking to rattling as if some pieces had broken off the fan blade and were rattling around inside the inducer blower compartment. On one occasion, the inducer motor would not even start turning unless I gave it a twist. So I started thinking that there was not enough inducer air and that the pressure switch was not kicking on. I disconnected the pressure switch wires and checked the pressure switch resistance, which was infinite if the blower was off and 7 ohms if the blower was on. I think that the pressure switch is OK."These blower noises were a clue that the blower was no good. It is amazing to me that Lennox uses an uncoated steel squirrel cage fan in a hot, moist atmosphere. The furnace exhaust must be hot and wet since water is a by-product of gas combustion. It is also possible that rain comes down the double walled exhaust duct and falls into the blower compartment. The blower blade does not have a chance.I thought that I was going to replace the squirrel cage fan blade but Lennox designed the unit like a tin can. You can't get the fan out of its housing and I do not believe that the fan is sold separately. It would be a lot less expensive than the whole blower assembly.I ordered a new blower unit. A new pressure switch is also required because the old one is not suited to the new blower. The new blower arrived yesterday and I discovered that Lennox has made some improvements. The squirrel cage fan can now be removed because the suction side has a drastically larger hole than before. The fan part is still naked steel, so it is going to rust out just like the old one did.I intend to paint the fan to increase its lifetime. I loosened the set screw that holds the fan to the motor shaft but I could not remove the fan blade. There must be a burr on the motor shaft. Forcing it off will not be useful because the fan is thin steel and it would get warped. So I am stuck painting the fan tediously by hand. I will use a rust proofing primer, claimed to be good to 300 deg. F, followed by black spray paint used on barbeques, good to 1200 deg. F that should protect the fan blade without increasing its weight very much.
I have a follow up to our correspondence of November. I took apart the blower assembly and found that there are two pieces. One is tin can housing and the other is a flat plate with a small hole. It is easy to remove the plate from the blower housing, at which point it became obvious that there was actually a large hole for sucking air into the squirrel cage. The plate has the small hole, which made me think that the squirrel cage was not removable. In principle, one can get the squirrel cage out and replace it. In practice, the set screw is rusted like crazy and it won't come out. The squirrel cage was so rusted that it broke in half when I worked on it. I now have the challenge of getting the squirrel cage off the motor shaft, which will take a Dremel tool or something. If I can buy a new squirrel cage from Beckett Air, I could refurbish the blower and have it ready for the next failure. I have another furnace that uses the same blower. The blower is subject to heat and moisture so it is disappointing that Lennox uses a bare steel squirrel cage. In addition to this, carbon dioxide is a product of combustion. It combines with water to make carbonic acid, which would corrode the blower even faster.I installed the new blower assembly that I had purchased and had no difficulty. First, I painted the squirrel cage with rust preventative primer followed by high temperature barbeque paint. Getting the new blade off its shaft was not so easy because there was a burr on the shaft but I eventually got it. I reassembled everything and it works great. Here is a tip that you might wish to convey to future users of your web site: paint the squirrel cage to enhance its life. My only remaining challenge is to see whether Beckett Air will sell me a squirrel cage. I have a spare used but working circuit board that I will keep around.