coleman furnace no ignition

Paul_69

i got a coleman gas furnace 98%. brand new out of box 120 vac to glow ignitor,tried another with same results. is it possible i am getting false voltage reading.the lockout code said something about pressure switchclosed b4 inducer was on.but i am baffled that with a reading like this that it wouldnt glow.

Tim McElwain

I am not to clear on what

you are saying. Do you have 120 VAC at the igniter and it is not glowing?



What is the model and serial number and what is the number of the integrated control?

Paul_69

not glowing

hi tim

correct, i had 120 volts ti ignitor at molex connection. model #cp9c060b12mp12c. i dont have serial # but can get it as well as control board number if needed. i am surprised it reads 120 volts and wont glow. ohmed out ignitor ok and tried two other ones. thx paul

Tim McElwain

Paul, I do not have a

manual for that unit but e-mailed Coleman to send me a PDF of the manual. I need to look at how the igniter is wired in. These units use a different setup for different White Rodgers gas valves. Lets see what they send me tomorrow if you can wait.

Paul_69

pdf

hi tim

yes,i can wait. just finish installing furnace and tried to fire it off. not cold here right now,using more for ac but need to find out why this didnt light off. thx for the help

                                                                                              paul

Tim McElwain

Couple of things to check:

Make sure you have a good ground and that the polarity is correct.



Did you power up the igniter outside the furnace and see if it glows. I usually just go plug the wires into a wall receptacle to test them. If it glows that way then something is wrong with the electrical on the unit. Make sure the control board is grounded.

j a_2

120 vac

Hi   I was just thinking about this as well..and will be interested in the findings....What I have found is the connect er pin/contact if you will, being pushed back and not making good contact...once you reinstall it...Use a set of thin leads or file yours down to a point....it will help...If possible, when its calling for ignition wiggle the molex and see if it starts to glow...Be careful....JA

VictoriaEnergy

Believe the control

The 120 may be a sensing voltage behind a resistor so it reads 120V under near zero load of your meter, but with the ignitor connected it drops way down.  You could check the volts across the ignitor when its in or,use a clamp on current meter and see check the current flow.  Do all that to satisfy your curiosity, then go back and check the fault code.





When the inducer is off, there is no pressure across the pressure switch so it should be open.  During the ignition sequence the control is expecting it open before it powers the inducer, and to subsequently close within the allowable time after the inducer powers up.  Your control is telling you the switch is closed when it should be open and it should not be attempting to lite the furnace.



Shut the gas off to the furnace. Disconnect the pressure switch.  Start the furnace. about 5 secs after the inducer starts running, jumper the pressure switch wires together.  Expect the control to run the inducer for a few seconds as a "pre-purge" and then it should power the ignitor.  If the ignitor then starts to glow, then shut her down and start looking for something to be clogging airflow to the pressure switch, a short in the wiring to the switch, or the switch itself(rare).

Paul_69

pressure switch

found problem today went back to fault code and found a pressure switch that was bad. brand new furnace.this switch was tied into the modulating gas valve which is white rodgers. thanks for all the help from posts. the 120 volts at the ignitor still isnot clear. if i had 120 volts to a motor, light etc. and it didnt work you would replace part. is this different for readings coming from a control board circuit? for future trouble shooting am i missing something or testing incorrectly? thx for all the inputs folks!!  paul

VictoriaEnergy

Elec-trickery explained.....maybe

It's a little convoluted, but here goes my best shot to explain it:



A good way to picture it in your mind is to think of electricity like water flow.  Voltage equating to pressure, amps are movement of water, resistance as restriction in the pipe.



So imagine a 2" pipe that is supplying water to our circuit with 120 PSI.  There is a valve that allows water to flow to our igniter.  The igniter has resistance, so imagine it being a length of 3/4" pipe that restricts the flow (amps).  When the control wants to light the furnace the valve opens.  Between the Valve and the igniter there is a small 1/8" tube that is always allowing a trickle of water to flow through the igniter, even when main the valve is off.  Because the the 1/8" line is so small, there is not enough flow to accumulate any pressure in the igniter.



Now if you remove the igniter, there is no longer anywhere for the water from the 1/8" line to flow to, so the pressure builds up to the supply pressure.  When you connect your volt meter to it, the current flow through it is tiny (think pilot orifice) so the pressure stays up at 120.  You have full pressure, but not enough current available to do anything with.  If you were to re-connect the igniter and use a clamp on current meter to essentially listen to but not interrupt the flow of water through the igniter, you would see only the control flow from the 1/8" pipe.  If the Main valve was opened up you would then see the much higher flow to power the igniter.



Does that help?

Paul_69

it helps

thanks very much for that explanation. so i guess the best bet with any test is to apply volt meter into load with connections still on for true reading or use amp meter as well.thanks for your interest, i am a 30 yr. tech but sometimes you still get thrown thru a loop when something looks true but is  false!!