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Phil, Mechanical Engineer
Category: HVAC
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Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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I have the wonderful Lennox Pulse 21. Model #G21Q3-80-1. The

Customer Question

Hi! I have the wonderful Lennox Pulse 21. Model #G21Q3-80-1. The control module is not sending 24 volts to the gas valve. More like 11 - 14 volts. Once it tested at 3 volts, and then it started working at 22 - 23 volts. Is this a control module issue or the relay?
Submitted: 1 year ago.
Category: HVAC
Expert:  Jason Whatley replied 1 year ago.

Hi my name is ***** ***** I'm here to help you out! If the control module is intermittently putting out random voltages then it is failing. To triple check, remove the wires to the gas valve (when the unit decides when it doesn't want to work again - and it will) and measure voltage. This will confirm the gas valve is not intermittently causing an issue. Very rarely is this the case, but it can happen. I've seen many failing controllers on pulses do exactly what you are describing here.

Customer: replied 12 months ago.
Hi Jason;
I took your advice, ordered the control module for over $100.00. It took a week to arrive. I put it in - very easy - and it worked for 1 cycle. Now, the voltage being sent to the gas valve during the start cycle is 1 to 3 volts instead of 24 volts. (same problem as before)
I took an internet course on this particular furnace and it seemed quite easy and logical. I figured if the flame sensor was to blame, it would at least start for a second. I'm now thinking it may be the pressure switch, because the induction fan seems to be working okay. BTW, I ran a jumper to the gas valve during the start cycle and it started just fine.
Do you have any further advice or ideas?
Expert:  Jason Whatley replied 12 months ago.

Okay, is this a g14 or g21 pulse? Is the light on the control module "on" when trying to light? Can you hear the inducer "surging" (on....off....on...off)?

Expert:  Jason Whatley replied 12 months ago.

Also did you remove the wires from the gas valve and see if you were getting voltage to those wires while trying to light?

Customer: replied 12 months ago.
It's a G21.
There is no light on the control module. There IS one on the ignition module, though. It is ON continually until the furnace fails to light about 4 times. Then it is on "lock out" and will blink. The draft inducer motor runs smoothly with no surging or 'on and offs'.
I tested The voltage at the gas valve. With the wires connected The voltage measures 1 to 3 volts from hot (orange) to ground. With both wires OFF, the voltage is 4 to 6 volts. (during the lighting cycle - otherwise the voltage is always zero)I tested the pressure switch, too. With the wires disconnected, the draft inducer motor doesn't come on. There is continuity between its terminals. I tested the resistance and it wasn't quite zero. It bounced around a lot at just a few ohms.
I blew into one of the hoses while testing the terminals and heard contacts open and the continuity was lost. When I stopped blowing, the resistance almost returned to zero.
Do you have any more ideas? Thank you!
Customer: replied 12 months ago.
I did test the voltage to the gas valve.
While the furnace was trying to light I tested voltage to the gas valve.
With both wires disconnected I got a reading of only 4 to 6 volts.
With the wires connected, I got a reading of 1 to 3 volts while the furnace was trying to light.I'm beginning to diagnose the pressure switch now. Looks promising.
Expert:  Jason Whatley replied 12 months ago.

The pressure switch is a normally closed switch - if there is a draft blockage, the switch will open and drop control voltage. If the switch opens (has no continuity) then the inducer will not run or surge. The surging will be apparent. This is the opposite of most furnace pressure switches that prove draft. This switch proves a blockage.

Expert:  Jason Whatley replied 12 months ago.

Reading your last post I think I see the mix up - the board in question was the ignition module, not the fan timer board. The ignition module sends 24v to the gas valve to open... This is the board I was referring to originally. You stated control module and I was interpreting that as the ignition module.

Expert:  Jason Whatley replied 12 months ago.

I'm sorry for the mix up- it's a little challenging to troubleshoot with this method so we do our best!

Customer: replied 12 months ago.
Do you think it may be the ignition module?
I noticed, too, that the control module is also called the fan timer board.
It has the direct output to the gas valve, though, so I naturally thought it was the defective part.
Does the power to the gas valve come initially from the ignition module and pass through the fan timer board to the gas valve?
Expert:  Jason Whatley replied 12 months ago.

Yes the common to the gas valve comes from the fan timer board, but the 24 volt hot comes from the ignition module.

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