Ignition Control Module Question


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Old 11-07-08, 09:31 AM
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Ignition Control Module Question

I have an older Lennox G16 gas furnace that has a ignition control unit that is not supplying any voltage to the spark ignitor to light the pilot. I have tested the ignitor's terminal with a voltmeter, and also by hooking a jumper wire to the terminal to see if I could get it to spark off a ground, but I get no voltage and no spark when the thermostat is calling for heat.

Then, inexplicably, the furnace will suddenly start working again, and will continue to work for a day or more, only to fail to provide voltage for the ignitor when I go to turn the unit on again after it has been sitting cold for a while (I have never had the ignition fail if the furnace had recently been in use, and was still warm).

I am wondering if this means that my ignition control unit is to blame, or if it might be some other kind of proving switch, like the air pressure proving switch.

Can anyone think of any other test or process of elimination I can go through before buying a new part?

One other thing I can add is that when I turn the thermostat on, the pilot gas valve does open, and I can light the pilot with a lighter. However, with the pilot lit manually in this fashion, and the flame sensor glowing red, the main gas valve does not open. So, I am assuming that there is something that is causing both the ignitor and the main gas valve to be denied voltage. Any suggestions? TIA
 
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Old 11-07-08, 05:43 PM
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Those encased ignition modules require line voltage* to supply power, and will not be activated unless the draft inducer motor runs, and the safety circuit is completely closed through every device. When the low voltage (24v)signal gives it the go-ahead, it enables line voltage to go into the ignition process.

By knowing this, this may help you test.

Be certrain every connection is good. And that if you have any of those connectors that are more like pins, that have a split in them by design down the middle, as oposed to spades, make sure the pins are spread some so that you have good connection with their female recepticles. I had a problem in that regard, preventing ignition go-ahead, and I caught that, which spared me at the last minute from buying expensive part. That was a good or lucky 'catch'. Also if spade connectors, slip them off one at a time, and reconnect, making sure you are getting a nice scratchy connection. If sloppy, squeeze connector closed some before reinserting.

*line voltage might be only 24 volts, that becomes transformed in the module. HSI's though I know have 120. Either way, you will be able to read what all your connectors say (often are abbreviated), and be able to tell which terminal is receiving the voltage to the module. Some actually SAY "Power". That is incoming. And if you are getting power, there, no matter if 120 or 24 volts, and only when the inducer comes on, then you know that everything is good up to the module.

Modules also need to be grounded, directly or indirectly, and you really should back off any grouding screw(s) and retighten. That could surely prevent one from working!
 

Last edited by ecman51; 11-07-08 at 06:00 PM.
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Old 11-07-08, 11:16 PM
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Thank you for that response. All of my connections are spades. I had connected and reconnected yesterday, but I did it again just now also. I have no corrosion, and all the spades feel nice and tight. I also backed off the ground screw as you suggested, and re-tightened it.

I turned the thermostat on (blower turns on, but no spark, as usual) and did some more checking with the multimeter. First, I put a probe on the ground screw, and used the other probe to check each of the two wire terminals on the air pressure switch. Both wires read 24v (I assume this means the air switch is closed, and the circuit is being completed as it should be).

Next, I checked all the wire terminals on the Robert Shaw 780-715 U ignition control unit. I got voltage readings from only three wires. A terminal labelled "24v" read 24v on the voltmeter. The flame sensor wire read 120v, and the PV wire read 24v. I could not get any voltage from any of the other terminals.

Would this serve to indicate that my ignition control unit might be to blame, given that I have 24v input power coming into it, but no voltage to the ignitor, and also no voltage to open the main gas valve if I light the pilot manually?
 

Last edited by penr0295; 11-08-08 at 12:42 AM.
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Old 11-17-08, 11:38 AM
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New ignition controller fixed problem

A new Robertshaw 780-715 U ignition controller did resolve my problem. Thank you.
 
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Old 03-21-09, 12:54 PM
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780-715 U Ignitor

OK I got the same problem. No Spark. I purchased a new unit over the internet (1/2 price of local supplier @ $105) But, being a dedicated DIYer I simply had to find the problem with the old unit. It does not have fancy modern ICs, but good old fashioned discretes. Googled for schematic. No Luck. OK sat down and wrung it out myself. see attched picture of same. During the process discovered a poor solder connection at lug E10 where an external jumper is supplied. This is due to the inadequate area on the PC board trace to retain the wire holder insert. Get a good connection between E10 and E4 and you'll be sparkin right up again. Enjoy! Unfortunately this reply does not allow me to insert an image. Contact me at glibert@cox.net and I will send you a jpeg.
 
 

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