Furnace Ignition Module Terminal Question

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Old 01-11-21, 10:09 PM
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Question Furnace Ignition Module Terminal Question

Howdy!

I'm a little stumped after diagnosing my furnace issue (not firing off the main valve) to a faulty ignition module from Robertshaw; pilot lights up okay and stays lit btw. I thought it'd be as straight forward as ordering another module and then plug 'n play. That is until I noticed the new module has a soldered-in jumper and a cap around it on the sensor terminal (see pics below). Assistance from anyone familiar with the Robertshaw 780-715U ignition module is much appreciated.

Am I suppose to cut that jumper and cap off since I'm on a 2-wire igniter & flamer sensor setup?


Old module (jumper not used)



New Module (jumper soldered on)

Thanks in advance!
 
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Old 01-12-21, 05:10 AM
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If the module model numbers are the same, the white wire should plug into the plastic jumper. Were there any installation info with the new module indicating what P338 means?
 
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Old 01-12-21, 10:19 AM
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You more than likely no longer need the extra flame sensor.
780 install notes
 
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Old 01-12-21, 11:11 PM
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Thanks for chiming in guys.

Turns out that jumper plug simply pulls right out. The extra leg had me fooled thinking it was hard soldered onto the PCB.

Unfortunately my problem persisted even after having replaced the box, so we can most likely rule out faulty ignition module.

Faulty thermocouple perhaps?

I'm hesitant to jump to this conclusion just yet, since pilot lights right up shortly after ignition sparks. Got a beautiful engulfing flame that gets the thermocouple red hot, after which the sparking stops and the pilot stays lit. From that it can deducted the thermocouple is correctly sensing the flame and relaying the signal to the module. I've also counter tested this by disconnecting the thermocouple to the module then allow the pilot sequence to begin, which it did but sparking continues despite having a good flame.

Curiously, I measured the P/C/M terminals of the gas valve and got the following readings:

P(ilot) to C(ommon): 15V (when pilot is commanded by the module)
M(ain valve) to C(ommon): 0.8V (when main is commanded by the module)

Hmmm, both readings appeared low? Should I not be expecting closer to 24V?

15V appeared sufficient enough to activate the pilot solenoid, but 0.8V likely was not enough for the mail solenoid. I artificially jumped P-to-M and bam, the furnace fires right up. So it appears the gas valve is also good.

What could be causing the low voltage? Can't be the ignition module, because I just replaced it.

Bad power supply?

I'm at a loss!

 
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Old 01-13-21, 06:49 AM
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Your spark control module is powered by 24 vac generated elsewhere on the appliance so check elsewhere why you are only reading 18 vac. Did you check the MV terminal on spark module for 18 vac? Since the pilot and main valve signals come from the spark module and it is new, check the wire connecting spark module to main valve (red wire) for continuity.
 
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Old 01-13-21, 07:37 PM
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There is a flame sense rod....... no thermocouple.

You need to check the 24v terminal on the ignition module for at least 24vAC.
There is no power supply. Just a transformer.

You should have the wiring diagram there. Follow it back from the transformer.
If you have a power vent.... it will pass thru there.
Post the diagram for us.
 
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Old 01-13-21, 10:28 PM
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This is an older furnace that uses thermocouples; no flame sense rods. You are however correct on transformer, not power supply. Brain fade.

The output of the transformer checked out at 28V as was the R-terminal to the thermostat. I've also gone over continuity check of the various wires and saw no signs of shorts. I did note the W-terminal (Heat) when energized was only hitting 0.8V (we knew this from my 1st post). Faulty thermostat?

I try jumping R to W to simulate a Heating command. Low and behold, I had sparks, pilot, and then full flame!

I've got an Ecobee3 smart thermostat with all the bells and whistles all appeared to be in working order. Maybe one of the pull-up resisters on the output pin fried. Heavens!

Going to Lowes for a replacement. Will report back.

 

Last edited by THX723; 01-14-21 at 12:46 AM.
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Old 01-14-21, 10:56 AM
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The 780-715 instructions say it can work with an existing pilot light with thermocouple. That's fine but when there is a thermocouple involved..... there is no 24v pilot valve.

I've only ever seen that module used as an intermittent pilot.
 
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Old 01-14-21, 02:39 PM
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@Pjmax I stand corrected. This is in fact flame sense rod not thermocouple. I have them backwards. And yes, intermittent pilot is the setup here.

Now for the update...

Son of a gun, it was the Ecobee smart thermostat! The world is alright once again (and warm) after popping in a new one.

In hind sight, thermostat should have been the first thing I fully diagnosed. I was blind-sided by its ability to initiate heating and blower. The ignition/valve control module clearly got the message and got on with the on-demand pilot only to choke on opening the main valve. It was easy then to suspect the flame sense rod first. When that checked out okay, the module was next on the chopping block and on with the wild goose chase you've all witnessed. Many of you more savvy folks likely cringed, but it was a good lesson learned for me and hopefully for someone out there too.

Thank you gentlemen for the pointers. I shall now go pour me some Scotch.

Cheers mates!
 
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Old 01-14-21, 02:58 PM
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If it was the Ecobee...... there may be too much current draw. The nest and ecobee's are two lines of thermostats that use electronic switching vs relay switching.
 
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Old 01-14-21, 03:52 PM
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I replaced it with yet another Ecobee (previous one has been working flawlessly for the past 5 years). Whatever it was that went bad inside, it sure caused me a few gray hairs and several cold nights.
 
 

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