No spark

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Old 11-23-07, 09:39 PM
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No spark

We have a Magic Chef model EG8A74D-10 furnace that has a RobertShaw 7100 DER gas valve, and recently had a RobertShaw SP715A ignition control module, and a cheap old Duracraft thermostat, approximately 18-20 years old. We rent from my in-laws, who are footing the bill but don't have much disposable cash.

The background (kind of long): About a month and a half ago the furnace would no longer come on. We live in upstate NY and can't go without a furnace at this time of year. I looked at it and the gas would come on, but no spark. I took voltages on the ignition control module, and there was no voltage output on the ignition terminal, so we replaced it - the only replacement we could get was the RobertShaw 780-715 universal control module, on which model the thermocouple is redundant.

This worked for about a month, then suddenly once again no spark. Now, however, we had 120VAC on the igniter output on the control module. I found that the wire to the igniter had impedance that ranged from 0 ohms up to 2-3 Megohms when it was wiggled. The wire and contact/ceramic insulater are constructed as all one unit, so just replacing the wire is not an option.

We ultimately found a replacement igniter with a whole new bracket all in one unit. The first thing I noticed was that, while the original unit had a 12 awg wire with very thick insulation (approx. 1-1.5mm), the new one was about 18 awg with much thinner insulation. The guy who sold it to us assured us it could take the current without overheating, and we saw few options so we went ahead with it, but still no spark. We did try to call the person who originally installed the furnace, but couldn't get ahold of him and have received no calls back from him.

I tested the output to the igniter from the ignition control module, and found 120VAC on the igniter electrode. I also tested the AC current to the igniter, and found 1Amp AC. I set the gap (as best as I could get my feeler gauges in under the hood) to the 7/64" specified on the original SP715A ignition module (the replacement doesn't mention a spec). The hood has good continuity back to the ignition module ground, common to the furnace case (this is where the original igniter sparked to to get the pilot to light, so that's where I looked for ground - I presume this is correct). I also checked the DC current to the ingiter, and found about 12 ADC. The DC voltage I can't check, because my meter only goes to 600 V, so it just shows out of range. We have 24VAC into the ingition control module, out from the module to the gas valve contact that controls the pilot gas, and 1.2VAC on the gas valve contact that controls the main gas until it senses a good pilot on the igniter, then we get 24VAC to the main gas valve. We can (and have been, to get required heat) light the pilot by hand, which does open the main gas valve, allowing the burners to ignite.

All of the feedback controls work: we can get it running and set the thermostat to the desired temperature, and it will run until the temperature hits the set point, shut down, cool down, and turn the gas back on to the pilot - just no spark. We only tried this as a test, though. I'm scared to death if we try to use it like that I'll forget and have the equivalent of a gas leak in the basement when it gets cool enough to turn back on, but not cold enough for us to notice. Therefore, I turn the thermostat to its highest setting, turn power and gas on to the furnace, light the plot by hand, let the furnace run for awhile, then go down and turn off the main gas and power to the furnace until it gets too cold, then repeat the process (chilly, semi-sleepless nights).

While trying to find out why it still won't spark, my father-in-law was dusting off the furnace (dusty cellar), and managed to touch a wire to the chimney (some of the system wiring was kind of jury-rigged), causing a spark and shorting out the thermostat. We replaced the damaged one with a HoneyWell thermostat which gives us a new thermostat, an almost new ignition control module, and a new igniter/flame sensor. While troubleshooting exactly what my father-in-law shorted with the extra wire - which has been re-wired so it can't short again - I tested the transformer and the limit switch, both of which are working fine.

This is now very frustrating, as everything else is working, we have voltage at and current to the ignition contact, continuity between the pilot hood and the ignition control ground, but we still have no spark. Admittedly, I don't know for sure if the igniter voltage and currents I found are correct, and I can't find the output specs for the 780-715 module.

I even went so far as to make sure all of the electric was hooked up, turn off the gas, make sure we had 120VAC at the igniter contact and continuity between the hood and ingition module ground, used a pair of needle-nosed pliers with the jaws taped with electrical tape to slowly move the contact closer to and further from the hood to see if changing the gap would work - I also grounded a flat-bladed screwdriver and tried to get a spark to that, still with no luck.

If anyone has any advice on where to go next, we'd very much appreciate it. It's getting cold, and I'm getting tired. Thanks in advance.
 
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Old 11-24-07, 11:32 AM
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Hi k:

What happens if you remove the ignition cable from the module and try to produce a spark at the module to ground? [IOW- not using the new cable]
 
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Old 11-24-07, 03:14 PM
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Thanks

Thanks, daddyjohn.

Am I correct in assuming that you mean directly from the output terminal to ground? If so, no. Is it possible this way, or do I need to do it more like checking the spark on a car (i.e. shunt a cable, for example the old cable, to a screwdriver for example, and try to spark that to a ground)?

Someone told me that I need to correctly reset the ingition module before it will spark - turn off the gas and power, turn down the thermostat, wait 5-10 minutes, and bring it back up in a specific order. Am I correct in assuming that, if I have voltage at the output terminal, the ignition module is correctly reset? What we've been doing is leaving the thermostat up and just turning off the main power and main gas - the gas valve on the 7100 DER doesn't have a "pilot" position and doesn't depress - it's just on or off.

Thanks again.
 
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Old 11-24-07, 03:30 PM
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Turning off the furnace power resets everything. Don't have to worry what position thermostat is in if you do that.

Is your control module grounded good so the spark can ..........>module.......>to ground. ?

Do I understand correctly you already installed a new ignition control module?

It can't be too many things (possibilities) wrong here since the furnace is accepting certain information enough to open the pilot.

(Spark may even BE trying to be there, but can't jump the gap due to poor connection or distance from sparker to pilot.)
 
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Old 11-24-07, 03:59 PM
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ecman51'

We do have a good connection to ground from the ingition module, which has indeed been replaced - it is slightly over a month old. I did make the mistake during our processes of not actually connecting the safety ground wire (green one) securely to the furnace case, and we didn't have "good" ground to the ignition module (6-8 ohms instead of 0 with the safety ground not connected), so I learned my lesson there.

My suspicion now is that we may not have enough DC voltage or current to make the spark - perhaps some feedback signal to the ingition module is there, but not as strong as it needs to be. As I said, I can't check the actual DC voltage to the ingiter as my meter won't go high enough. My research indicates that it should be somewhere between 13KV and 30KV, but as I mentioned, I can't find the specs for this specific ingition module.

Thanks for your help so far.
 
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Old 11-24-07, 04:18 PM
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One thing I forgot to mention that we had tried was to artificially indicate to the system that we had a good pilot using a propane torch, to see if we could get 24VAC on the main gas control wire to the gas valve, with the gas turned off. We got the ignition contact red hot, but couldn't fool the ingition control module into believing it had a pilot - there has to actually be a pilot lit for the module to initiate the main gas valve to the burners.

I don't know if this is helpful or useful information, but I figure any detail I can provide may be useful.
 
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Old 11-24-07, 04:29 PM
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Originally Posted by kscullin View Post
... so we replaced it - the only replacement we could get was the RobertShaw 780-715 universal control module, on which model the thermocouple is redundant.

We ultimately found a replacement igniter with a whole new bracket all in one unit. The first thing I noticed was that, while the original unit had a 12 awg wire with very thick insulation (approx. 1-1.5mm), the new one was about 18 awg with much thinner insulation.
Replaced both the control module and ignitor from the original and are not exact duplications of original? Hmmnm. Probably you are onto something about the impedance and the two just do not jive, I'm thinking.
 
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Old 11-24-07, 04:32 PM
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Yes the pilot has to be lit in order for the model to prove the pilot flame. It uses a process called flame rectification. Yes, what I mean is take the spark to the cabinet straight from the module but not using the new cable. I'm also suspicous of the old cable as well. So use a third wire, maybe even an automotive ignition cable.
 
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Old 11-24-07, 08:33 PM
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I don't have an old spark plug cable, especially not one with a spade connector. So I cut the anode off the old cable, made sure the rest of it was good and continuous, and tried that - still no spark.

I. R. officially not the brightest tool in the shed (sharpest bulb that doesn't go all the way to the top? - God, I love mixing metaphors ). Anyway, I got the bright idea that perhaps I could re-attach the good section of the wire to the original anode - my father-in-law got the anode out of its ceramic collar. Anyway, I spent some time trying to un-solder the old section of wire so I could solder the good wire to it but couldn't get it hot enough. Eventually, I did figure out that my quest was impossible - even if I were successful, when the anode got red hot under the pilot, it would just melt the solder. They must bond that with some kind of conductive high-temperature epoxy (or other type of adhesive). I doubt that the metal is flexible enough to snip some ears into it and crimp it onto the old wire.

According to Magic Chef, their furnace parts manufacturing is now all done by Allied Air in South Carolina, and I can't seem to get the right part number from them for an OEM igniter, if they even make one. They may just make the substitute we got, as the HEP guy we bought it from supposedly did get the correct part number - the part number we got from them was for the thermocouple.

It seems entirely possible that, now that we've replaced most of the ignition system, we may have to try to convert it to a "universal ignition system", which has been suggested as an alternative to fixing this one. I don't know what exactly that would entail, but I'm guessing new transformer, limit switch, ignition module, gas valve and igniter unit.
 
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Old 11-24-07, 10:09 PM
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Hi ks:

No spark striaght off the module and you're sure your test cable was good? Recheck your grounds and all your inputs. You had the t-stat calling for heat right? Is the t-sat digital or analog?
 
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Old 11-25-07, 01:34 PM
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I have been re-reading this entire post. It is interesting, and fun dealing with a very intelligent DiY'er. I wish everyone here would always think of being so detailed.

Since the new ignitor is not stuck, perhaps the gap the spark needs to jump is too wide? If you have new module, the wire is good? (IS it?Check resistance in one of your car's spark plug wires to compare!), the hood/other end of what receives the spark is grounded, you'd think it just HAS to spark. But it won't if that particular replacement module internally has a different (say lesser) d/c secondary voltage conversion and the gap the spark must jump say is too wide.

First compare resistance with one of your spark plug wires. Then if you feel the wire is good, then can you toy around and close that gap?
 
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Old 11-25-07, 03:52 PM
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daddyjohn - the cable I was using is continuous with no breaks at just about 0 ohms. I did re-check my inputs and (most of) my grounds, and found something interesting: Inputs are all good, the grounds from the ignition module and the gas valve common terminal to the furnace case are all 0.1 ohms, but the case of the gas valve - where I was trying to get spark - is at 3.1 ohms compared to the furnace case and the ignition module grounds. I didn't get a chance to check the pilot hood to furnace case before I had to go to work, but I'll check that when I get home. Oddly, this wasn't the case a few days ago when I checked them - everything was common. Not sure what changed - perhaps too much pipe dope last time I re-connected the gas line. I'll also try the spark to something more reliable.

The new thermostat is analog, as was the old one.


ecman51' - thanks. Anything to make the job easier. I did do some playing with the gap, making it larger and smaller - down to as close as I dared without touching the hood - under the concept that, since the igniter is constructed differently, the gap won't necessarily be the same. Your assumption about the new cable is correct - I haven't actually tested its continuity. Since I was getting consistent voltages, I assumed it was good. I will also test that when I get home.

I admit I never thought of comparing the cables with an automobile spark plug cable - I had my brain set to 0 ohms is correct, and may not necessarily be. I can't remember a standard resistance rating for an auto spark plug cable, so I'll have to check one.

Thanks for all the input, guys.
 
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Old 11-25-07, 05:26 PM
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Pilot hood to module ground is what counts as well as pilot hood to cabinet/burner and from there back to module. [ground path that is IOW- you don't the pilot hoodback to module ground trying to jump a seam in the cabinet. The primary reason I wanted you to check the grounds is so you can take the module outpit to gtround. We need to know if the module is puttimgout. Readig the volatge on the igniotr teminal isn't a good enough check. Next thing to verify is the module. Just because it worked before we can't ass/u/me it's still ok. That's also why the inputs had to verified. If the t-stat circuit isn't working, or a safety is open, the module won't spark.
 
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Old 11-25-07, 11:49 PM
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Some measurements:

Ignition cable, spade terminal to anode 0.6 ohms.
Hood to ignition module ground: 3.6 ohms.
Ignition module to furnace cabinet: 0.1 ohms.

It does seem that I may have a grounding problem somewhere between the hood and ignition module, as well as the aforementioned possible gap discrepancy. Perhaps cleaning up all of the contact surfaces good will solve it - the burners are on a unit that slides into and out of the furnace cabinet, held in place by four small "L"-shaped clamps, with the pilot unit on a clamp in the middle between the burners. The primary grounding point for the hood seems, at this point, to be the gas tubing to the pilot itself, which is only connected directly to the gas valve. I believe some sandpaper is likely in order on the pilot unit clamp and the bottom of the burners/top of the plate they sit on to make a good ground to the furnace cabinet.

It's raining heavy and about 40 degrees, so somehow I just couldn't get the motivation to pull a spark plug wire off my car at 1:00 AM to test the resistance of one of those . Admittedly, it seems that since the original wire wavered from 0 ohms to 3 Megohms, it should be 0 when it's working correctly - I've never seen a single wire unit that had both an intermittent open and an intermittent short (although I stopped using the term impossible with electric and electronic systems long ago).

If I'm barking up the wrong tree, or if my conjecture seems plausible, please let me know. I can't think of any other way to establish a decent ground between the pilot hood and cabinet at this point, although it seems the sandpaper idea is just inviting another corrosion interruption incident in the future and that there should be a more reliable solution.

My head hurts now.
 
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Old 11-26-07, 04:36 PM
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I hear your frustration. Let's run a wire from where the pilot burner is attached back to the module. IOW- bypass depending on the cabinet, burner attachment, etc. The only reason I was suggesting using an automotive ignition cable is becasue of the insulation of the cable and not it's resistive value. We need to verify that the module will produce a spark and I didn't want you to get zapped in the process. The next step is verifying the module will still spark. The feedback circuit to the module is a an alternating current that has been rectified to a pulsating direct current uing the pilot flame as a conductor. That's how the pilot flame is proven. You wouldn't want to open gas to the main burners unless there is a way to lite the gas. The pulsating direct current is in microamperes [one micro amp = one millionth of an amp], so 3.6 ohms on that circuit is a lot of resistance.

My best guess on a wire that is both high resistance and no resistance if manipulated is a break inside the insulation.
 
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Old 11-26-07, 05:14 PM
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Take a screwdriver and see if you can draw a spark off the spark terminal
 
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Old 11-26-07, 05:20 PM
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IMO, 3.6 ohms, by allowing current to pass from some solid object's tip, to it's base (the vent hood-pilot tube/ground line), is little resistance = good. It is nearing 0 resistance. 3600 would be higher resistance.
 
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Old 11-27-07, 04:10 PM
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I honestly don't know if what ecman51' indicated or daddyjohn's comment is the more accurate - whether 3.6 ohms from pilot hood to ignition module ground is too much or not. I do know, however, the larger the difference in potential between the anode (igniter) and cathode (pilot hood) the better your chances of a spark that'll jump through air, so following daddyjohn's recommendation I ran a ground strap from the mounting screw on the pilot unit back directly to the safety ground mount at the ignition module - 0.3 ohms from the pilot hood to the ignition module ground. Grabbed my test lead (with good insulation - I don't want to get zapped either ), and tried for a spark at the hood. Nothing. I now most strongly suspect that the ignition module isn't putting out what it needs for spark, since my test lead is the good section of the original ignition wire (you could actually see the frayed wires at the point where it connected to the anode, so I cut that back a bit and stripped and tinned the end).

However, since a new ignition module is about $120, and the return policy of the company where we bought it is to return it to the manufacturer for testing, receive credit from them if it's bad, and then issue credit to us - in the meantime we purchase another module to try - we want to be absolutely sure.

Unfortunately, as nice as the weather has treated us so far, we're running out of time to play with it before we run the danger of pipes freezing. My father-in-law called an HVAC man who comes highly recommended as someone who will let you know exactly what's wrong with it and not just try to sell you a bunch of junk you don't need or a new furnace. We'll see what he says (I can't wait to see his face with all of our ground straps added to the mix - hee hee). I'll let you know what we find out there, to add to your advice aresnals.

Perhaps from all of your advice and feedback, I can even help him isolate it if he gets stuck. I'm sure you'll admit, this does seem to be a sticky one.

BTW, daddyjohn - I probably should've clarified this earlier, but what does IOW stand for - I can't puzzle it out?
 
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Old 11-27-07, 06:01 PM
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mbk3 - tried that, with a screwdriver touching the ignition terminal on the module to ground and from the igniter (spark terminal) to a screwdriver in contact with the pilot hood. No luck - unless you're describing a different technique and I'm just being dense. Bear in mind, I don't have any "test leads" (wire with a clip or clamp on either end) per se, so it can be tough to get a screwdriver in proximity with a certain ground other than the handiest ones (pilot hood, gas valve body).

Now I'm just trying to use an igniter wire to various grounds, as it's much more versatile to get it to reach to the specific ground I want for that particular test. Thanks, though.
 
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Old 11-27-07, 06:21 PM
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IOW = I owe wampum. Just kidding. = In other words....
 
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Old 11-28-07, 09:25 AM
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Lol. thanks - don't know why I couldn't figure it out, now that it seems so obvious (after getting the answer, of course).
 
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Old 11-29-07, 01:10 PM
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Well, it turned out that the problem may have been twofold: It certainly was the ignition module as you suspected. The guy who came down also said that the "igniter" we were sold wasn't an igniter, it was a flame sensor for a warm ignition system (which may or may not work as an igniter).

He replaced both - the RobertShaw ignition module he replaced with its Honeywell counterpart. He also left on our ground strap that we had added, and advised us to clean out the burners good and back the adjustment screw slightly out to give them more gas - some were apparently partially blocked and none were as high as they should be.

Thanks for all of your help and advice. Hopefully I'll never have to darken your doorstop again, but if I do, I know where to come (it's in my favorites now).
 
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Old 11-29-07, 05:29 PM
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Well the wrong part thing expalins a lot. I'm glad you've got the heat back. Thanks.
 
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