Intermittent Pilot Not Sparking


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Old 02-24-08, 08:58 PM
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Intermittent Pilot Not Sparking

Hi,

I have a Rheem RCDD-04NC-CR gas furnace with a Robertshaw 780-715 U Ignition Control Unit. There's no spark, but I measured ~100V at the ICU's igniter terminal. Is that an adequate voltage? I'm guessing either the spark plug / flame sensor unit has to replaced or the ICU needs to be replaced (hopefully not both), but I don't know if it's not sparking b/c of the spark plug or b/c the ICU is putting out too low of a voltage. Also, I found it a bit disturbing that the flame sensor wasn't actually wired to the ICU, but it looks like sensor input on the ICU was jerry rigged wired to a via on the PCB right next to the sensor input terminal.

Thanks in advance,
Joe
 
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Old 02-25-08, 05:32 AM
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I'll leave the flame sensing to someone else this morning as I don't recall on that unit at the moment.

Cut power to the unit and check for dust bunnies or any critters that may have committed hari kari on the igniter. Anything in there will short it to ground and give you no spark.

If you think it looks clear then with the power still off go ahead and cut the gas valve off and get out your meter and an aligator jumper wire or two. Clip your aligator wire on to the spark ignitor rod and then on to one of the meter leads and then your other lead to ground on the chassis. With the gas still OFF and making sure your meter lead with the aligator on it from the ignitor isn't touching ANY metal cut the power back on and do a call for heat and see if your getting decent voltage spikes from that end.
 
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Old 02-25-08, 10:56 AM
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Hi Mike,

Thanks for the reply. No dust bunnies or dead critters.

I got ~35V at the spark plug ignitor rod, but I'm not sure how accurate of a measurement that is. The problem is that I can't get access to the the spark plug without unscrewing it from it's mounting point and that's probably how the ground is connected as the only other connection is an aluminum gas tube for the pilot. The resistance of the spark plug ignitor to the end of the cable/terminal where it connects to the ICU is 2.2 ohms, so I think it should be basically what is at the igniter terminal on the ICU.

I'm not sure what "decent voltage spikes" means. From reading around online/what I've heard, I've gotten conflicting statements that I should be seeing around 120V or around 15kV. Either way, the ~100V is a bit below the first (I'm not sure if it's enough to be a problem) and far below 15kV which definitely would be a problem.

As for the flame sensing, I've also read that it's redundant and maybe that's why it's not hooked up. I've also read something about local sense (via the spark plug ignitor?) as opposed to remote sense (via a second terminal aka flame sensor?).

Any further advice? I really wish I had the manual to the ICU so I could see if it's behaving within specs and go through the appropriate troubleshooting steps. Thanks again.
 

Last edited by ryogajyc; 02-25-08 at 10:28 PM.
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Old 02-25-08, 12:48 PM
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Don't take voltage reads at the igniter terminal. That is high voltage. Up to 15,000. Remove the high voltage wire at the module, take a screwdrive and touch the igniter terminal, next draw the screwdriver tip away about a 1/4" and see if it will draw a spark. Remember the thermostat must be calling for heat. Can you light the pilot manually? If yes, it sounds like the module needs replacing. If no, more troubleshooting is in order.
 
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Old 02-25-08, 04:42 PM
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Originally Posted by mbk3 View Post
If no, more troubleshooting is in order.
---like making sure the electrode or whatever you have that sparks inside has a good ground. I have been running into a rash of band grounds on electrode type sparking units. Also your spark plug wire may be draped across metal as it enters the burner area, and over time has caused the voltage to leak out of the rubber insulation there. Try repostioning the wire.

No need to worry about flame sensors at this stage, as you are not even making it to the spark stage.
 
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Old 02-25-08, 10:43 PM
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Looks like a bad ignition control unit, but I'll post an update once I get it replaced to confirm. Any ideas how long ICUs normally last or what sort of warranty is on a Robertshaw 780-04NC-CR Ignition Control Unit (ICU)? It was replaced around 3 years ago and I would hope these things were designed/warrantied to last a bit longer than that.

Here's a blow-by-blow analysis which I hope can help others troubleshoot their problems.

First, I forgot to mention that I could control the call for heat from the automatic shutoff switch that disconnects power if you open the blower panel. Using that, I could hear clicking when the call for heat was switched on/off. Probably the relay connecting high voltage in the ICU?

Per mike2501, I went back and tried measuring the voltage from the spark plug tip to the chassis ground since my earlier measurement was using the multimeter probes which may have had a intermittent connection. With alligator clips, I got ~100V AC and 6-7.5V DC. I wasn't sure which it should be, so I went for both, but I'm guessing AC. Since is shy of the expected up to 15kV, this probably indicated the ICU was bad.

Per mbk3, I tried sparking the igniter terminal on the ICU using a screwdriver. I could get the ICU to click, but no visible spark, even in relative darkness.

Per ecman51, I had previously checked the ground which seemed good, but I double-checked after reassembling the spark plug ignitor/flame hood back to the pilot gas tube and mounted it back in place. Continuity and resistance around 1-2 ohms, which meant the ground was probably good. BTW ecman51, did you mean bad ground? If not, what's a band ground?

Per mbk3, I tried lighting the pilot light. I was a bit apprehensive about this since I'm not use to working with live/high voltage or gas and there was a clear warning on the furnace saying not to try to light the pilot light. But after a couple of attempts the pilot light went on, and after the main burners came on, I can see why the warning is there. Fortunately, I was well clear by then and already putting the panels back on. At least I can get the house warmed up before letting it cool for the night.

Thanks to all three of you guys for your advice! I'm pretty sure it's the ICU now, but I'll let you guys know for sure after I confirm it.
 
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Old 02-26-08, 03:28 PM
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Originally Posted by ryogajyc View Post
BTW ecman51, did you mean bad ground? If not, what's a band ground?
A typo.

Although, a double-meaning could apply in the case of an old Coleman furnace that has a band that attaches the electrode kit to the burner tube. Grounding of the center electrode must go through that band and into the tube and then into the chasis of the furnace (which is grounded). So in this case a band ground COULD be a bad ground.
 
 

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