I have a 14 year old Heil CGK075N908 natural gas furnace that won't spark up.
When the thermostat demands heat, the exhaust blower turns on, but no spark ignition follows.
From reading in this forum, I checked the rollout switch, the exhaust temperature switch, the high limit control, and the exhaust pressure switch. They are all wired in series, and each safety measures zero Ohms with the power off, and I get 24VAC across the circuit with the furnace trying to turn on.

I checked the ground on the spark igniter, from the panel right through to the place where the spark would jump to if there was one. The electrode looked clean, but I polished it anyway with a bit of sandpaper just to make sure. The spark gap is about a quarter inch.

The gas valve has three electrical connections on it, marked PV, MV/PV, and MV. A few seconds after the exhaust blower starts up, the PV terminal goes to 6.1V, while both MV/PV and MV stay at 0V. I can hear a tick over the sound of the exhaust blower as PV goes to 6.1V which I take to be the gas solenoid opening. Just in case, I tried using a barbecue lighter but it would not light.

The gas valve has a small 1/4 inch pipe that goes to the spark lighter, and a larger 1" or so pipe that goes to the three burners.

The furnace uses a Honeywell S8600M ignition module. A few days ago there were a few instances where the main blower would not turn off. Cold air was coming from the registers, but when I cycled the main power on the furnace the blower fan turned off.

Any advice would be most appreciated.

 

If your safety and limit switches are good, I would suspect the Honeywell ignition module. The gas valve operates at 24 volts. It won't open at six volts.

 

Thanks very much for your reply.
I replaced the ignition module but still no spark.
I checked to see if the ignition module was cracked or somehow shorted and it is not.
On the new ignition module, I checked that the ground for the valves was common with the spark ground and the 24vac ground.
I also verified that the 24vac line was live and going into the ignition module.
The new S8610 ignition module is replacing an older S8600, but according to Honeywell, it is a direct replacement.
I'm also not getting any status led's on the module.

 

Did you check the pressure switch hose for cracks and run a paper clip into the pressure switch port and also on the inducer side.
spiders make nest in them as well as water crusting up.
With low voltage I would suspect a pressure switch issue.

 

Look over the diagram again and make sure everything is connected properly. Use a test light and see if the s86 is getting power. If it doesn't spark with good power, I would start to suspect the s86. It would not be the first time I've seen bad one right out of the box.

 

Thank you jim-connor and mbk3. Your help is very much appreciated.
When I replaced the S8600 with the S8610, I moved the connections over as instructed in the manual. I missed the little asterisk that says to not connect the +24VAC line, but instead connect 24v to the TH-W connection. On the 8600, there is no TH-W lug.
Thank you again for your assistance.

 

So it is going now we can presume? I had the same thing maybe 1 month ago with one of these control modules I had to repalce, then convert according to directions. Same thing with the TH-W. Lots of reading to do/pretty thick little booklet that came with it.
The directions also said it cannot be mounted a certain eway, even.

 

Funny you had this problem .....Yesterday morning I get a call from a friend who replaced his honeywell module. No go..he calls me and I had him do exactly what you did. And then I ment him for lunch. Good pay for phone service.
Glad you got it.