Coleman furnace question


Hello, I'm new to your world here at alt.home.repair
I've searched the internet and newsgroups looking for help on a gas furnace issue.
Background: One morning this past winter when I woke up, I noticed that the house was getting cold. Having replaced the ignitor 2 or 3 times over the past 10 years, I went to the furnace expecting to find my culprit. I then noticed that my vent blower was still running and was apparently had been for a while(motor felt hotter than expected). I then recycled the furnace to try to troubleshoot the problem. Everything seemed in order: thermostat activates the sequence of vent blower, then ignitor activates, gas valve opens, ignition, ignitor deactivates(acts as flame sensor), air mover activates, so far so good. About ~60 seconds aftern the air mover turns on, the gas valve shuts off. Eventually the air mover shuts sown, but the vent blower continued to run.
2 or 3 days before this, my hall thermostat had taken a pretty good hit with a ball. I wasn't sure why, but I suspected these these events were related. The heat anticipator in my round Honeywell was a possible candidate, because the adjustment lever had been knocked to one end of it's range. The problem was intermittent, and it was the end of the heating season, so I brushed it off. Last week, the locally cold night-time temps activated our furnace, and I then found out that my problem was still there, and it was not going away. This weekend we are supposed to get temps in the low 30's, so I began to try to fix it. I tried a new thermostat....no fix. Tried to bypass the limit switch, still no luck. Seems like most of the other switches would keep it from getting this far into the sequence.
In the 60 seconds or so that I actually have heat being moved buy the air mover, it seems cooler than what I remember. Is there something on this furnace that activates a 2nd stage in the gas valve? The gas valve is a RobertShaw 7100. I saw on the internet that there was a 1991 recall on this gas valve, but I couldn't access the article. The furnace needs to(as Emeril would say), kick it up a notch. As you can probably tell, I'm not an HVAC professional, but I did stay at a Holiday Inn Express last night. It would make sense that something is trying to happen once the furnace reaches a certain temperature. Possible issue with the gas valve?
Thanks for any help.
Michael Harris
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Have you checked the sensor that lets the furnace know that the gas has ignited. On a Coleman furnace I believe this sensor is a light sensor of some sort. It can get dirty and shut the gas valve off after a short period of time. Wipe the lens off.
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I just posted below about getting this fixed. Look at the thread for the HEIL issue. Anyway, after checking all switches and the thermostat, I read about the gentleman's Heil sensor/ignitor issue. I had replaced the ignitor(burned out) 3 times in 10 years. I replaced it often enough that I kept a spare. Anyway, I stuck the new one in there today and it fixed it. Even though it would ignite, the flame sensing wasn't working correctly, cutting the gas off.
You were on the right track. Actually the sensor/ignitor isn't a light sensor, but a porous type of glow plug. More of a winding loop type design.
Thanks again.
Michael Harris

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