Some of these problems sound similar to mine.

I have a Coleman furnace Model 2880E766 with a Robertshaw HS780 ignition control module on it.

Thermostat calls for heat.
Draft fan comes on to purge the system.
After the purge, the gas valve comes on and I have a flame.
Gas valve cycles off and on even though there is a flame.
Sometimes the main blower will come on but usually it goes through it's cycle until it locks out.

I am an electrician so I got my meter out and checked a few things. There is 25 VAC at the gas valve except when it cycles off and on. I get a constant 25VAC at connection TH where the safeties connect. There is also a wire connected at terminal RS but the wiring diagram on the furnace doesn't show what this is. I've also been told that my unit probably doesn't have a flame rod so at this point I'm a little lost. It seems to me that there would be a flame rod and it just isn't sensing the flame. I did find a way to force it to run but I don't like doing it.

Any suggestions?

 

The inducer motor fan should turn on

the the ignitor should switch on for 30-60 seconds and get white hot.

The gas should turn on for the main burners and should light off the igniter within a second.

After a couple of minutes the main fan should turn on

The burners should remain lit until the thermostat turns the furnace off.



You say the burners are turning on and off, but please describe that in more detail, especially the timing.


Try turning the thermostat way up --- to 80 degrees or so to insure that the thermostat isn't shutting off the burner.


Some ignition systems use the ignitor itself as the flame sensor. In such systems, the ignitor can fail to detect the lit burners properly, which might require that the ignitor be replaced.

Double check to make sure that there is no separate flame sensor.


Also, it would be worthwhile to measure the flame rectification microamps flowing (presumably) through the ignitor and back to the ignition control through the ground. Measure the DC microamps flowing from the furnace chasis to the ground plug on the ignition module.

A good read is 5-10 DC microamps. A marginal read of around 1 microamp will often cause the burner to shut off and recycle as you describe. Checking the microamps is the specific method to check to see whether that circuit is working properly.

 

Ok, I'll check those things. The cycling is more like the valve is on and there is flame. The valve turns off for just a milisecond then back on. When it goes off the flame never goes completely out. What am I doing to make it run? When the gas valve is cycling I check to be sure that there is flame then I put 24 volts (jumper to the transformer) to the valve forcing it to stay open. Less than a minute later the main blower starts. I remove the jumper and the unit stays on, turning off once the therostat is satisfied. I do make sure that there is flame first because I don't want to flood the unit with gas.

 

I'd also check the pressure switch to make sure it's not opening and shutting off the burner. The pressure switch is a normally open switch that closes, and should stay closed, when the inducer motor developes an adequate pressure through the vent system.

It might be marginally defective or the vent system might be marginal, causing it to open up intermittently rather than staying closed.

 

I'm pretty sure it's not the pressure switch. While the gas valve was cycling I took a jumper wire and jumped around all of those switches just to see if it would stop cycling, but it didn't. I will double check though and let you know what I find out.

 

I didn't have the time to look at my furnace last night but will look at it tonight. The HVAC supervisor at my work is giving me a bunch of extra sensors and ignitors that he has laying around at home. The furnace worked all day yesterday but I'm still going to take it all apart and clean it, then check the things you suggested. Again thanks for the help.

 

I started looking at some of the things you suggested then I just went ahead and took things apart, cleaned, and adjusted the sensor a little closer to the flame. Turned the furnace on and it started right up. No cycling of the gas valve. I'm keeping my fingers crossed though. I'll be convinced in a few days. Thanks again for your help.