Coleman Presidential II gas furnace problem

[msufroggy]

Hello,
I have a Coleman Presidential II gas furnace (downdraft) model # 7700-856.

Here's what's going on (trying to be as specific as possible). It does have A/C attached as well.

When I turn the thermostat up to the desired temp, I do hear a "click" in the furnace, followed by the draft/combustion motor running. But the motor continues to run, and nothing else happens. The pilot does stay lit. My brother cleaned the burners off, and they look fine.

I'm thinking it's a bad centrifugal switch since the motor keeps running, and it's possible that the switch is never closing to open the gas valve.

The blower does work if put in Manual mode, but not on auto. So the blower motor is fine.

I was also thinking it may be the fan relay switch, but I have to still check if the burners light (I don't hear them lighting). My brother said he's getting 24vac to the gas valve, but said the furnace wasn't running???? Shouldn't there be NO voltage to the gas valve UNLESS the furnace is calling for heat? I will have to double check with him to see what he said.

What is the relationship of the transformer to the booster/combustion motor.

Does anyone have a wiring diagram? Another forum said a common sign of a bad booster motor/centrifugal switch (since they are not separate entities in this furnace) is that it would continue running and running. (which it does).

My brother is experienced with electrical/hvac, and I don't have much $$, but I wanted to ask if someone can help me troubleshoot voltages. A wiring diagram and/or sequence of events in the furnace would be helpful.

Thanks!
Heather

[HayZee518]

two things to check for. 1) is the fan loose on the shaft [combustion blower] 2) is the sail switch making up?

[msufroggy]

Thanks for your speedy response to my problem. It is appreciated. The fan wheel looks like it's spinning when the motor is running. I can also manually turn the fan wheel when the motor is off.

Re: sail switch...I am not familiar with that switch.

The booster motor just runs and runs. If the burners do not light, I can rule out the fan and limit switches, correct?

Booster relay would be ruled out since there is power going to the booster motor, correct?

I hear a "click" when I turn the thermostat up so which I was told means the transformer works. I know there is 120 v at the load of the transformer, and I believe my brother told me that he is getting 24vac at the line of the transformer.

[HayZee518]

yeah, if you didn't get 24 volts which is the control voltage nothing would work. look in the output ducting of the combustion blower for a microswitch connected to a movable vane. this is an air flow proving switch hence the name sail switch. your brother's got it mixed up 120 is the primary and 24 volts is the secondary. if it is the booster relay that pulls in, this could be the click you hear. ok, the ignition module. white rogers makes one that uses 24 volts and generates 18,000 volts for a spark plug ignition. it also incorporates a purge timing circuit before the burner kicks in.
The coleman site says that if any of the doors are open or loose then the cover switches are not made up.