Williamson furnace blower won't turn off

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Old 08-17-14, 02:37 PM
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Williamson furnace blower won't turn off

We have a 1978 Williamson gas furnace. The blower on it services our A/C. We have not had a lick of problems with either unit until last week when the blower fan decided it would not turn off while running the A/C. Now the fan stays on regardless of the thermostat position, even when we set the thermostat to "OFF" (rather than Heat or Cool option). Right now the only way to turn off the fan is to throw the circuit breaker, or the toggle switch on the outside of the furnace that also controls the fan (which we just discovered).

We thought the easiest trouble shooting would be to replace the thermostat. That didn't work. We have voltage testers, continuity testers etc. whatever is probably needed to trouble shoot this but honestly we aren't sure where to start. We cannot find a Williamson wiring diagram that made sense to us. I will attach photos of the different control units we see once I figure out how.

Any help is greatly appreciated. Again, we are looking for advice on how to troubleshoot it.
 
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Old 08-17-14, 03:08 PM
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Old 08-20-14, 08:14 AM
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Here are the photos. I hope.











Thanks.
 
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Old 08-25-14, 08:55 AM
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Your attachment in post #2 won't open. I am not familiar with Williamson furnaces but most of the older furnaces used a combination fan and limit control like the ones in the photos I attached. The "probe" on the rear of this control is a bi-metal design that sticks into the plenum and turns the fan on and off based on temperature rise and fall. Many times the contact inside the fan side of the device sticks and causes the blower to operate non-stop. I suggest you look into the compartment where your burners are located to see if you have this device in there. Most of the time the metal cover will have a white button sticking out. Sometimes it will not have a cover as shown in one of the attached photos. Many times you can tap the device with the handle of a screwdriver to see if the contact inside will release and shut the blower off. Or you can use your fist to hit the cabinet near the device to see if the contact inside will "unstick". If you don't have a fan limit control as pictured I would then suggest you turn off power to the furnace and trace the wires from the blower motor backwards to determine what controls it. If you find the fan and limit control post back and I, or others, can provide some troubleshooting methods.
 
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Last edited by firedawgsatx; 08-25-14 at 09:51 AM.
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Old 08-25-14, 11:23 AM
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Another possibility that might cause your blower to run constantly is the fan relay. They are also prone to having the contact sticking closed causing the blower to run constantly. It is difficult to troubleshoot a system without a schematic to help trace the wiring. Another issue HVAC technicians run across is some "creative" fixes done by others who previously worked on the system. They might have re-configured wiring, replaced parts with others that weren't correct but would make it operate and etc. Normally, on the older furnaces the fan relay wass used to operate the fan in a/c mode on high speed and the fan and limit control the blower on low speed for heat. I have attached a photo with the fan relay circled in red. I can't make out the number on that relay. If you get a chance it would be helpful if you could post the numbers on the relay to help identify it.
 
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Old 08-25-14, 11:30 AM
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I don't see any means for the furnace to turn on the blower. Nor do I see any type of thermostat or safety stats.

Can you take a picture of the entire upper part of the furnace. We're looking for additional wiring not already seen in the pics.
 
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