York 90 Gas Furnace High limit open/blower malfunction

I am trying to get my furnace working before winter hits too hard up here in Alaska. My high limit keeps opening, it seems my blower is not operating properly causing the high limit to trip. The blower runs for 20-30 seconds and turns off causing my high limit to trip. Since the blower runs can it still be a bad blower causing my problems or a secondary casuse such as run capacitor??

Thanks Sean

 

most likely it is the motor. However, a capacitor may cause those symptoms. For the low cost of a capacitor I might throw one on to find out before replacing the blower motor.

 

Start by measuring the voltage being applied to the motor to be sure the power to the motor isn't being shut off.

If the voltage stays on, is the motor hot to the touch --- blower motors usually have a thermal limit switch that turn the power off in the motor if the motor overheats.

Does the shaft turn freely, or is it stiff and hard to turn due to lack of oiling or another defect?

 

im at work right now, but evrything turns freely and it comes on but it turns off after the 20-30 seconds. I did notice that it is hot when i touch the motor. Is that a sign the motor is bad?

 

Also when i turn my T-stat to fan only no heat i can hear a faint hum but the blower does not turn on. i bridged the limit switch that is on the blower but it didnt make any difference if i had disconected, bridged or left it hooked up.

 

NEVER bypass a safety device!

 

isnt that limit switch meant to shut the blower off if it gets too hot? I fugured if i bypassed it and it worked then the switch was bad, causing the blower to shut off at the wrong temperature...i didnt leave it bypassed, just checked to see if the switch was bad.

 

Its meant to shut off the burners and run the blower if it gets too hot.

 

As hvactech said earlier, very likely it's a bad motor that is overheating and shutting off on the thermal limit.

Generally speaking, if the motor will start it isn't a bad capacitor, since the capacitor is switched out of the circuit and isn't used further after the motor comes up to speed.

 

Wrong, It is a run capacitor on a PSC motor and stays in the circuit the entire time.

The PSC motor gets it's name from the fact that there is a "RUN" capacitor connected in the motor circuit at all times. This device assists in maintaining a high efficiency and power factor, and decreasing the amount of power consumed for the same power output.

A weak capacitor may get the motor started but because it is weak will not get the motor to full speed and cause the motor to overheat even though it is running. It still could be a bad motor though. For the low cost of a capacitor (less than $15) I would start there.

 

thanks, ill look at the specs of the capacitor and get one. If the capacitor does not fix it then the motor needs to be replaced?

 

yes, then the motor needs replaced

 

Well I was getting ready to pull the capacitor but I fgured iwould try it one more time, I put my t-stat can control to on, went in the crawl space made sure everything was put back together right. The fan kicked on and started runnig perfect, waited for the house to warm up just incase it was a fluke, then I switched it to auto on the thermostat. When it reached 70 the furnace shut down, i set it for 72 and went down to watch it, the pilot lit, but fan didn't go. I opened the door spun the fan blade. When I closed the door it kicked on full speed. Is that asign of the capacitor not having the juice to start it???

 

A good capacitor is necessary to get a motor started, after which a switch takes it out of the active circuit.

So if pushing a motor will get it started, a bad capacitor is a possible cause of the problem, along with a low supply voltage or a bad motor.

The odds are the motor is bad. But replacing the capacitor is worthwhile since it's a cheap part.

Check to verify that the supply voltage to the motor is correct is a low probability cause of the problem but should be the first thing to check.

 

Seattle, you are wrong, sir. A PSC motor does NOT have a switch to take the capacitor out of the circuit. A blower motor is a PSC motor not a capacitor start motor which does have a switch inside to take the capacitor out of the circuit.