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When my thermostat reaches the desired temperature, the furnace fan shuts off briefly, then cycles on and off a few times. Several internet posts pointed me to the limit switch, which seems to make sense, so I replaced it. The new switch exhibits the same problem, so I'm not sure whether I got a bad switch or there's something else going on. Is there a way to test the limit switch to rule that out? If so, where do I look next? There seems to be a resistor inline with one of the limit switch leads, but I don't know what it's rated at.

The furnace is a 25-year-old Carrier. The model number and serial number are illegible, and I don't have a manual for it.

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Scary. I would be scared the fan was going on because there was still heat that needed to dissipate and it's cycling on and off because it cools to the right temp and then heats back up and then the fan pops on again to cool it down. –  tooshel Mar 1 '11 at 23:25
    
Is the limit switch adjustable? –  Tester101 Mar 2 '11 at 12:53
    
No, there's no adjustment on the limit switch. This is the switch I installed: shortyshvac.com/… –  chrispix Mar 2 '11 at 20:46

2 Answers 2

up vote 1 down vote accepted

grab a multimeter and figure out what the normal operation of the switch is. it's probably 12 or 24 vdc. leave the multimeter hooked up and watch it while the system is off, operating normally, and experiencing the problem. the resistor is probably a supervision resistor, so there are at least 3 possible outputs from the switch: open, resistor load, closed/short.

you can then decide the best way to diagnose the problem.

once you know how it should be operating, you can, for example, remove the switch and short or open or use the resistor to trick the furnace in to thinking it is still hot and the fan needs to keep running. if the fan doesn't run properly, then you know the problem is not the limit switch.

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Thanks longneck, I'll try that this weekend. –  chrispix Mar 3 '11 at 18:26

It can be tested with a volt/ohm meter this most likely has a board on it if so the limit switch serves one purpose to turn of gas valve when the unit gets to hot to keep it from over heating. It would have to be a old fan limit switch to turn blower of coz is should always be the last thing to shut down. They only use 24 vac on furnance that would be in transformer. A limit switch has a normally closed set of pionts so with omh meter dial up 200 if has scale it should read o to a max of like .8 and reading should be on right side of meter if it stays 1 on left lets you it is open and bad. On the board is a blower relay which most likely is bad which will give you this problem and need to replace board. Can check to see if two leads coming out going to motor had voltage is so the you blower motor has a over load built into it if it gets to hot will shut motor down once cooled it will turn back on. With you furnace being this old would say good idea to think about replacing with a new 90 to 98%. The money you safe in running the unit and the cost of motor will in 3 -4 year just about pay for it self

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This answer is difficult to read, please consider editing it with some punctuation and paragraphs. –  Tester101 Jan 20 at 16:01

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