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Testing Limit Switch

Posted: Wed Feb 04, 2009 3:39 pm
by boboleeenk
Would some one explain to me, how to go about using a multimeter to see if a limit switch is good or bad? Thanks in advance.

Posted: Wed Feb 04, 2009 4:49 pm
by Mark
Set your meter to the ohm/resistance setting. This is the setting that when you touch the two probes of your meter together, either the needle jumps to the other end or the meter beeps. This tells you that you have a complete circuit.

To test your limit switch, first shut OFF the power to your furnace. Then remove the wires from the limit switch. Put your meter on the two terminals. If the meter beeps or the needle jumps to the other end, that tells you the switch is closed (which is what you want).

If the meter doesn't beep or the needle doesn't move, then you have a bad limit switch. This test will only tell you that the switch is open or closed. It will NOT tell you if a switch is working properly to their set temperatures.

By the way, fan switches work just the opposite. When tested, they should test open (no beeps or flying needles).

How can you tell the difference between a fan switch and limit switch? Look at the markings. A fan switch will have an F before the temperature. F130-20F or F150F, for example. A limit switch will have an L before the temperature. L140-40F or L160F. The F marked at the end indicates farenheit. In some cases it could be a C for celcius.

The first number is the temperature that causes the switch to open or close. The second number is how cool the switch needs to get before it resets itself. For instance, a limit switch with a marking of L150-40F indicates that it will open at 150 degrees, and reset itself at 110 degrees (or 40 degrees cooler). If your switch has no second number, then it means you have to manually reset it with a button.


RE: Testing Limit Switch

Posted: Wed Feb 04, 2009 11:30 pm
by boboleeenk
:wink: Thank you, very much, Mark, for the reply! That was very helpful. I appreciate it very much. Thanks again.
I'm 65 , and still learning something every day.