I have an old Sears Power Miser 80 Furnace and the fan blower won't turn on when the thermostat is set to "Fan Auto". The furnace ignites, the Fan Limit Control spins around while heating up and when it hits the fan notch, nothing happens. Eventually the furnace will reach the shut off notch and turns off. If the thermostat is set to "Fan ON", the furnace works fine (but of course the fan never will shut off).
A little history on the furnace. We just had it inspected this spring by Centerpoint and the tech said it looks OK and it passed all checks. Later in the summer we had an issue with the Central Air and the tech had a difficult time troubleshooting it - was messing with wires inside the furnace and thermostat. Eventually he found the wall thermostat was bad, so he replaced it. The Central Air has been working great since then. But the first time I fired up the furnace today, I have this problem.
First Question: Could the Central Air tech have messed up something with the wiring that would cause this furnace fan issue?
I've tested the Fan Limit Control with an ohm meter - spun it by hand and when it hits the fan notch, my meter shorts. So looking over things and based on some research I believe the next cause could be the fan Relay? Any ideas ?? Thanks
Furnace Info: Sears Kenmore Power Miser 80, model 867.768020
Post some pics of the furnace with both doors off and the fan relay.
Your ohm meter tests continuity and does not short per se.
The fan control should send either 120 volts or 24 volts to the fan relay depending on what that relay coil is rated for.
Try check for voltage at one side of the fan control when it is warming up and go to ground or C on the terminal strip where the thermostat wires attach.. One side should be live.
When it hits the fan on setting then you should get voltage on the other side of the switch to ground.
If none then the fan switch is no good.
If the relay is getting voltage to the coil then it should close a circuit and turn on the blower.
If there is a wiring diagram on one of the doors take a pic of it and post it here.
Use photobucket if the site does not let you attach pics until after 5 posts. Put the links here.
As I mentioned, a new thermostat was installed over the summer - technician was troubleshooting a difficult starting AC unit - after some frustration, he finally changed out the thermostat and the CA has been working great. But this is the first time we fired up the furnace and we're experiencing this problem.
Here are some pictures of the furnace. I noticed last night there was a little water (very little) inside furnace and I see it turned to rust as shown in the pic (must have been dripping in for the summer?). We've had an extremely rainy summer with some pretty good winds - so the water must have come through the stack (hopefully not dripping from the condenser above!).
Wondering if this might have caused my fan issue?
As far as further testing of the Fan Limit Switch (beyond the continuity test I already performed), I'm not sure where I should put my probes for the voltage. Again, the burner lights up so I thought that side of the Fan Limit Switch is good (white wire), but the fan side is what needs to be tested (yellow side), correct? I believe I need to connect one probe to the yellow wire contact and the other probe to the "C" terminal?
One last thing, based on some research I checked the voltage between terminals R and C for about 27 volts.
You may have damaged the fan/limit switch be moving it by hand. After figuring out the problem, you may have to get a new one.
Were you testing resistance across the fan side?
If yes, the problem is downstream of the fan/limit. Unless the dial isn't rotating when the burners come on.
You'll need to check all the wiring downstream of the fan/limit, compare to diagram. If the wiring isn't messed up, either the normally closed contacts on the fan relay are bad or it's the heating speed tap of the blower motor. Both are pretty unlikely.
The Fan Limit Switch still works and moves normally when the burners heat up - the dial rotates to the activate the fan - so I don't think its damaged. I've seen a few videos showing that checking resistance is one way to check the Fan Limit Switch operation as shown in this video https://www.youtube.com/watch?time_continue=190&v=Pyz9MxgHn5k.
I am concerned about the water and rust. Looking over the furnace, I noticed the clear drain hose for the AC evaporator pan has a lot of rust in the hose. Same color rust that is inside the furnace. I'm starting to think the evaporator pan is rusted through and leaked all over the furnace this summer. Wonder if it damaged something related to the fan.
Could the water have shorted out the low speed winding on the blower motor? Is there an easy way to test the blower motor with my multimeter?
My next issue will be to repair that AC pan before next summer...
OK, I can't believe I fixed it. In my original post I mentioned that an AC tech was messing around with the wiring inside the furnace - I could tell he was getting frustrated, he was at my home a long time. Well he was the cause of the issue.
I opened this box (in the pic) below the fan relay to trace the yellow wire from the Fan Limit Control switch, and I noticed the yellow wire went into the relay, and another yellow wire came out of the relay but it didn't look like it was fully pushed it. So I grabbed the wire and it just fell out of the hole. I reseated the yellow wire, and tested it. Sure enough the fan works now.
:vs_clap: :vs_cocktail:
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