I've got an older Carrier Weathermaker SX; 58sxa080 series 100 furnace.

It won't turn back on or cycle after one heat cycle; it comes up to temp. turns off then won't kick back in, on it's own, to reheat the house.

Every morning and evening (no one home during the day), we have to turn the power off to the furnace or open the blower door to cycle power to the unit. We leave it off for 30 sec. then back on and the furnace kicks in and runs fine.

I cleaned the drain tube and collector / trap; it wasn't staying running first. Today I noticed before doing anything there was a "humming" noise inside before turning it off.

I also noticed that the transformer (110v to 24v) appeared to have been replaced at one time (it's not mounted to anything); I unplugged it and it had 24v on the blue / red(yellow) wires.

Question:
Are the R and W wires for the T-stat suppose to measure 24v with the T-stat connected? I only got 24v with the wires disconnected. Trying to see if the T-stat is bad.

Any suggestions as to what to check or what's gone bad?

TIA
bfordz

Well, I took a guess and replaced the transformer and that didn't fix my problem.
It still is humming when it won't turn back on or kick in and start another heat cycle.

I'll keep trying!

After replacing the transformer I noticed one time that the burner didn't kick in after starting (resetting the power) it back up. The pilot lit OK but the burners just wouldn't kick it like they usually did. I recycled the power a 2nd time and all went well.

I got some advise to clean the flame sensor, it could be dirty or carboned up over time.
Thinking with the above no flame starting issue maybe they were on the right track.

I removed the pilot/flame sensor/ignitor assm. It appeared to be all one piece, but I also couldn't get at it to clean it; I had to remove the gas line and pulled the unit partially out to gain access to it.

I cleaned the sensor with a mild nylon pad and reassembled. I didn't disconnect any wires while doing this as the connectors seems too old and brittle, last thing I wanted to do is break one.

Now the pilot won't light. I can hear the ignitor clicking but no flame. I've tried several times by cycling the power but no go.

What type of voltage readings should I have at the sensor wires? There's 3 wires, brown, yellow and white I believe.
Also, how can I tell if I've bumped the ignitor electrode out of adjustment? How should it be facing and is there a specific gap I should measure?

Does this unit have a intem pilot?. ( sparks and lights a regular looking pilot burner ,like one you would see on your water heater) I think it does if memory serves. It's a block with 3 wire connections... those are a royal pain in the A**.. lol.. you need to replace the whole pilot assmy .the flame sense on them is a bimetal and just cleaning won't work sorry to say there not cheap but could be worse sorry but in 24 years that's the only sure way ( if its after market kit it gets a bit more complicated ).. good luck. OH PS make sure you don't get ANY dust on the pilot orifice there VERY small and clog easily

Nvdragon,
Thanks for the reply. Yes, your correct in your description.
This has been a real royal pain in the "A". You mention the pilot orifice... and I read where some have taken a wire to clean them out.

My pilot is rather large, is it possible that the orifice comes out? I'm thinking that it might have fell out the first time I took it out to clean the sensor.

The second time, I took it back out to re-examine; I didn't pay any attention to the orifice the first time, thought maybe that's what I missed the first time, when looking from the gas line end it's basically a straight shot through... no small hole to get plugged with dust, it's a 1/4" (approx) passage. It would have to be a pretty big "dust bunny" to plug that hole.

If the "small" orifice is missing would that explain my "no start/light" of my pilot?
Would a large passage cause too much gas flow and shut something down?

Don't TRY TO FIRE WITHOUT THE PILOT ORIFACE!. sounds like it fell out when you had it apart there it has to be there.. if not the gas will just pour into the campartmet . Ingition and blast off.. the hole in the oriface is very very small a wire from a wire brush might be to large to go through.. just make sure when you replace the module it lines up in the same place. The oriface looks like a little cone only about a half inch long.. really small.. and where the pilot flame touches the burners there is triangles that make a slot all the way across the burners those are for starting.. make sure the slots are clean ( you get build up where the pilot flame touches and it will cause a hesitation when firing anf blow uot a bit when it fires.. so get a new ign mod and you shopuld be good..

Nvdragon,
Thanks again for the reply,
I've got one ordered and I'll look the rest over good before I install it.
And I guess I'll Thank the good Lord I didn't get 'blast off', I did try to turn it on before knowing I was missing that.
I'll post what happens.