79' Carrier/Bryant Furnace keeps cycling and blower does not shut off ..

happy holidays, friends.

not only does furnace continually cycle on/off (never generating any real heat) but blower neverrr shuts down.

i've checked air filter, cleaned 3 wire pilot assembly, and i've even tried temporarily jumping/bypassing limit sensor ... to no avail.

any suggestions would be greatly appreciated!

thanks,
mike

 

If you can get the model number I think we can help you.

 

Bryant/Carrier
model no. 394GAW036100
product AAEA

thank you!
m

 

That model # doesn't come up, not surprised with a 35 year old furnace. Does the pilot light come on?

 

I corrected the model number. Looks like a 1978 vintage furnace.

This is the closest I can find to a real manual....
394GAW/heater.pdf

 

When the main burners shut off, does the pilot shut off too, and then the entire ignition cycle begin again with the pilot lighting, then the main burner?


Or does the main burner cycle on and off by itself? If so, the pilot orifice needs to be taken out and cleaned.

 

thanks for your efforts, all

when burner shuts down (after around 5 mins), pilot goes right out. i've cleaned pilot orifice as well as gas line end nearest 3 wire assembly.

high temp. limit switch does not read open. fusible link isn't open either. as an experiment i disconnected lead from one side of limit switch: spark immediately ceases. i think this tells me problem not w/ either switch or fusible link ???

re- blower running continuously: looking at manual, seems only possibility i haven't explored is "defective heat relay" on PC board.

that doesn't explain cycling burner though.

thanks again,
mike

 

One thing to check is make sure the ground for the ignition module is clean and no rust.

Thanks PJ.

 

between C and R I'm getting 30v
C and Gc getting 0

when i disconnect R and W from thermostat, blower still runs but no spark.

 

That furnace has a strange feature. The heater blower is wired so that the blower is on when there is no power to the relay. It's the opposite of just about every other furnace. I actually thought you were going to tell us you were missing the 24vac.... because that could cause the problem.

The schematic shows a time delay circuit. Most likely that is your problem.



It looks like module should allow power to flow thru it to activate relay. When burner gets power it starts a time delay for the relay to turn the relay off.

 

thank you, pete

so i've been on steep part of learning curve all day ... it's been fun.

do you think i should order a new control board ??? could a faulty board result in constant cycling AND blower that doesn't shut off?

best,
m

 

It's hard to put my finger on the problem. If you have an open limit or a limit opens.... the burner shuts down, the blower continues to run and there is no 24vac between R and C. You've said that you have the 24vac there.... therefore I have to agree the board is bad.

 

new board installed and it took care of blower remaining on <constantly> yes : )

furnace is STILL cycling though. doh!

a clue might be that just BEFORE burner shuts down i can hear igniter, sparking.

eventually i get flame and whole process begins again.

suggestions ???

thank you!
mike

 

If you hear the igniter sparking.... you have a pilot or pilot sensor issue.

I left you the link to the manual for your furnace. Towards the bottom is a very good description of the heating sequence.....and what is supposed to happen. If you haven't down loaded it.... do so as these manuals are not always available.

It gives three different heating explanations based on the make/model number of the valve.
Can you post what model number gas valve you have.

 

gas valve 646A-W
301273-720

replaced pilot assembly/ignitor and high temp limiter ... furnace is still cycling.

thanks for help
m

 

Start again.


Carefully observe the series of events that occurs when you turn up the thermostat, and describe that sequence here, in order and in detail.

 

getting somewhere i think ...

it'll run all day long w/ R + W jumpered ... but with even just a few feet of brand new thermostat wire and a brand new 24v thermostat, it'll do the fire up for 3-5 mins and stop ... repeat.

INCREDIBLY frustrating.

even if i run it with only a short length of wire with ends jumped: same thing !?!

happy holidays
mike

 

I moved your post to your ongoing thread. If the threads are separate.... everyone will start in with the questions that were already asked and answered.

The length of the stat wire has no bearing because it's operating a relay. I see you posted the valve type. Let me peruse that further.

 

thank you for collapsing threads.

i had a professional come by this morning. after checking furnace, voltages etc ... he removed stat and jumped wires ... furnace <appeared> fine. he said you need a new stat ... told him i can do that : ) after installing exact same 24v honeywell stat, i realized furnace was still intermittent. this told me it was cable between furnace and stat. this is how i ended up putting a small run of new wire to new stat then eventually, wire only, jumped.

furnace runs if jumped w/ small jumper but remains intermittent if any real length is used.

hope I'm making sense.

edit ... jut received an email from an acquaintance. he says transformer could be dying. ???

thank you!
m

 

Without going back and reading.... do you have a voltmeter ?
Transformer could be a possibility.

 

i do have a voltmeter
thanks
m

 

Ok...good.

You can put it on the R and C terminal. Start a call for heat and watch the voltage when the furnace is lighting. There should be very little sag.

You can also put the voltmeter on W and C. On a call for heat you should be getting almost the full voltage you measured above.

If the voltage goes below 24vac then you'll need to locate the low voltage leads of the transformer and check it there.