Symptom: The furnace turns on and runs normally for approximately 10 minutes at which time the gas valve begins to cycle off and on. The burners will go out but then immediately re-ignite then go out again. The furnace will continue to run but due to the cycling it will never heat the house to the target temperature and will just keep running.

The unit is a Carrier 58DRC and is 30 years old. It has a pilot light with ignitor and two limit switches (overheat and flame roll-out with reset) it has no flame sensor probe. I replaced the filters and then tried a temporary bypass jumper of both limit switches but it does the same thing.

One thing I have noticed is that when the gas valve turns on rather than making a clunk sound when the solenoid engages it makes a sort of chatter noise as if it's weak or undecided. It also makes this noise when it's doing the short cycle on/off symptom. So could be bad gas valve but not sure how to test that.

Any tips or ideas?
Thanks
Paul

 

If you have a voltmeter, you could check the voltage at the gas valve. If you see the voltage going up/down in sync with the burners turning on/off, it would point to something other than the gas valve.

 

Thanks Bob, I had tried that but ended up scratching my head because I got no readings. Then I realized I had the meter on DC so tried again and sure enough the 24V to terminal 1 for the MGV drops off momentarily when the symptoms occur. The voltage at terminal 4 for the HOLD relay remains constant. This voltage appears as soon as I set the thermostat to HEAT as is accompanied by a solid "clunk" sound. However when the voltage appears on T1 it is accompanied by a clicky-chattering sound as though the relay (or the signal to it) is weak. It then takes anywhere from 10 to 15 minutes before the problem starts to occur and it really doesn't seem to matter if I let it cool down or start it up again almost immediately.

Anyway, I think I've narrowed it down to the pilot light assembly which apparently has some sort of flame sensor but I'm not sure how it works. All I can see is the burner and the ignitor but there's a connector at the bottom of it with three wires and well I guess my next step is to get that thing out and try figure out if it can be cleaned or serviced in some way. Gotta be cheaper than a gas valve which are about $180.

 

Welcome to the forums.

I'm seeing vintage 1985 on that furnace. If that's close make sure you have that furnace cleaned and inspected. At that age the heat exchanger needs to be checked for integrity and rust thru issues.

Yes it does.
It's part of the spark ignition. More than likely the pilot and spark assembly needs to be cleaned.
The pilot orifice may be partially plugged too.

58DRC downloadable manual

 

Update: The pilot light assembly has a bi-metallic strip that does the flame sensing duty. No real way to take it apart or get in there so I just sprayed a bunch of Deoxit in then blew it out with compressed air. Also ran a pipe cleaner through the gas orifice and after reinstalling it the first thing I noticed was that the relay clicked in with a normal sound. Heater has been running for an hour now with no troubles. So I'm confident that's the culprit though not sure if my fix will last. I put it on my Amazon wish list - sixty bucks - just in case. I would go ahead and order one but it's like the third most expensive probable part after the gas valve and fan control board so I'll just wait and see....

Pete: thanks, I already had the manual and yea I figured out the flame sensor. There has to be some sort of contact down in there so the Deoxit spray is probably only a temp fix. Also I've had my head up in there for the past two days and have cleaned/inspected pretty much everything. It's a good ol' unit that should keep me comfy for another 30 years if I'm lucky ;-)

 

Further update: While the cleaned pilot assembly continues to work I decided to order a new one just to be safe. I got it on Amazon for $60 but when I installed it - NOTHING. I pulled it back out and with an ohmmeter I compared it to the original and found an open circuit on all three wires whereas the original had a closed circuit on the YEL to GRN. So I returned it and they sent me another one which checked good with the ohmmeter and worked - for one day at which point the pilot would not light. The ignitor would just keep ticking. So I returned that one for a refund and the original one is back in and working.

Point is that there seems to be a real issue with quality. Maybe it's just my bad luck but two faulty units straight off the shelf?

 

Unfortunately there is no guarantee the amazon seller is selling prime parts.
I've only bought a few of those from a supply house and didn't have a problem.

 

I have a theory that it may not actually be about manufacture quality but rather that these are NOS parts and could very likely have been sitting on the shelf for a decade or more.. I read somewhere in my research that they can have a problem when not used for a lengthy period. I've also found that they are NLA from many vendors and are superseded by a different part number.so I suppose I'll try that.