Blower Motor Causing Hi-Limit Switch Error on Furnace Board?


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Old 01-10-10, 06:17 PM
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Blower Motor Causing Hi-Limit Switch Error on Furnace Board?

Blower Motor Causing Hi-Limit Switch Error on Furnace Board?

This is a Lennox 80MGF3/4-100A-1 circa 1995, configured horizontally set up for LP w/ Honeywell Chronotherm III T8600 TStat.

Yesterday AM my furnace refused to heat up the house whatsoever. From inside I could hear the burners kicking on and then the vent motor but after a bit they would turn off and no hot air exited from the ducts. Up in the attic I noticed the LED flashing "4" which was the Hi-Limit or Rollover open code.

Checked continuity on the rollover switches and main hi limit switches - OK. I took the burners off, cleaned them and removed some remains around two semi-burned turbulator retainer clips in 2 heat exchanger tubes. I generally cleaned things up and as a final step gave a few taps to the capacitor on the main blower motor. Oh and the air filters were quite overdue and also changed.

Now it works intermittently. From what I can tell here's what is happening:

1 The vent motor kicks on for 45 seconds or so.
2 The ignitor lights the burner.
3 After 45 seconds I hear a distinct 'click' from the controller board. The main blower motor does NOT start at this time as it should. Nor is there any AC voltage at the 'ACB HEAT' connector.
4 After about 20-30 more seconds the hi-limit circuit goes OPEN and the LED starts flashing "4" AND the gas supply to the burners is immediately shut down. (The vent motor continues to run...)
5 sometimes after about 5-8 minutes or so - the blower motor does kick on (sometimes)... Once it gets blowing the secondary hi-limit switches mounted near the blower will close shortly thereafter and the burners ignite and voila! I have plenty of heat again... until next time it needs to cycle anyway.

Right now my best guess is that the extended delay in the blower motor starting (if it ever starts) is causing 1 or both of the 2ndary hi-limit switches to open. This in turn sets off the "4" code on the board and shuts down the flames as it should.

As to what is causing the motor to not turn on promptly within 45 seconds of burner ignition - I could not get VAC at the ACBHEAT connector. Eventually if/when the blower motor kicks on 5 or 10 minutes later I could get voltage at the ACBHEAT connector. I may have also triggered a few blower starts by gently tapping the soldered relay on the board controlling the blower.

QUESTION: I'm thinking the problem is with the board - probably one of the relays soldered to the board??? Do you agree?

QUESTION: Also it appears that the 2ndary Hi Limit circuit is opening up about 60 seconds after the burners ignite. Basically that leaves a 15 second window between 45 seconds and 60 seconds for the blower fan to start functioning. That seems pretty tight - is that about what it should be?

QUESTION: How is the 'new' 19W9401 Controller Board an 'upgrade' over the OE 20J8001 (3MC4) board?
 
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Old 01-10-10, 06:58 PM
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Searching Google, someone mentioned the air filters which you said were overdue & needed to be changed. Since you changed them, there was an improvement. What about a reset button? Does it need an electrical reset to reset the codes?
 
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Old 01-10-10, 07:18 PM
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No reset. Code clears as soon as the hi limit circuit closes (cools down) when the blower fan kicks on and cools the furnace down. Or enough time passes and it cools to within spec. Then normal slow flashing returns as it should.
 
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Old 01-11-10, 05:07 AM
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Sounds like your main blower is not starting up intermittently. When the problem happens, check for 120v at the motor or feel the motor and see if its hot to the touch (with power removed). also see if you can spin it by hand and it spins freely and doesnt make any noises. if it seems to be locking up the bearing is failed and the blower motor must be replaced. if all ok, there is a capacitor in the circuit that is used for start up. replace it and see if it helps. if you're getting no power to the blower the control board is bad or wiring/connections.

chain of events that should occur in a call for heat:

- stat calls for heat
- vent motor kicks in, pressure switch confirms
- ignitor lights burners, flame roll out sensor confirms flame
- main blower turns on after a small delay (usually 10 sec)

if the main blower never turns on the heating element will get hot and the high limit switch will eventually trip (open) and remain open until cool. if you power cycle and the blower decides to work the system will turn the main blower on to cool everything down then reset itself once the high limit switch closes.

good luck, hope this helps.
 
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Old 01-11-10, 07:09 AM
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Originally Posted by zz2h33
Sounds like your main blower is not starting up intermittently.
It IS starting late/intermittently. It always takes anywhere from to 5 to 8 or more minutes to starts. Sometimes it just doesn't start up at all. I would call that intermittent.

Originally Posted by zz2h33
When the problem happens, check for 120v at the motor
WHat I did was check for voltage on the motor leads on the board - stated above as ACBHEAT. THere is NO voltage (until 5 or 8 minutes later). I didn't see any point in checking at the motor since the board wasn't outputting 120V.

Originally Posted by zz2h33
or feel the motor and see if its hot to the touch (with power removed). also see if you can spin it by hand and it spins freely and doesnt make any noises. if it seems to be locking up the bearing is failed and the blower motor must be replaced. if all ok, there is a capacitor in the circuit that is used for start up. replace it and see if it helps.
Spins freely and quietly by hand and seems to work fine - when it does get power from the board.

Originally Posted by zz2h33
if you're getting no power to the blower the control board is bad or wiring/connections.
This was my original diagnosis in my first post - more specifically I believe an electronic relay on the board has poor connectivity and thus the board will need to be replaced.

chain of events that should occur in a call for heat:

Originally Posted by zz2h33
- stat calls for heat
- vent motor kicks in, pressure switch confirms
- ignitor lights burners, flame roll out sensor confirms flame
- main blower turns on after a small delay (usually 10 sec)
According to my manual there is a 45 second delay before the main blower turns on. As stated in my original post I hear a distinct click from the board at 45 seconds. However I do NOT see 120V at the board connector (ACBHEAT) for the main blower.

Originally Posted by zz2h33
if the main blower never turns on the heating element will get hot and the high limit switch will eventually trip (open) and remain open until cool. if you power cycle and the blower decides to work the system will turn the main blower on to cool everything down then reset itself once the high limit switch closes.
Yep that's pretty much what happens but the hi limit switch does trip pretty quick - at about 60 or 70 seconds of the burners running without the blower.

So it sounds like you agree it's the board.

QUESTION: How is the 'new' 19W9401 Controller Board an 'upgrade' over the OE 20J8001 (3MC4) board? What additional or modified functionality does it offer to make it an 'upgrade' over the OE board?

thanks
 
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Old 01-11-10, 08:39 AM
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I'm sorry I cannot answer your questions about the board differences but do agree sounds like a board issue. If you hear the relay close in the control module but get no voltage then thats the only conclusion unless you are not measuring at the right terminal for the voltage. I had responded from my own experience as I have a similar issue but different model unit. Mine seems to be the cap not providing enough "kick" to get the motor started. my motor will get hot to the touch and if i use a stick to give it a little push it runs fine. i just have to source a replacement cap and see if it fixes the issue. good luck. for now to keep my house warm i have put the fan at constant "ON" to keep it running. You may want to try that until you source your parts.
 
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Old 01-11-10, 06:09 PM
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I suspected the cap on the blower motor early on too. There are 3 wires going to the motor: neutral directly from the house power supply, AC and HEAT. I measured right on the board across HEAT and AC and nothing. Eventually after 5 or 10 minutes the blower fan spins up and yep, you guessed it, the same terminals THEN have 120VAC. The wiring diagram also confirms these are the right connecting points for power to the blower fan in heat mode.

I'm fairly certain it's an electronic relay that switches current to the blower fan that is intermittently not working on a delayed basis.

Until I get a replacement I can wire 120VAC directly to the fan and bypass the board or set the TStat to FAN and if/when the fan eventually kicks on - turn up the heat - weather is mild right now anyway.

thanks
 
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Old 02-24-10, 08:38 AM
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As I suspected - the relay soldered to the controller board for the fan was 'intermittent'. A replacement board solved the problem.
 
 

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