Lennox G40UH troubleshooting

Reply

  #1  
Old 04-04-13, 01:13 PM
Member
Thread Starter
Join Date: Apr 2013
Location: USA
Posts: 7
Received 0 Votes on 0 Posts
Lennox G40UH troubleshooting

Hello DIYers,

I have a Lennox G40UH-60C-110 natgas furnace that is not working properly. This system has the 'SureLight' control board, but it is not marked for either a 10M93 or a 56L84, so I do not know which board it is or even if there is a difference between the 2 boards.

All winter good heat, no problems. We have had mild weather here for the last couple of weeks after the winter. So, we have not needed heat. We just had a recent snap into some cooler weather and needed heat, but nothing.

We have just had a contractor up in the attic putting in new insulation and I noticed this morning when up there in the attic that the manual natgas switch we have (it goes to a 4 position breakout manifold) was turned to about the 80 to 90 percent off position. They must have kicked it or brushed up against it when they were working. I turned that back to full on.

I have the Lennox "Unit Information" PDF for the G40UH(X) that I downloaded from a link I saw somewhere here in the forum. I have used the flowchart on page 31 to run down items.

The indications I see (this was for a cold system):
* When there is no demand for heat and the system is powered I see the 2 LEDs blinking at 1 Hz, as expected.
* When there is demand for heat, the 2 LEDs go to blinking fast for a moment (as expected).
* Then I hear the combustion air inducer start, then quickly shut off.
* The indoor blower comes on.
* The LEDs show the indication that the primary or secondary limit switches are open. DS1 - 1 Hz flash, DS2 - On.
* I have checked the primary and secondary limit switches with a DMM and both show closed, not open (the system is cold).
* Leaving the flowchart, I read about the rollout switches and checked them and they show closed.
* Then I checked the combustion air pressure switch and it showed open (as expected). (This is the same as the Prove Switch, I believe).

I am wondering if the system "ran out" or "ran lean" of natgas and went into a protective mode of some sort. I am looking for but have not found if there is a way to purge the gas lines and restart the system.

That is where I am currently stuck. Any help would be appreciated.


Thanks,
Mike
 
Sponsored Links
  #2  
Old 04-04-13, 04:43 PM
Member
Thread Starter
Join Date: Apr 2013
Location: USA
Posts: 7
Received 0 Votes on 0 Posts
Further steps:

I checked the flue for obstruction and did not find anything there.
I jumpered around both of the limit switches, in turn, and got the same result.
|---> That appears to point at the control board.

Again, any suggestions appreciated.

ml
 
  #3  
Old 04-04-13, 10:10 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 53,651
Received 421 Votes on 394 Posts
Welcome to the forums.

The indoor blower comes on.
Does that occur in 45 seconds from the call for heat time ?

If the blower comes on in 45 seconds and the draft inducer only runs for a few seconds then the problem sounds like the control board.

Wait for the pro to stop by. He knows these units.
 
  #4  
Old 04-04-13, 10:44 PM
Member
Join Date: Oct 2002
Location: Hamilton County, Ohio
Posts: 4,296
Received 2 Votes on 2 Posts
I'm sure that you did turn off the 120 volt feed to the furnace when you were doing your testing, but in the off chance that you did not, switch it off for a minute or two and then back on. That may reset the system.
 
  #5  
Old 04-06-13, 01:35 PM
Member
Thread Starter
Join Date: Apr 2013
Location: USA
Posts: 7
Received 0 Votes on 0 Posts
Thanks for your reply, PJ.

No, there is no wait for the large blower to come on. The sequence is pretty rapid.

System on and at idle with no fault on LEDs,
Heat is called for,
inducer kicks on for a moment, both LEDs quick flashing
inducer blower turns off after maybe 3 to 5 seconds, fault is signaled on the LEDS
and the indoor blower comes on for 90 seconds.

ML
 
  #6  
Old 04-06-13, 01:45 PM
Member
Thread Starter
Join Date: Apr 2013
Location: USA
Posts: 7
Received 0 Votes on 0 Posts
Thanks goldstart,

I did try to cycle power to the unit and I get same result after letting it sit for a while.

However, while I was watching the system cycle through I had the interlock switch held closed. I have respect for AC power and I was careful...
 
  #7  
Old 04-06-13, 02:05 PM
Member
Thread Starter
Join Date: Apr 2013
Location: USA
Posts: 7
Received 0 Votes on 0 Posts
New observations:

The unit has a Honeywell gas valve. However, the control board is marked for a White-Rodgers gas valve. It is has been running for 10 years without trouble, but that seemed odd.

The circuit board is marked as a 32M8801. Anyone know where to get a schematic? Bunch of them available on-line. I have the board out of the unit now to take a look at it and there are some very slight over-temp brown marks on the back but nothing really indicating a major breakdown problem. One of the brown areas is at the pin for the over-temp limit switch power on the 12 pin connector (pin 1 on J58)

ML
 
  #8  
Old 04-06-13, 02:49 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 53,651
Received 421 Votes on 394 Posts
The blower fan is a timed response. It is supposed to come on 45 seconds after the call for heat. If it's coming on before that.....it's an issue.

The board schematics are proprietary information. I doubt you'd find a schematic for it. That's the reason the boards are usually just replaced.

Usually a control board malfunction is caused more by a defective/poor soldered connection(s) as opposed to an actual parts failure.
 
  #9  
Old 04-10-13, 04:13 PM
Member
Thread Starter
Join Date: Apr 2013
Location: USA
Posts: 7
Received 0 Votes on 0 Posts
Control board replaced and there is now heat. Sweet! I watched the system cycle through a number of times and it all happened as the flow-chart said it should.

However, the board that came in had a different dash number for the PIC micro-controller chip. The old board was 150-0743, the new board is 150-0818.

Any Lennox folks out there that can tell me if this is a minor dash number change not to worry about?


ML
 
  #10  
Old 04-10-13, 09:34 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 53,651
Received 421 Votes on 394 Posts
You're talking about the actual processor IC chip ?

Old parts, old boards, old programs are constantly being improved and upgraded.
If your furnace is operating as it is supposed to.... the program must be correct.
 
  #11  
Old 04-11-13, 08:35 AM
Member
Thread Starter
Join Date: Apr 2013
Location: USA
Posts: 7
Received 0 Votes on 0 Posts
Yes, that is the part number on the microcontroller. This same number is also reported on the part number label for the board itself.

The system works fine, so I assumed it was a minor rev change.
 
Reply

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
Display Modes
 
Ask a Question
Question Title:
Description:
Your question will be posted in: