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Old 12-16-2010, 02:21 PM   #1
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My furnace is mis-behaving

Hi,

My Suburban SH-42 2346A furnace in my 05 Itasca Sunsruiser 35U has two problems. It has a Coleman Mach Trueair thermostat.

We are on the road from NS to Texas and got caught in the cold, cold weather along the eastern seaboard.

Lost night I woke up at about 0400 and the interior temperature was 39 in spite of the thermostat being set at 60. I moved the sliding selection button from gas to electric and back and the furnace started, So I slid the slider back and forth numerous times in the hope that it would clean the sliding contacts - so far it has worked.

The second problem is perhaps more serious. Over the last 3 or 4 days, including today, when the furnace attempts to start the majority of times it does not.

Often it will try to start, go through the purge cycle (about 10 sec) and shut down without lighting.

Other times, it will go through the approx 10 sec purge cycle, light, and 4 or 5 seconds shut down.

It might recycle 15 or 20 times then light up - it will then shut down 2 or 3 minutes later well short of the selected temperature setting.

This occurs when on battery or plugged in - at the moment, plugged in, my house batteries measure 13.7 volts

Help and thanks,

Preston (waiting out a snow storm on I-81 at Falling Waters RV park at Exit 23 in West Virginia
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Old 12-16-2010, 05:54 PM   #2
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Quote:
Originally Posted by prsmith View Post
Help and thanks,

Preston (waiting out a snow storm on I-81 at Falling Waters RV park at Exit 23 in West Virginia
prsmith, We were having the exact same problem with our furnace. The diagnosis was a bad thermostat.

Remove the shell on the T-Stat. Look at the slider switch and see if it has any perpendicular movement at all. I did and I found mine was loose. Push the button toward the circuit board (with your thumb) while in the OFF position "Gently but firmly" and see if the switch moves at all.

Taking 2 fingers slide the switch to the furnace position and center the switch over the furnace position as much as possible and wobble it over the detent position. Hopefully the furnace will fire.

We woke up in 25 weather with no heat one day while on the road a few weeks ago. The tech came into the rig. Centered the switch using the method I described and the furnace worked for a week. I never touched the button again, just adjusted the temp up or down.

Now if the furnace starts running and stays running you got it made. I wish you good luck - ain't easy getting a T-Stat but not impossible.

When I got home and checked to see if it was a fluke - the furnace kicked on but didn't fire for what seemed like 3 minutes or so but then the burner kicked in.

When you move the switch from OFF to Furnace you should hear a relay "click" in. The click will be brief but you need to hear it. Without that audible click - I don't expect the furnace will fire.

We had 2 electric space heaters .... they couldn't keep up, we froze.
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Old 01-01-2011, 08:12 AM   #3
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Happy New Year everyone!

Thanks for your reply DriVer!

Finally got the darn thing sorted out.

After our incident in mid-December, I stopped at a dealer near Knoxville. He did not have a new 'stat card, but he pulled the existing card and we could see that the contacts on the slider switch were corroded. We slid the switch back and forth many times and used electronic contact cleaner on it - more sliding and the switch contacts were bright and shiny. This solved the switch 'location' problem and at least the furnace wanted to start. It did not solve the recycling problem

Yesterday a mobile tech was in our park in Mission TX. He thought the problem was with the thermostat card and he did have one with him - we installed it with no change to the recycling problem. We could measuring a DC signal from the card at the furnace control card so we knew that the 'stat was calling for heat. So we replaced the control card on the furnace - no change. Then we pulled the furnace and checked the limit switch that was at the back end of the furnace - it was stuck open. Replaced it and all was well. Put the original cards back in and the furnace worked like it should

Our problems were caused, first by the dirty slider contacts on the 'stat card, and secondly by a malfunctioning $7.70 switch.

Hope this helps somebody else.

Preston
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Old 01-26-2011, 06:41 AM   #4
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Regretfully, our furnace problems continued and were finally solved yesterday

Our problems were two fold - the dirty thermostat sliding switch contacts as previously explained and a bad furnace motor

Our furnace is a Suburban SH-42 used only by Winnebago (I think). The furnace is located in the rear cap and must force air the length of the coach - of course the SH-42 uses a unique motor to do this.

What was happening is that the motor was overheating and an internal non-replacable thermal switch was opening and removing power from the control card - by the way there is also a replaceable thermal switch on the rear of the burner plenum (rated at 190F)

When I used the 'stat to shut off and restart the furnace, the card would recycle and the furnace would run until the malfunctionning motor internal switch shut it down again.

Until we installed the new motor there was no other indication that the motor was dieing. Once we installed the new motor and fired up the furnace, the sound was totally different - faster and quieter with almost double the 'blow' from the registers. Last night it was 39F here in Mission TX and it was nice to have a working furnace for the first time (we bought the rig second hand in June 2010)

Hope this helps someone else.

Preston
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Old 01-26-2011, 06:04 PM   #5
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Bad furnace...bad furnace...bad furnace.

The problem may be the t'stat or possibly the controller board.

Ken
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Old 01-26-2011, 07:48 PM   #6
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No - the thermostat and control board were replaced with new ones and the problem persisted - so we reinstalled the old ones. The new motor fixed the problem
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Old 01-27-2011, 08:42 AM   #7
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Thanks for posting the final resolution. I will tuck this away in case mine starts acting up.
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