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Old 04-10-2006, 07:48 AM   #1
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1979 31' Excella 500
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Furnace woes, how to debug?

Ok, I have the Suburban furnace on my 1979 31' International. Here's the symtoms. Turn up the thermostat and the furnace lights just fine brings trailer up to temp, and turns back off. I have observed this and had the thing running for several hours while fixing up the trailer inside.

Here's the problem. I've left the furnace on overnight just to make sure it's going to work once I get this thing on the road. I've done this twice with the same results. In the morning when I go to check on it, furnace blower is on and blowing nothing but cold air and trailer is cold.

What do you guys think. Since the blower is on this kindof tells me that the thermostat is ok and calling for hot air. So is it the ignitor or the ECU? How do debug?
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Old 04-10-2006, 11:41 AM   #2
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It is likely the spark board is marginal. You might try cleaning the ignitor terminals where the spark is generated. You might also pull off and push on the terminals on the boad a couple times to clean them. You might also clean all terminals on connections from the battery to the furnace. Original boards do not work well if the voltage gets low be that poor battery or excessive voltage drop due to poor contacts. If you have digital voltmeter check the voltage at the spark board if you can get to it. If not check the voltage at the furnace when it is running. It pulls a bunch of amps and voltage drop is a potential. If all that fails new boards from Dynasaur (bad spelling ) are about $119 and are more tolerant and reliable that the Suburban units.
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Old 04-10-2006, 11:49 AM   #3
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I guess there are two ways to get into such a "lock out" situation. One is no spark and the second is the sail switch is stuck. I guess I was looking for a way to narrow it down to which one it is the next time I run into it. I guess I'll start by cleaning up the contacts as much as I can. It is strange though that the thing will startup cold just fine and run, but only after entended running it fails.
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Old 04-10-2006, 12:38 PM   #4
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Originally Posted by wacnstac
It is strange though that the thing will startup cold just fine and run, but only after entended running it fails.
The joints and components in circuit boards don't like being repeatedly heated and cooled. Your furnace symptoms are very like those of a failing board. As well as their excellent replacement Universal Ignitor Boards, Dinosaur makes an "Extend-a-life" kit, which enables one to position the new board a few feet away from the furnace in an area with a more constant temperature.
Nick
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Old 04-11-2006, 06:38 AM   #5
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1979 31' Excella 500
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Where does one find a universal ignitor board and (or) extend a kit.
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Old 04-11-2006, 07:03 AM   #6
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Most better RV suppliers will carry them. I am a Dinosaur Dealer and have them in stock. You can also upgrade your furnace to a 'Fan 50 Plus' board that will turn your furnace blower off after a 3 try ignition failure so the blower motor will not run constantly and drain your batteries.

The Dino boards are of MUCH higher quality than the original Fenwal boards as they are made on a military spec G-10 base with both top and botom terminals soldered thru. You can twist and bend them and the contacts won't come loose; plus you get a 3 year warranty. Can't do that with other boards!!

PM me if you can't find one locally.
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Old 04-11-2006, 07:16 AM   #7
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Where does one find a universal ignitor board and (or) extend a kit.
http://www.dinosaurelectronics.com/Online_Sales.htm gives a list of suppliers, as well as loads of information about the products.
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Old 04-11-2006, 07:35 AM   #8
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Before replacing the board (a bit pricy), take out the sail switch and give it a good cleaning with compressed air. If it works, you're in business. If it doesn't, no money lost.

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Old 04-11-2006, 07:58 AM   #9
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Can the ECU board be replaced without taking out the furnace from under the sink?
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Old 04-11-2006, 08:11 AM   #10
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If you have an outside door......yes. If you only have the intake and exhaust ports visible from the outside....no. The igniter board is usually located on the left side of the furnace when looking AT the intake/exhaust, next to the blower cage. The sail switch is there also. BTW, it is the sail switch that controls the voltage to the board and hence, the ignition system so if your sail switch is bad, it may be the problem. You can check that once your furnace is out by doing a continuity test.That would be the first place I would look.
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Old 04-28-2006, 12:19 PM   #11
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What is your phone number. I might be interested in ordering a new furnace controller board for my 1979 Suburban furnace.
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Old 04-29-2006, 05:24 PM   #12
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I just looked. The heater in my '79 has a small, apparently potted circuit board on the right-hand side as viewed from the rear with the cover removed. It has a couple of screws securing it.

This heater has never given me trouble, but I've not been impressed with the heat output at the rear of the trailer or the flow from the tank heaters, which I recently got to test when I had the shower out for repair. I'm wondering if the fan is as powerful as it once was - after all, the front vent was blocked off when the cat heater was installed. All of the output is going to the rear, and all it has to provide heat for is the bathroom, the tanks, and the rear bedroom outlet. The sail switch is obviously being activated, so the fan is at least putting out what it requires.

My Argosy heater stopped heating once, and I found a dead mouse in the fresh air intake tube. He had to really squeeze up to get in there, and then I guess he couldn't back out.

Lamar
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