Furnace issue

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Danno13

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Hi all,

November was my last camping trip, furnace wouldn't fire. Fan would come on, but wouldn't trip the ignitor. After tinkering, pulled the high limit, joined the wires with a marrette, worked just fine for the weekend.
Over the winter, ordered a new high limit and sail switch. Just replaced both, fan will spin, won't trigger ignitor. I'm testing 12v on one side of the sail switch, but only 0.25v on the other with the fan going.
If I pull the sail switch and manually squeeze it with the fan running, it triggers the ignitor, and the furnace fires every time. What would cause the sail switch to fail to activate inside the blower assembly?
 

Gary RV_Wizard

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The sail switch is activated by the force of moving air against the "sail". But it should be either open (0v due to low air flow) or closed (enough airflow to push the sail and make contact). That 0.25v is either a measurement error or a defective switch.

Generally the sail switch doesn't activate if the voltage is low, say less than 11.5v. You say you are reading 12 (exactly 12.0?), so that's not it.
 

Danno13

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Hmm, well if you say low voltage would cause the sail switch not to activate, then maybe that's a possibility. If I recall, I think it could've been 11.35v. I just hooked the battery back up from winter storage too, so maybe it's a little low on charge. Would that be enough to stop it from activating?

It's a brand new sail switch, and if I pull it out of the housing and pinch the sail closed manually, the furnace fires up. So I know it all works.
 

Mark_K5LXP

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Not sure I would attribute a measured quarter volt anything more than "leakage", since any DMM has a pretty high input resistance. Would have to review the schematic to see if maybe there is a circuit that might be a path for this.

More specific to Gary's point is that it's not "low voltage" that would directly inhibit the sail switch from activating, it would be the secondary effect of the blower motor not blowing hard enough to fully push the sail switch due to low voltage. "Low" in this application is probably around 10.5V, or whatever the operating voltage limit is for this model of furnace. So by definition, the sail switch should activate any time the furnace is operating within it's specified limits. In this case, even 11.35V would be enough.

My guess is that something is preventing the "sail" from moving fully, either mechanically or perhaps something is actually blocking the air flow. The fact you can externally operate the furnace by manually closing the switch says the fault lies with the air flow or mechanics of the sail.

Mark B.
Albuquerque, NM
 

Danno13

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I was wondering if it was air flow as well. I went back today, i checked the battery, and it's good. I hooked the 9 pin up to my truck, left it running, and tried it that way. Good voltage on one side of the sail switch, 0.25 on the other when the fan is running. I removed the sail switch, used my compressor and blew out the fan and blower housing, no change in results. This is a brand new sail switch. I get the same results if I put the old one back in.
 

Kirk

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I just hooked the battery back up from winter storage too, so maybe it's a little low on charge. Would that be enough to stop it from activating?
Yes low voltage to a direct current motor causes it to run but slower than the higher voltage. It should run fast enough to trip the sail switch as long as the voltage is above 11V but there may be other issues as you clearly have described a situation where the air movement it not sufficient to close it. The same motor moves the air through the RV with a squirrel cage fan on one end of the motor shaft and combustion air with another squirrel cage on the other end of that same motor. The sail switch is a safety device to insure that there is enough air movement through the combustion chamber for safe use of the furnace. Since the air though the RV is supplied by the same fan motor, something like a crushed heat duct on the inside could restrict air movement enough to slow the turning of the motor and so not close the sail switch which is activated on the combustion side. It could also be caused by a motor with bad bearings, or even some type of nests on the inside of the squirrel cage of either blower. The fact that you can make it work by manually closing the sail switch clearly shows that it is the problem, so the next step is to figure out why it isn't moving enough air.
 

Danno13

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Yes low voltage to a direct current motor causes it to run but slower than the higher voltage. It should run fast enough to trip the sail switch as long as the voltage is above 11V but there may be other issues as you clearly have described a situation where the air movement it not sufficient to close it. The same motor moves the air through the RV with a squirrel cage fan on one end of the motor shaft and combustion air with another squirrel cage on the other end of that same motor. The sail switch is a safety device to insure that there is enough air movement through the combustion chamber for safe use of the furnace. Since the air though the RV is supplied by the same fan motor, something like a crushed heat duct on the inside could restrict air movement enough to slow the turning of the motor and so not close the sail switch which is activated on the combustion side. It could also be caused by a motor with bad bearings, or even some type of nests on the inside of the squirrel cage of either blower. The fact that you can make it work by manually closing the sail switch clearly shows that it is the problem, so the next step is to figure out why it isn't moving enough air.
This almost scares me more than an electrical issue, harder to chase down the source of the problem. Would have to track all heat runs through the guts of the trailer.
 

Henry J Fate

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I was wondering if it was air flow as well. I went back today, i checked the battery, and it's good. I hooked the 9 pin up to my truck, left it running, and tried it that way. Good voltage on one side of the sail switch, 0.25 on the other when the fan is running. I removed the sail switch, used my compressor and blew out the fan and blower housing, no change in results. This is a brand new sail switch. I get the same results if I put the old one back in.

Curios as to how you are checking for voltage at the sail switch. If you put a meter across the sail switch with the fan running, the meter should read zero. If it reads 12 volts, the switch is open and not working properly.

The other way to check the switch is to find a dependable ground for the ground probe of the meter and connected the probe to the ground and check each side of the switch using the other probe with the fan on. 12 volts should be found on both sides of the switch. If 12 volts is only on one side of the switch, the switch is open.

Which method are you using?

I would use both to see if the 2 methods agree.
 

Danno13

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Curios as to how you are checking for voltage at the sail switch. If you put a meter across the sail switch with the fan running, the meter should read zero. If it reads 12 volts, the switch is open and not working properly.

The other way to check the switch is to find a dependable ground for the ground probe of the meter and connected the probe to the ground and check each side of the switch using the other probe with the fan on. 12 volts should be found on both sides of the switch. If 12 volts is only on one side of the switch, the switch is open.

Which method are you using?

I would use both to see if the 2 methods agree.
I'm using a dependable ground with the black probe, and touching the red probe to one prong of the sail switch, which reads 12v with the fan running, but only 0.25v on the other prong of the switch. If I pull the switch out of the fan housing, and manually click the sail closed, I read 12v on both sides. That means the switch works, and does its job completing the circuit, activating the relay which starts the gas, and fires up the furnace.
 

Danno13

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Might be low batteries or a less than optimal power lead connection. or a weak blower motor.. in short not enough breeze to sail the switch.
It's definitely starting to feel like not enough breeze to sail the switch. Has anybody heard of a blower motor being weak enough to cause that?
 

Mark_K5LXP

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Motors can go bad but it's generally not as subtle as reduced output. Before I went through the throes of changing that out I would conclusively verify that it's not something mechanical inhibiting the sail switch operation ("dirt", bent, etc), and that the ducting isn't restricted.

From a Suburban service manual:

"Dust and lint should be removed from the room air blower wheel and sail switch. A build up of dust and lint on the blower wheel can cause the motor to drag and not generate enough air flow to engage the sail switch. Dust accumulation on the sail switch will restrict the travel of the actuator arm and prevent operation of the valve circuit."

Mark B.
Albuquerque, NM
 
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Gary RV_Wizard

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Isn’t that telling you the new switch is bad?
No. It says that the switch circuit is not closing, but there could be various reasons for that
  • Insufficient air flow to move the sail
  • Mechanical interference with the sail lever
  • Bad connection between switch and wires
  • Defective switch
The chances of two defective sail switches is extremely low, so I'd be checking the other possibilities thoroughly.
 

Danno13

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The sail switch definitely isn't blocked by anything preventing it from closing. I checked the cavity, it's wide open, switch is free and clear. I blew out the fan housing with my compressor, and there was basically nothing that came out of it, no dirt or lint etc.
 

Kirk

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The sail switch definitely isn't blocked by anything preventing it from closing.
Then something is still causing it not to move enough air for safe furnace operation or there is some type of mechanical bind. That is a safety device and to bypass it would be very risky!
 

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