SAIL SWITCH VOLTAGE CHECK

[Colorado_Dave]

Hoping for help on troubleshooting a Furnace Sail Switch. Furnace was intermittently failing and now progressively worse. Blower would come on and after about 30 seconds would shut off - no audible attempt to open the gas valve nor ignite. I could see the sail switch blow over and using a skewer, I could feel the sail switch close/open when furnace was off. I finally pulled the entire furnace (Dometic/Atwood DFSD20121) and extracted the sail switch. I attached one of the leads to a 12V battery and using a multimeter, detected 12V when it was closed (blown over) and 2V when it was open (not blown over). Should I be getting 0V when open?
I know the furnace needs to have an open sail switch at the beginning of the cycle, and a closed sail switch when the blower blows it over. Could this 2V be causing my issue???
I do have a new sail switch on order but I have nothing to compare my current switch to. HELP!

[Gdetrailer]

enblethen wrote:
Not a very good drawing but may help.Wiring diagram
Interesting the sail switch is tied to thermostat.
You could have thermostat issue.

Fan would not turn on if it was a "T stat" issue.

T stat delivers 12V to sail switch and terminal on control board marked "air".

Sail switch contact must close to complete circuit to control board (apply 12V to the control board input terminal) to initiate ignition.

I doubt the issue is the sail switch and is most likely a fault in the control board. Sail switch is not carrying any more current than the T stat so contacts should not be burnt and as long as the sail switch contacts are enclosed, dirt should not an issue.

You could test that theory by jumping in a manual switch to bypass the sail switch which you can flip on after the fan motor has started.

If no joy, then the issue is not the sail switch but rather the control board.

Not unheard of to have a control board go south.

[enblethen]

Not a very good drawing but may help.Wiring diagram
Interesting the sail switch is tied to thermostat.
You could have thermostat issue.

[BFL13]

So the issue is that the OP can somehow see the sail switch blow over (I can't see that on my 8531) and that should mean it is closed to let voltage pass along to the next thing in line. The question is whether the switch is bad so it doesn't pass that voltage along.

During the 30 seconds before it shuts off, can't you simply put a voltmeter on the gas valve to see if it has power? If it does then that would prove the sail switch is working right?

[ktmrfs]

the proper way to check the sail switch is to measure the open and closed RESISTANCE. open resistance should be in the megohm range, closed resistance should be in the few ohms range.

Measuring the voltage at the sail switch with nothing for a load other than the DMM will NOT insure any validity to the switch. The DMM has to high of a resistance and won't show a voltage drop from a high resistance closed switch.

Measure the voltage across the switch when it is in the closed position and connected in the furnace when the blower engages is another valid way to check the switch. voltage across a closed switch should be millivolts.

Also the resistance of the sail switch in open position must be measured and it should be in the megohm range at least. Ideally it will be high enough that a DMM with a 20Mohm scale will read as open. It must be high enough that the board sees that the sail switch is open.

[dougrainer]

BFL13 wrote:
I don't understand why it matters what the voltage is when it is open. The thing is, when it is closed and the squirrel cage is turning, the gas valve does not open. If it is not rotating fast enough, the sail switch does not close, but it does, the OP reports, so it is turning fast enough.

So the question is why doesn't the gas valve open. I assume it quits because there was no flame, so that part works.

This problem "got worse" so maybe wasp nests? Except there was no gas valve opening he said.

READ my response. THAT is why voltage matters. IF you close the sail switch manually BEFORE the furnace operates(to test the sail switch), nothing will happen because the CB "sees" that the switch is closed and will not operate the furnace. Older models with older style CB's you can test a sail switch this way. Doug

[enblethen]

Could be the four wire connector being corroded at circuit board. Clean with white soft pencil eraser. Apply some electrical de-oxidation grease to connection.

[BFL13]

I don't understand why it matters what the voltage is when it is open. The thing is, when it is closed and the squirrel cage is turning, the gas valve does not open. If it is not rotating fast enough, the sail switch does not close, but it does, the OP reports, so it is turning fast enough.

So the question is why doesn't the gas valve open. I assume it quits because there was no flame, so that part works.

This problem "got worse" so maybe wasp nests? Except there was no gas valve opening he said.

[ktmrfs]

I'd say there is a high resistance but not infinite resistance path around the sail switch. typical DMM input resistance is about 10Megohm. use the ohms function on the DMM an see what the open and closed resistance is. Open should be >>10Megohm, closed should be less a few ohms.

Also, the DMM is reading the voltage with no load applied. a closed resistance of even a few kohm with a 10Megohm input DMM will still read 12V, but be a high enough resistance to have the system think the sail switch is open,

[dougrainer]

Current Furnace Boards can "see" a fault in the Sail switch. If you have leaked 2 volts, the circuit board will sense this and not operate correctly. A defective Sail switch is very difficult to determine on newer Furnace's due to this. Furnace Circuit boards rarely fail. People NEVER understand this. They replace a circuit board and the furnace then operates and they think they found the problem, BUT, when you mess with the furnace you may be touching/jarring a defective marginal part and the furnace operates and you think the CB was the answer. There is a current thread on a 2/3 year old Atwood where the RV'er replaced the CB last year and now is experiancing the same type problem and thinks it is the CB. Probably a bad sail switch from the symptoms he lists. When you have failure of fan motor or ignition sequence, replace the CHEAP sail switch first. Doug

[goff1256]

Order a high temperature limit switch also that’s probably the problem sale switches and limit switches are cheap and not worth the frustration of trying to pinpoint the problem from what you have said and educated guess would be a bad limit switch

[wa8yxm]

It should be zero but your meter is what we call a high impedance load (means it is a very tiny load) so if there is any leakage due to moisture on the wires, or perhaps a leaky transistor on the control board it will detect SOME voltage.. I doubt 2 volts is a problem (but may be wrong)

It is possible you have a control board problem... But you are not yet at diagnostic level IF, and I stress IF, you have a control board problem may I recommend Dinosaur boards.. Very well built.

[DrewE]

Checking the switch with the voltage setting on a digital multimeter with nothing else connected (other than the 12V supply) is tricky because the multimeter has a high impedance input, and any stray voltage, capacitance, etc. can cause misleading readings. More useful is to check the switch with the resistance setting, with no battery or power supply attached. It should read an open circuit (i.e. infinite resistance) in one setting and a short circuit (a fraction of an ohm reading, usually) in the other setting.

[Old-Biscuit]

When Sail Switch is OPEN should be ZERO DC Voltages on output side (to Limit Switch)

Sail Switch is a 'micro switch' and do fail, hang up, get dirty etc

Atwood AFS wiring on pg 7

Fan starts then shuts down 30 seconds later......W/O any attempt at ignition?

After start of fan.....
15-17 second purge cycle
Then Circuit board should be sending DC to gas valve and spark electrode for ignition
No attempt at ignition ...circuit board shuts fan off


Sounds like Circuit Board issue
See pg 30 for Sequence (AF Models have the use circuit boards with blower relay/control incorporated on circuit board)

ANY Error Codes.....Blinking light on circuit board??

[enblethen]

Could be a poor connection in the 2 volt side of switch. Voltage you are seeing can be simply induced.