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Discussion in 'Heating / Cooling Systems' started by EdZilla, Sep 19, 2017.
Fan is blowing into the camper.
Generally, the 12VDC wire coming into the furnace from the thermostat has two functions - first, to complete the circuit that allows the fan to operate and, second, to pass through the limit and sail switches before it connects to the control board. If either of the switches aren't working, the control board will not signal the gas valve to open and the ignitor to spark.
You can temporarily by-pass each switch by connecting the two wires that goes to that switch together and trying the furnace. If the furnace works after connecting the wires together then you know that switch is bad. Disconnect the two wires and replace the switch. By-pass each switch separately to isolate the bad switch. If the furnace doesn't work during either of the by-passes, then the control board or gas valve may be bad.
DO NOT leave the two wires connected to each other - it is a safety hazard!
My furnace is very similar and I've been working on it solving other problems, so I've studied the basics of the Atwood units. With what you describe is happening - the furnace fan comes on but you never get an ignition sequence - we know that the components up and through the fan are working: thermostat, time delay relay and blower. We know everything was working at one point, so unless you've been re-wiring things, the odds of the blower running backwards are pretty slim. In the operational flow of how Atwood set up their furnaces, the next items in the pre-firing order are the limit switch and then the sail switch. (Atwood describes the sequence one way, but their wiring diagrams reverse the positions of the limit and sail switches, with the limit switch first in the circuit sequence from the relay, and the unit wiring matches the diagrams.)
The DSI board pulls voltage from the circuit completed through the limit switch and sail switch with the WHITE wire running to the DSI board connector.
You need to run the following tests with the thermostat turned up and the furnace fan running:
The limit switch is right there on the front of the furnace with the two white wires running into it, right behind the relay on the right wall of the furnace box. The limit switch only opens when the furnace exceeds a set temperature and normally is closed. Get a multimeter and set it to 20VDC range. Put the black (-) lead on the grounding bus on the right wall of the furnace where all the black wires mount - an alligator clip for the meter probe comes in handy for this. Put the red (+) lead first on the left and then right poles of the limit switch. You should get ~12VDC from each side. If you get no reading from the left pole, the time delay is probably actually bad. If you get a reading on the left pole but none on the right pole, the limit switch is bad. 12V readings on both poles means the limit switch is good. Next check is the sail switch.
You may have to improvise a back-probe with a pin or small paperclip to make a connection for the meter lead into the DSI board connector if your meter probe won't reach the connector pin. It's better to probe at the back of the connector where the wire goes in than at the board side so you don't distort the contacts. Just don't accidentally let the back-probe pin touch ground.
With the red (+) lead of the meter, probe the WHITE wire going to the DSI board connector. You should get a constant reading of ~12VDC if the sail switch is working properly. If the voltage is jumping up and down significantly or cutting in and out, the problem may be a restriction in the airflow through the blower or the sail switch is failing or not positioned properly. Either of these conditions won't allow the DSI board to start the timing sequence to open the gas valve and fire the ignitor - it wants a steady voltage.
If you have steady voltage through the switch circuit to the DSI board and no sparking or gas valve opening, the DSI board is bad.
Hope this helps.
The switches are wired in series and the return line from the sail switch goes right into the DSI board connector providing the board power. To bypass the switches entirely you have to get a good back-probe to the DSI board wire and jumper it to the relay side of the switch circuit. It would work, but not recommended for the electrically-faint-of-heart.
ETA: you could check each switch individually with variations of your method, but it would take more jumper wires and still have to back-probe the DSI connector.
Very good advice.
And I'm not really electronically faint of heart.
Ive been a ham operator since I was 12 and I started my engineering career as an oilfield instrumentation technician after graduating from electronics school, so I think I follow what you guys are explaining and it mostly makes sense to me. I have a good DVM, probes, and plenty of alligator clips.
The only thing I don't understand is the term "back-probe".
And the description of where the limit switch is. Is it "right there on the front" or "on the right side behind the relay. "?
I may not be able to work on this again until Friday because tomorrow I'm taking my two teenage boys to Red Rocks to a "Get the Led Out" concert.
Thanks again and I'll update as soon as I can.
Yeah, I thought that description sounded "off" when I read it.
Look at the last pic you posted; the limit switch is on the front of the furnace housing, small round cap, held in with two screws, two white wires connected to it at 3 and 9 o'clock. Just above and to the right of the ignitor that has the big red wire and smaller black wires going to it.
"Back-probing" is going into a wiring connector from where the wire goes in to the connector body (the "back" side), rather than in the openings at the face of the connector (front-probing) where the pins actually engage their connecting partner. Back-probing is usually safer in terms of not damaging the connecting points of the interface - stuffing a big meter probe into the pin slot can distort the connector lug and cause poor contact afterwards. It's really common in the automotive world to back-probe sensor connectors to get readings while operating since you don't have to disconnect anything to do so. I learned the hard way a few times, messed up some connectors front-probing and then had connection problems. I'll back-probe any chance I get.
Going to see anybody at Red Rocks is always better than working on a furnace!
And it sounds like a Led Zep revival, so his kids have good taste in music as well!
Just a thought, check the voltage at the heater with the fan running. I found mine dropping to ~10v at the heater. Like yours, my heater would blow are but with the low voltage the fan wasn't blowing hard enough to trip the sail switch. The low voltage was the result of the heater circuit running through the top safety/kill switch and probably 10 of their "quality" splices. I ran a dedicated circuit from the fuse box to the heater, bypassing all the obstacles the factory wiring had to go through and now my heater works flawlessly with almost no voltage drop.
Should be a great concert! Get that furnace working and take it with you; you'll need it.
Cold front coming in Friday afternoon with rain in the foothills and snow in the high country!
there is always a chance of slow air flow from mud dubers/mice nest, if the air in or air out are partially blocked.
I haven't yet been able to make time to bypass the switches and do the tests you guys described, but I did notice that the sail switch is in a strange position and because of that I doubt that it is effective.
See the photo below. Is that the sail switch?
It is a switch, and if you look closely, it is marked with "5 AMP". It's very easy to depress by hand, but the fan doesn't effect it.
Unless I'm mistaken and this is not the sail switch, it is not in the path of the airflow and therefore the switch will never close.
IMG_6221 by EdZilla posted Sep 22, 2017 at 10:27 AM
The fan air flow is in the large opening on the left and the sail switch is in the channel to the right of it, where there is little or no airflow.
Nope, rabird is spot-on: circuit breaker. Normally mounted inside that metal housing right below where it is shown in your pic. The sail switch is located in the rear of the furnace with the blower unit and is not visible.
I turned on the 12V to start another round of troubleshooting and the fan came on as usual, but no heat, also as usual. Then I decided to connect it up to "shore power". When I flipped the converter switch from battery to converter the Fan sped up and a few moments later the Heat came on. Hooray! I think I'll have a beer.
So now I'm trying to figure out why it works on shore power but not on 12V. Don't ask me why I hadn't previously tried it with 120VAC.
I turned down the thermostat and it turned off as expected, and then back up, and seems to work pretty normally when on 120VAC.
Next I'll switch it back to battery and see if I can get it to work. The voltage is 12.34 at the battery with no load, and the panel voltmeter I installed reads 12.2. Maybe the fan isn't spinning enough on 12V for the Sail switch.
Well then, it seems that Adam H. was onto something with "check the voltage".. Check the voltage at the top red lead on the time delay relay - that's the main power feed. 12.34V is a little low for a no-load battery that's fully charged; is the battery getting old? There still also may be a flow restriction or problem with the sail switch.
By the way, the backstory on this camper is that it was gifted to me a couple months ago (August 2017) by an old friend who's kids are grown and on their own now, so he has little use for it now. We were at an outdoor concert, and I told him I'd bought a Toyota 4Runner so I could pull a camper, which I was shopping for.
He said "You can have mine. Take your kids and go have fun."
How cool is that?
It had been in his mother in law's garage for exactly 10 years, so had been out of the elements. He said the electrical system did not work, and that he had seriously tried to figure it out with no luck.
I took it home, bought a new battery and found that the electrical all worked. I have to assume that the converter breakers weren't in the right positions "push on, pull off". I replaced the incandescent bulbs with LEDs and took the boys camping!
I've been out with it a few times and now that it's fall, I figured that I should figure out the rest of the systems.