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Phil
Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 5940
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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Have a Duo Therm 66112 furnace in a truck camper. Furnace

Customer Question

Have a Duo Therm 66112 furnace in a truck camper.
Furnace is in an 89 model but looks like new.
Pilot lights ok. When I turn furnace valve to on, no main burner.
Thermostadt starts furnace fan, but still no main burner.
Removed propane supply line yesterday and sprayed WD40 into furnace valve.'
Figured years of no use could have cause valve to
freez up?

Any ideas?
Submitted: 2 years ago.
Category: HVAC
Expert:  Phil replied 2 years ago.

Phil :

All work to be done by licensed professional. WE KEEP GOING UNTIL YOU HAVE THE INFORMATION YOU NEED, I come and go. This is step ONE.

Phil :

Hello

Phil :

Look on the wiring diagram for the gas valve, tell me if two wires come from the pilot light area and to the gas valve or not.

Phil :

Tell me if the knob on the gas valve has an ON, OFF and Pilot positon.

Phil :

we can go from there

Phil :

Phil

Customer :

no wires anywhere from pilot

Customer :

Knob has pilot on and off

Customer :

took some pics, but wouldn't tell you much more than i have

Phil :

Hello again,

Phil :

look this over please, tell me if its the model you have

Phil :

and if the wiring diagram on the next to the last page is accurate

Phil :

http://bryantrv.com/docs2/docs/operating/dt6600.pdf

Phil :

it it is, tell me if you are getting 12 volts DC across the two gas valve terminals.

Phil :

We can go from there

Phil :

Phil

Customer :

yes this is the furnace.

Customer :

I printed out the diagram, WHERE are the two gas valve terminals???

Phil :

That diagram shows then on the top of the gas valve

Customer :

Blue wire lights a 12v bulb

Customer :

Black wire has sml voltage, but won't light bulb

Customer :

does that help?

Customer :

thermostadt OFF

Customer :

I have 2 great pictures of gas valve showing all connections

Customer :

but can't paste here..

Customer :

got an email???

Phil :

JA does not allow email, it bypasses the peer review and quality control set up.

Phil :

but you can attach JPEG files by clicking the paper clip icon in the bar just above the the dialog box

Phil :

Phil

Customer :

duotherm661
Full Size Image

Phil :

Thanks, the black and the blue wire should have 12 vdc across them if none of the safety controls are tripped, and the pilot is lit.

Phil :

if not, then the flame sensor rod shown in the diagram next to the ignitor is dirty and not sensing flame, so the control card is holding power off of the gas valve... or one one of the other furnace safety controls is wired into that circuit and tripped.

Phil :

You can also remove the black and blue wires and check to see if you have 12vdc between the two with the pilot lit.. if not the problem is in the controls as noted just above

Phil :

If you do have 12vdc to those two wires, use your ohm meter to check for continuity through the two terminals on the gas valve...... the ohms read should be in the 100 to 900 ohms range roughly.... if you get an open circuit the gas valve coil is failed.

Phil :

neither terminal with the wires disconnected should read to ground.

Phil :

Phil

Customer :

12 volts in blue, a little less in brown (with some clicking) when it was clicking main burner lit several times, but went out....

Customer :

12 volts in blue, less in brown but clicking made light to full brite and main burner

Customer :

lit

Customer :

any ideas what is dirty or need to replace? almost there...

Phil :

Hello, if all you did was do the tests I asked for, thats checking for voltage across the black and blue wires.... and then it lit. the problem is most likely lose or corroded wire connections in the area, or perhaps inside the gas valve.

Phil :

the way to track that down is to very very gently. and cautiously. notice which wire or terminal you touch that causes the click. or the gas valve to light. it will be that wire or terminal or device that has the bad connection.

Phil :

if you go too fast. you will not know what wire or terminal or device it was

Phil :

Phil

Customer :

back up, when I touched just my probe to the black wire every time I heard clicks

Customer :

and then got a light. Almost like a bad relay????

Phil :

depends on how firmly you touched the black wire, and precisely where exactly you touched it.

Phil :

tell me where you touched it, if you barely touched it where the wire terminal attaches to the gas valve, and that spade connector is tight, then its a frail and failing wire connection where the wire from inside the gas valve attaches to the brass terminal itself,, that may or may not be repairable... you will have to take the closest of close looks

Phil :

try touching it with just a screw driver, not the meter probe.. there is a very slim chance that its an electronics issue, affected by the test meter voltage itself

Phil :

if its a bad wire connection a screw driver touch will cause the changes

Phil :

Phil

Customer :

Looks like the gas valve itself as all wires intact.

Customer :

Any ideas??? more WD40?

Customer :

big question for me is ordering another gas valve, I see them on EBay

Customer :

and think I could put one in?

Customer :

Is that the route you would take?

Customer :

Sorry to be so much trouble...

Customer :

Don

Phil :

Hello, the bad wire connection could be inside the gas valve where the wire solders on, not repairable.

Phil :

If you can get the exact replacement valve on ebay it will have a 90% chance of curing the problem. These furnaces can be unsafe however, you should have the work inspected by an RV gas appliance repair outfit... be sure all the gas fittings are tight and do not leak, and test it by blowing out the pilot and making sure the gas valve shuts down properly.

Phil :

before you change it, make sure its not just a loose connect on top of the valve. you may even want to try twisting the terminal one or two degrees... just slightly.

Phil :

If you like you can click the big green accept button now, thats how I get credit. you can then follow up days or months later on the same dime if necessary.

Phil :

Phil

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Phil
Phil
HVAC Technician
5940 Satisfied Customers
Retired HVAC/ Electrical & Boiler contractor. Industrial