7900A601 No Heat

[danielrogers1]

Today I noticed it was getting very hot in my trailer, I turned the thermostat down 5 degrees or so and the heater shut off, it has not come back on since then.

About 8 hours since it had last come on and at around 50 degrees I began to get curious, checked the pilot light and it's good (gas) checked the breakers and they're good too, so I unscrew the thermostat from the wall and notice 1 of the 2 wires going to the back of the thermostat has become broken. I stripped down the wires, reattached them, replaced the thermostat and still nothing. The switch is on Auto not Continuous. When I switch it to Continuous it does blow air, but it is not hot. I can switch the temp switch from Heat to Cold and turn the thermostat down and it will engage automatically and begin moving air, but with the switch on head I can't get it to do anything.

Any ideas? I do have a multimeter and am familiar enough with electronics and wiring to test for voltages if you can explain what I need to be testing, as it is it's like 10 degrees there so I will go back tomorrow during the day and it will hopefully be a bit warmer and I can take some pictures if needed and do any testing that's needed.

[Robert]

Set t-stat below room temp for a no call for heat. Test across the Red and White or whatever the two colored wires are if different at the t-stat.


Do you read 24-28vac ?


IF so, set t-stat for call for heat, wait a minute and test across the gas valve terminals. Do you read 24-28vac ?
If so, reply back. If not, trace back from there to the t-stat at all connections to find loss of low voltage circuit.


Thanks,
Robert

[danielrogers1]

Robert: Thanks a ton for your reply, at the thermostat I am getting 25VAC.

By the gas valve terminals you mean the terminals on the little box with 2 wires which the gas line runs in to? If so, that has no current going through it.

*edit*

Ok, I have a little more information for ya. What I believe to be the gas valve has 2 brown wires going into it, one front center and one a little to the left of that. The one on the left runs up by the control panel, and tested there has 24VAC, the other runs up on the right hand side of the furnace to a little round thing which says SYNCHRO on it around the front, and has a red card/filter looking thing coming out the back of it inside the unit. This had no current, but I'm unsure if it's because I just got a bad read because the plugs are encased in plastic, but I'm pretty sure I got good contact and it's just dead.

[Robert]

The 7900A601 is not a valid number. Look on data plate or sticker on front facing of furnace for furnace model number.


The gas valve is a med.-large component on front center of furnace.

It has a gas line coming in and out, usually two wires coming to top and a knob or lever for lighting pilot if you have a standing pilot furnace.




Thanks,
Robert

[danielrogers1]

Sorry about that, the model number is DGRT070AUA and the serial is 930777280.

The current seems to be dropping at a device on the right hand side of the furnace, 2 screws hold it in place, inside there is a piece of metal attached to a spring, and a small black box where 2 wires go, 1 is the brown wire coming from the gas valve. There is a number 2TM-4A-YU above the hp and voltage for the device which is 10A.1/3hp.125.250VAC

[Robert]

Hi,

Go to this page:

http://www.mobilehomerepair.com/ColemanGas.html


Combustion Air Damper Assembly # 7956-5771 on left side .


Gas Valve # 7956-336P on right side .


Is it either of these ?


If not, where on right side, give better description of where or post a pic.




Thanks,
Robert

[danielrogers1]

Yes those are the correct parts, I should have just looked off the schematic diagram to get the names of what I'm talking about.

At the gas control valve there's the two grey wires, there is no signal across the two terminals but testing the terminals individually they both have 24VAC. One wire runs out to the limit switch, testing across those terminals I get 24VAC on the limit switch. The other runs to a switch on the damper assembly. Again there is no signal across the terminals but each wire has signal. The damper motor itself is working because it will open and close when call for heat changed on and off.

[Robert]

Hi Daniel,

Sorry for delay in replying. We usually spend w/e's at another home we've bought and are working on.

I got a laptop a couple months back, but just now finally got a set up to work from both homes.

This is my very first post under new set up, LOL.

By what you stated, you have an open limit switch. Turn power OFF, jumper across the limit, turn power ON and see if it works.



ALL other test readings were correct .


You can also and easier, turn power OFF, remove wires after writing down where they go from Limit, then OHM across it to see if it shows continuity.



Thanks,
Robert

[danielrogers1]

Hey no worries, just glad to have the help

Ok, so I jumped the limit switch, turned power back on and immediately the flame jumped up and a minute or two later the blower kicked on, I again have heat

So I just need to replace that switch?

[Robert]

Hi,

Yes, that should take care of it. It has been off long enough that the limit should have reset and it hasn't.


They do fail sometimes due to weakened contacts.



Make sure filter is clean and if you have a/c, make sure A-Coil is clean also fro good airflow.



You want to be sure it was just a faulty limit and not a faulty limit and an overheating problem.


In that case, it would have tripped doing its job, then failed to close after furnace cooled down.


Can send ordering info tomorrow if needed when I get to my parts books at other home/office.


Thanks,
Robert

[danielrogers1]

Hello,

The filter is clean, and I do have a/c but I'm not sure where the a-coil would be located? I just bought this place at the beginning of november so I haven't had cause to use it just yet.

[Robert]

For a downflow gas furnace, a-coil will be located at bottom under the furnace.


The bottom door panel of furnace, look for where the two refrigeration lines enter unit, that is where the a-coil is at.


http://www.mobilehomerepair.com/article ... ioner.html


Thanks,
Robert

[danielrogers1]

Sounds good to me, I made sure the a-coil was clean, i'll have to wait till this summer to get some of the actual cleaner since I can't run it with it this cold out.

[Robert]

Will try to send the info tomorrow. Severe storms dictated we remain in the DW rather than go back to SW right at end of county where the tornadoes come across.




Thanks,
Robert

[danielrogers1]

No worries, safety first. The furnace is up and running for now, I'm keeping an eye on it to make sure it's not getting too hot, and it's warming up outside too so I don't need to run it as much anyway.