Coleman 7680B Furnace Won’t Shut Off

[Dave_L]

Hi,

We just bought this manufactured home in February and I’ve got a Coleman furnace which acts weird. When the temperature is 45 degrees or below outside, the furnace won’t shut off. However, when the outside temperature is in the 50’s, it will operate correctly. I thought it was the old White/Rogers mercury type thermostat which was bad, so I bought the LUX DMH110 thermostat and I’m pretty sure I connected the 4 wires correctly as I labeled them and followed the simple instructions.

The thermostat is set to FAN-AUTO and to HEAT. I set the thermostat to 73 and it operates correctly for several hours but then it goes crazy. About 2 or 3 in the morning (colder outside) it will come on and won’t shut off. A few weeks ago I thought I woke up in hell because the temperature hit 88 degrees!

Also, when I go to the thermostat and turn the MODE SWITCH to OFF.. nothing happens. I have to shut off the blower/motor at the wall mounted switch next to the furnace. Also, many times if I pull the cover of the thermostat off and press the RESET switch then reset the temperature to 73 the furnace will operate correctly for a few cycles then won’t shut off. By the way, this same condition was happening with the old W/R thermostat and that is why I thought it was the thermostat.

My wife usually goes to bed at 10:00 p.m. and I’m a night owl. I’m also retired as I’m home 90% of the time. Sooooo, the way we’ve been operating is we let the furnace run and when it operates correctly we’re happy. Then, when I go to bed about 4:00 a.m., I turn the blower/motor off and when she gets up at 6:00 a.m. the house is cold so she turns it on for a couple of hours prior to going to work.. then shuts it down for me so I won’t go to (you-know-where) again before I wake up.

Last week I shut the unit down completely. I turned off the power at the circuit breaker. Turned off the gas, and opened up the “A” coil box and used a powerful light and inspection mirror. The coils looked clean and I could see no problem.

I really need your help and I sincerely appreciate it. I’ve read through most of the other posts but couldn’t find a specific answer to this weird problem. Below are the particulars on this Coleman unit.

Bless you and thanks for your good help.

Coleman Mobile Home Furnace
Model No. 7680B Series 856
Serial No: 087764567
Fan Switch: F120-3 770-328
2601F134-191 L7V
Lower Limit Switch: L155-2 7680-359
20601L9-718 L7V

Evcon 4-Wire Thermostat A/C Control Box
Number: 7900A6041
GE Capacitor: Z97F9001

Outside Unit is:
Coleman/Evcon Heat Pump
Model No. BRHS036B
Serial No: 940405857

DMH110 Thermostat Wiring:

» Y wire to Y connector.
» W wire to W connector.
» RH to RH connector.
» G to G connector.
» Jumper from RH to RC is ON (Factory Setting. I took it off but it didn’t help so I put it back on).

JP1 is Closed (5 Minute Minum run/Off Time – Factory default)
JP2 is Closed (Fahrenheit Display Format)
JP3 is Open
JP4 is Open (Gas/Electric fan operation). As this is a gas furnace, the instructions said: “Choose gas if you have a gas heating system, this will allow the furnace itself to control the operation of the blower fan.”

(Note, I opened JP3 to change the swing and the furnace/blower operated better but after a few cycles it still will not shut off).

[Robert]

Hi,

The burner is remaining lit and blower running after t-stat is satisfied,,,,correct ?


If so, when it does this, reach in with needle nose pliers and disconnect both wires connected to gas valve.

IF the burner does not shut down within a minute, the gas valve is malfunctioning and needs to be replaced.



Thanks,
Robert

[Dave_L]

Robert,

Thanks for your reply. You were 100% correct in your assessment that it is the gas valve making the problem.

I had to wait till it got cold outside before I could perform your test. The wires had "Y" terminals on them and were connected by 2 screws which I had to loosen. I set the temperature at the thermostat to 73 degrees and flipped on the blower/motor wall switch. The furnace came on and when it got up to 77 degrees I pulled out the two wires (which I had loosely screwed down) and held them in my hands for 3 minutes. The furnace never stopped nor the blower/motor. I shut off the blower/motor and the furnace stopped then replaced the two wires and tightened them down properly.

This gas valve is a rectangular box about 8" long by 3 or 4" wide and is mounted to the front of the furnace by a bracket. It has two numbers on it:

7660-326

and

7680-556

It has a 1/2" pipe which enters the right end from the natural gas supply. There is another 1/2" pipe which exits the side of the gas valve and goes into the burner. The thermocouple comes out of the top and enters the burner. There is a pilot feed tube which enters the lower left end of the gas valve.

Do you have this particular gas valve available which I can purchase or can you order it? I looked at the different models on your website and I didn't see one which might fit. I'm really only concerned about the length of the 1/2" pipe from the natural gas supply being able to fit a new gas valve and the 1" to 1.5" run of 1/2" pipe coming out the side going into the burner.

Also, which thermocouple would you recommend? Would it be a good idea to replace it at the same time?

You did an excellent job in figuring out the problem and I appreciate your help.

Thank you very much!

[Robert]

Hi,

Does the burner just finally shut off on its own or do you have to shut the gas off to get it to stop ?


The burner will not light or stay burning without gas and when wires are removed, the gas valve should close, shutting off the gas and thus shutting down the burner.


With burner not shutting down, the gas valve must be remaining open on its own, in other words it is sticking open.



That's what I'm trying to determine is how it is being shut down.


Finally closing on its own or you shutting gas off to furnace ?


What is last part of furnace model number ?


7680B856 or 7680B859 ?


Thanks,
Robert

[Dave_L]

Hi Robert,

I'll try to be more specific.

1) When I remove the 2 wires from the gas valve as you requested, the gas burner IMMEDIATELY shuts down.

2) When the furnace has EVERYTHING CONNECTED NORMALLY (that means complete, normal, factory installed operation) the ONLY way to turn the burner off is to turn off the blower/motor switch. When I do this, there is NOT a cool down period but the burner immediately goes off.

3) I DO NOT turn off the gas to the unit. If I did, I would have to relight the pilot each time. I ONLY turn off the blower motor when it goes ABOVE the temperature set on the thermostat which is 73 degrees.

4) Again, to make it clear.. Sometimes I pull the Lux thermostat from the wall, press the RESET button and replace it. I then set the temperature to 73 degrees. Next, I walk to the furnace and turn on the blower switch (this operates the motor which spins the fan inside of the swirl cage called a blower). After a few minutes the burner comes on then it sends a signal to start the blower and it comes on normally and blows heated air DOWN into the ductwork.

5) After 4 or 5 PERFECT cycles the furnace will come on and STAY ON until the temperature in the house hits 90 unless I turn off the blower.

6) A cycle means: To operate correctly. The furnace comes on... heats the home to 73 degrees then turns off. That's one cycle. When the temperature inside the house cools down to about 71 degrees, the furnace comes on... heats the home to 73 degrees then shuts down. Again, after a few of these CORRECT cycles, it will come on and stay on unless I turn off the BLOWER SWITCH which immediately shuts off the burner. The pilot light always stays lit.

7) The part numbers I listed in my 1st post are EXACTLY the way they are written on the furnace by Coleman. The furnace says: Model No. 7680B Series 856. Therefore, it MUST be a 7680B856.

Thanks for your help
Dave_L

[Robert]

Hi Dave,


Ok, now we are on the trail and have the scent, LOL.


Ok, gas valve IS ok . When t-stat reaches set temp and is satisfied, sometimes the burner does not shut down and keeps going.


IF the t-stat IS no longer calling for heat, then there is a short somewhere still sending signal to gas valve.


The blower motor switch is where we need to start here to know the voltage there.


It does not have any input into burner at all, but since the burner shuts down when you shut blower off at switch, there is now a connection between them.


Go to the switch that you shut off and tell me how many spade terminals and how many wires are there.


IF only two, test each side to ground and give me the voltage reading.



Do this when furnace is running nonstop. Also, look at furnace where the t-stat wires enter side from t-stat.


Find the White t-stat wire and follow it to wherever it goes, either to a component, terminal strip or wire nutted to another wire, etc.


Test it to a good ground for 24VAC and see what it reads.



Do BOTH tests only when furnace is acting up.



Yes, that is correct model number.



Thanks,
Robert

[Dave_L]

Hi Robert,

First of all, thank you very much for your help in figuring out this mystery. I do appreciate it.

To help you better, I’ve uploaded photo’s to your email of the furnace and its wiring.

To answer your questions:

1) I couldn’t find the short.

2) The switch which operates the blower is a simple light switch type with two terminals on the right side and a ground terminal on the left bottom (see photo’s).

a) When the blower and furnace are off and the switch is in the OFF position my Fluke reads 0.0 VAC on the top right terminal and 120 VAC on the bottom right terminal.

b) When the switch is in the ON position and the blower is running and the furnace is operating NORMALLY, the top right terminal reads 118.5 VAC and the bottom right terminal reads 118.5 VAC.

c) When the switch is in the ON position and the blower is running and the furnace HAS NOT SHUT OFF (temperature is at 77 degrees while thermostat is calling for 73 degrees), the top right terminal and the bottom right terminal both read 118.5 VAC.

Note: When the blower switch is in the ON position and the blower is NOT running there is a CONSTANT hum. Sounds like it coming from the motor. I only see a capacitor on top of the blower swirl cage (capacitors don’t make noise do they?) the motor, and an auxiliary limit switch. Is this normal? Sounds like the motor has a small amount of current flowing to it but not enough to make it start.

3) I tested the white thermostat wire at the gas valve. It also goes up to the auxiliary limit switch on the left side of the blower. I was afraid to test it at the limit switch next to the blower because the blower is running and it’s hot up there (see photo). Therefore, I only tested it down at the gas valve. Here are my results:

a) When blower and furnace are NOT RUNNING white thermostat wire to a ground reads 0.0 VAC

b) When blower and furnace are NOT RUNNING, touching both the black and white terminals simultaneously read 0.0 VAC.

c) When blower and furnace ARE RUNNING white thermostat wire to a ground reads 0.0 VAC.

d) When blower and furnace ARE RUNNING, touching both the black and white terminals simultaneously read 25.0 VAC.

Thanks again for your great help.

Dave_L
P.S. If you didn't get the photo's via your email let me know and I'll do it again.

[Robert]

Hi Dave,

Haven't seen any pics yet. Do these tests please.


Furnace operating NORMALLY:


White t-stat wire to ground at gas valve = ?

Black wire to ground at gas valve = ?



Where does black wire come from ? Transformer maybe ?


Send pics to [email protected]



Thanks,
Robert

[Dave_L]

Hi Robert,

I don't know what is happening... I mean, on the 23rd I THOUGHT I sent an email to [email protected]

I sent you 9 photo's of my furnace and tried to answer your questions in the email below (copy).

On the 22nd of this month I also sent the 9 photo's to [email protected].

Sorry for the confusion, I'll try again. Below is a copy of the email I sent you on the 24th.

Dave
----------------------------------------------

When the furnace is operating NORMALLY the WHITE thermostat wire to ground at the gas valve reads 0.0 VAC.

When the furnace is operating NORMALLY the BLACK thermostat wire to ground at the gas valve reads 0.0 VAC.

The BLACK wire from the Gas Valve goes up to a Wire Nut which connects a Blue Wire which goes to the transformer and a black wire going to a Molex connector. Please see attached photo “Front_of_Furnace_01.jpg.”

Thanks!

Dave

[Robert]

Sent replies to new gmail address.



Thanks,
Robert

[Robert]

Sent replies to new gmail address.



Thanks,
Robert

[Dave_L]

Hi Robert,

The wife and some friends took me to Los Wages (Las Vegas) for my birthday. Just had time to run tests this morning about 4:00 a.m. Sorry for not getting back to you sooner.

As per your instructions:

When the furnace is operating NORMALLY (which means the blower is operating and the burner is functioning), here are the results:

1) White thermostat wire to ground at gas valve = 0.0 VAC

This is performed by setting my multimeter to the ACV setting and touching the black (COMMON) lead on the multimeter to the BLACK Gas Valve wire and grounding the multimeter’s red lead to the frame. I can also touch the MM’ Black (COMMON) lead to the WHITE Gas Valve wire and the Red MM lead to ground at the frame and I get 0.0 VAC.

I also reversed the test by touching the multimeter’s Red lead to the BLACK Gas Valve wire and grounding the Black (COMMON) test lead. Also touching the Red MM lead to the White Gas Valve wire. Any way I do it I still get a reading of 0.0 VAC.

2) Black Gas Valve wire to ground at gas valve =? It has 0.0 VAC (see above tests).

3) Where does the black wire come from?

In the photo titled Front_of_Furnace-01.jpg that I sent to you please notice that the black wire coming from the Gas Valve goes up to a box where it is tied in with a red Wire Nut to 2 other wires. At the Wire Nut we have 3 wires connected. One wire is blue which goes to the transformer. The other wire is black coming from the Gas Valve. The 3rd wire is black and it enters the Molex connector on the top right side of the box.

Here are further tests to help (or confuse us more).

It seems like the WALL SWITCH wants to control everything in CONJUNCTION with the thermostat. And, the WALL SWITCH will override the thermostat. Here is what I mean:

1) I turn Wall Switch to on. Thermostat is set to 73 degrees. House temperature is 69 degrees. AS SOON AS I turn on the Wall Switch I hear the blower motor energized but not turning. The flame at the burner comes on but the Blower is NOT on… yet.

When I check the voltage at the GAS VALVE by placing the Black (Common) MM lead to the Black wire and the Red MM lead to the White wire I get a reading of 25.5 VAC. I can switch the MM leads the other way (Red lead to black gas valve wire and Black lead to white gas valve wire) and although the polarity is switched, I still get a reading of 25.5 VAC.

2) At this time (before the blower has come on and with the Gas Valve getting 25.5 VAC), when I test the Black Gas Valve wire to ground I get 0.0 VAC. When I test the White Gas Valve wire to ground I get 0.0 VAC.

It seems the thermostat is calling for power and the Gas Valve is operating normally. In a about 30 seconds to 1 minute the blower comes on and the furnace will operate normally.

3) When the Blower and Burner are both operating NORMALLY and the Gas Valve is letting gas pass through to the burner, I get a reading of 25.1 to 25.3 VAC at the Gas Valve.

4) When the house temperature reaches 73 degrees EVERYTHING SHUTS DOWN NORMALLY. I test the Gas Valve and there is 0.0 VAC (no voltage) at it and it is shut off. I guess it listened to the thermostat. When I check for ground at the Gas Valve (same tests as described above) there is 0.0 VAC.

5) As soon as the house temperature drops to 71 degrees the burner lights and the blower comes on. I guess it got a signal from the thermostat to turn on.

6) But then it won’t turn off! Or, sometimes it’s a 50/50 proposition. What I mean is that sometimes it will go on and off normally like this for 6 to 8 cycles then all of a sudden it will stay on.

When the furnace is RUNNING AWAY WITH ITSELF (e.g. the Thermostat is set to 73 degrees and the house temperature is 77 degrees and the furnace WON’T shut off) I get the following:

1) Gas Valve reads 25.3 VAC. Touching the MM leads to ground at the frame gives 0.0 VAC.

2) Turning OFF the Wall Switch (NOT thermostat) the blower immediately stops and the burner immediately stops with only the pilot light lit. When I test for voltage at the Gas Valve (by touching the Black COMMON MM lead to the black wire and the Red MM lead to the white wire) I get 0.0 VAC. A test for ground is the SAME as above (0.0 VAC).

3) When I wait 5 minutes and turn ON the Wall Switch.. both the burner and blower IMMEDIATELY come on. I get a reading of 25.3 VAC at the Gas Valve.

If I don’t turn the Wall Switch OFF again (shutting both the blower and burner off), the house will heat to 100 degrees if I let it.

I hope this helps. It’s warming up here in SoCal at night (I live in San Diego county) and soon I’ll just turn the gas off till next winter. It’s driving me crazy as I can’t figure it out. You’re a much better man than me if you can understand it.

Thanks again for your help.
Dave_L

[Dave_L]

Hi Robert,

I have another question I'm hoping you can help with.

When the Wall Switch is ON and the furnace is operating Normally (that means the furnace is listening to the thermostat and it is going on and off at regular intervals to keep the proper temperature) the blower motor continues to HUM when the burner is off.

The blower motor works fine when it receives a signal to come on. I think that means the capacator is working... but when the motor shuts down why does it continue to hum?

The only way I can stop the humming is to turn off the wall switch.. and doing that turns off the furnace completely.

Thank you very much for your good help.

Dave_L

[Robert]

Hi Dave,


Hum is most likely from the transformer and echoing somewhat.




Thanks,
Robert