10 replies to this topic

[Budget Appliance Repair]

Coleman/Evcon forced-air furnace
Mod# DLAS056BDC
Ser# 941258626

Problem was flame wouldn't light, replaced bad ignitor and found another problem.

Original ignition control module: RobertShaw HS780/34NR-306A

1) Draft Inducer comes on
2) 34 sec ignitor warm-up
3) Gas valve receives 24 VAC and flame ignites
4) 6 second ignition proof starts

PROBLEM:
5) flame goes out, it doesn't see the flame

Cleaned flame sensor rod and burner, checked all wiring polarity - all OK

I have never taken the Micro-Amp readings from a flame sensor before so have no idea what range they should be in so did a bunch of research online and found good info on how to do and found that my meter is able to read Micro-Amps.

Found the attached PDF file online that explained what I need to know and gave some general specs.

If I check voltage on the flame sensor I get a good solid 119 Volts from flame sense terminal to ground and when checking for uA DC I get a good reading of 3.0 - 3.2uA DC -- I can't find any info on the RobertShaw controls as to what range of uA DC the flame sense circuit is looking for.

The general info I have found says these controls can run anything from .10 uA - 10 uA

I did find on one spec sheet for a RobertShaw control similar but not the exact same part of 1.5 uA which the 3.0 uA that I'm getting is well within the limits.

From everything I've done, (including running a ground wire directly from the module to the burner assembly), I conclude that the ignition modules flame sense circuit has failed and plan on replacing it with a S8910U1000 Honeywell ignition control module.

Would all the above sound like the ignition module is definitely my problem and does anyone have the actual specs of what the original control module uA DC reading should be.

Thanks for any help you can provide,

Attached Files

•  Flame Rectification Info.pdf   29K   9 downloads

[Dan Webster]

My friend a tv repairman that works at the wings of life as a volunteer told me he removed the cover on these modules and has had quite a bit of sucess in soldering a loose connection on the pilot flame sensor ground terminal. The solder connection comes loose. Remove that cover on this puppy and look for a loose solder connection:

Attached Files

•  robertshaw ig module.jpg   8.9K   8 downloads

Edited by applianceman18007260692, 02 January 2012 - 12:46 PM.

[Budget Appliance Repair]

I don't know why I didn't think to open the box and look for anything obvious like burnt solder connections, (I usally do that on range and other controler boards).

I've ordered the new ignition module and will install it but will take the old one apart and see what I find.

[Shootist]

Don't forget to check/clean the flame sensor and make sure the furnace is properly grounded.

[Budget Appliance Repair]

Thanks Shootist, check/clean the flame sensor was my first action to correct, it didn't change anything.

On a follow-up visit after some research on how to check the uA DC current of the flame sensor I also cleaned the burner itself and checked all grounding. All grounding is OK and the cleaning of the burner nozzle didn't change anything, (I had never had to or thought of the burner itself needing to be cleaned but it makes sense because that is the path of ground through the flame and to the burner as ground).

The new ignition module should be arriving on Friday so will know soon enough if that is my problem.

Edited by Budget Appliance Repair, 05 January 2012 - 06:44 AM.

[Budget Appliance Repair]

Installed new Honeywell S8910U1000 ignition module and all works well. Took the old module apart and no loose/cold solder connections, nothing obvious that was causing problem so must be in the flame sense circuitry.

Just for my knowledge I took the microamp reading of the flame sensor on the new module and it never showed a steady reading, (it kept jumping around from a low of .9 uA to 4.2 uA, generally staying in the 2 - 3 uA range but never a steady constant reading).

The old module that wasn't working showed a good constand 3 - 3.2 uA thru the complete six seconds of flame proof.

Can anyone tell me if an unstable reading is normal, (caused by the movement of the flame changing the characteristics of the flame envelope that the sensor is imersed in)?

[RegUS_PatOff]

sorry, I don't know,
but, are there any uA readings with either Module when there is no Flame ?

[Budget Appliance Repair]

The original bad module showed a solid and steady 0.00 uA when no flame was present.

The new module went into a negitive reading of around - 0.9 uA when no flame was present and module was energized going thru prepurge and ignitor warm-up.

[RegUS_PatOff]

Flame Rectification
some info:

... The Controller sends about 90v AC to the Flame Sensor .. check with the Controller mfg.

As for flame rod position, keep in mindit is not a thermocouple. Heat does not generate voltage for the signal.
If the flame rod is too close to the ground within the cone of the flame, you will actually get a weaker signal.
To set up the strongest potential, you actually want the flame rod out on the edge of the main flame cone
where the most positive ions are, teasing them into lining up so the signal can hop-scotch across to the ground.
Too close and it doesn't seem to draw in enough ions for stepping stones.
When cleaning the flame rod, don't forget the ground connections.

Another thing to watch out for is the position of the sensor in the flame.
I know of some techs who will bend the sensor to get a higher reading rather than clean the crossovers on the burners or the burner itself.
When I find a bent one I usually replace it and start from square one.
Knowing the history of past service calls sometimes reveals the need for a sensor or hsi replacement.
Carefull, if you find a bent flame sensor on a Lennox 80MGF or G24 furnace,
someone probably bent it as called for in a service bulletin.
Lennox had some problems with their choice of flame sensor position on those furnaces about 10 years ago.

Per Honeywell and others, there is a correct ratio of areas exposed to the flame.
The ratio is 5:1. That is, the area of burner or pilot hood (etc) should be 5 time that of the rod.
Therefore proper flame rod orientation is extremely important.
Early in the residential sector flame rods were often misaligned.
Now days a technician really has to work hard at incorrectly installing a flame sensor.
Most all are indexed by a tab or slot that correctly aligns the rod.
One thought on cleaning the rod, do not use anything that will leave groves on the rod.
These groves will only increase the rate of fouling the sensor rod.
For this reason sand cloth, emory cloth, and some steel wools are not reccomended.
http://www.amgair.co...on/Flamerec.pdf
http://www.amgair.co...ication0001.PDF
http://www.achrnews....fication-system
http://www.achrnews....on-service-tips
http://www.fieldpiec...e-rectification
http://www.ien.com/a...tification/7534

[Budget Appliance Repair]

This is something I wasn't sure about and have some questions regarding

.. The Controller sends about 90v AC to the Flame Sensor .. check with the Controller mfg.

And specifically this complete step:

Make sure there is AC voltage between the flame diode and the base of the flame. Measure AC voltage from the flame sensing rod to the base of the flame. The value varies by model (in neighborhood of 90V), but the important thing is to ensure that there is voltage present. If there is no voltage, check to see that the wires from the module to the flame sensing rod are correctly connected or if the flame sensing rod or the wire connected to it is grounded.

I measured from the sensor terminal on the ignition module to the ground of the furnace and got a reading of the full 119 VAC.
From the diagnostic statement above it sounds like you should actually be taking the voltage reading from the end of the flame sensor rod to the base of the flame, (meter probe tip in flame, NOT to the actual burner head itself)? Is this how you would read it to mean?

If that is the case, then no way to really test this Coleman unit in that manner since the burner is inside a sealed chamber with just a little 1" site glass window to see the burner and ignitor operation.

[Daveb1972]

I know the manufacturers say not to clean the rod with abrasives, BUT I'm sorry to say if you don't, they don't get clean. There is enough junk in the natural gas that coats that flame rod (sulfates, and hi temp leas to oxidation) that the rod becomes an insulator. Cleaning the flame sensor rod is part of normal maintenance, and often the source of midnight no heat calls (I work as the on-call tech for a local HVAC company). There isn't any magic in the rod, just a probe that uses the flame to act as a diode and conduct to ground. When the reading is stable and the rod has been cleaned with emery cloth, AND it wont hold flame, replace the module. Don't try to test HSI's for flame rectification (some modules will let you use the HSI as the flame rod) because you'll cook your meter. Newer Lennox units will give erroneous readings if you try to take flame sense readings directly (units made after 2010 with the 8 segment LED display on the board)
For your own checks, test flame sensor current before cleaning, then after. You'll easily see the benefit to cleaning the rod. The rod is mostly stainless steel, which isn't really stainless at high temp. The oxide layer that forms has to be broken mechanically,(with abrasive) but not aggressively. (My day job is in stress corrosion cracking of nuclear grade stainless steels, in high temps and in aqueous environments)