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justplumducky 11-21-2012 08:34 AM

MicroAmps-DC Testing for Flame Rectification
Ok, I've edited this post (eliminated all but one of the questions), because I discovered a really good youtube video by Goodman, or about Goodman - or maybe it was just a Goodman ad in the opening of the video? Anyway, it answered my previously posted questions here, except for the remaining one below. The video: Title of the video:Grounding & Flame Rectification.wmv, by instructor12b

The following paragraph (in italic) is excerpted from Fieldpiece's Tech Articles, located at (the excerpt is found toward the bottom of the page, below the drawings that illustrate how to hookup the fieldpiece multimeter.

"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."

How do you accomplish this test since the end of the flame sensing rod and the end of the burner tube are inside the heat exchanger ?.....

Thanks much for any help.

JJboy 11-21-2012 10:28 AM

Just put the meter in series with flame sensor. The meter needs to be in micro amps . You soils get around 2-4 micro amps not less than 2

Sent from my iPhone using DIY Forum

justplumducky 11-21-2012 01:15 PM

JJBoy, thx for your reply. I found a great video on youtube that answered all but one of my questions, so I edited my post except for one remaining question. I know how to hook up and do the uA's test, but would like to know how to check this voltage also, if you can help... Please see original post.

hvac5646 11-21-2012 02:47 PM

check from sensor lead to ground. that will give the answer you want.

justplumducky 11-21-2012 03:52 PM

Thank you hvac5646.

Houston204 11-21-2012 08:36 PM

It's much easier with an 80% furnace.

If I were to take the lead off of the flame sensor and connect it to the furnace body, then switch over to volts I could measure voltage.

justplumducky 11-21-2012 09:00 PM

Houston~, you are the man, Man! :thumbsup:

justplumducky 11-23-2012 07:09 PM

I separated the quick disconnects on the flame sensor lead, inserted meter probes and on the first try, the uA's-dc was all over the place... all under 1.0 uA. On the next try, it was still all over the place, but as high as 1.9 this time.

Voltage on the first and only try/measurement was 104 vac, measured on the lead coming from the control board, to cabinet (after I separated the quick disconnects).

This ignition failure (a repeating one flash & pause... from the board's green led light) happened at the same time as the igniter failure. The igniter had a small crack and hot spot (minimal hot spot, of course) around the crack. Possible that this igniter failure fried the control board (arcing at the crack?), since this ignition failure condition (flame going out soon after coming on) surfaced immediately after the igniter failure?

I installed new igniter and flame came on first time, but went out 5 seconds later, and continues to do so. There is no delay between flame going out and any click of the gas valve, or relay on the board - it's simultaneous. If there's any click from a relay on the board (when flame goes out), it's drowned out by a larger click/noise coming from the gas valve, I believe ( the "flame going off" click sounds more like the louder click/noise of the gas valve when it first lights up).

I took this flame sensor to a former tech at the supply house counter, and he said there was no need to install a new one. He ohms checked it end-to-end (very close to zero ohms), and from sensor rod to its mounting bracket (no shorting thru the insulator). . Insulator was not cracked or discolored. The rod itself was pretty clean before and after I scrubbed it up with steel wool. Still no help.

I'm confident that this flame is full (have seen a couple of weak flames before) and sufficiently enveloping the flame sensor.

The end of burner tube and its projecting tab are both clean and in very good condition.

Combustion air motor remained working as good as it was on startup sequence.

Is it possible this flame sensor still could be bad, despite its good appearance and ohms testing (and no shorting to mounting bracket)?

Ground connection from control board multi-pin connector to cabinet is good - took it off scrubbed it a bit to be sure.

If I haven't mentioned it yet, this is a mobile home furnace (ignore the "A" and "B" labeling in the pic):'s%20furnace%20pic.jpg

beenthere 11-24-2012 07:30 AM

Whats the manifold pressure.

justplumducky 11-24-2012 07:42 AM

Beenthere, have never done that yet - don't have a water column guage, if that's the correct guage.

You're thinking lack of pressure is possibly shutting down the gas valve, distorting the flame, or is responsible for the fluctuating uA's test, or something of that nature?

Thx for jumping in here...

beenthere 11-24-2012 07:55 AM

Bad grounds and or low gas pressure can cause low uA readings.

justplumducky 11-24-2012 08:32 AM

I just talked to the homeowner and had her ask Duke Energy if they would come out and check the gas pressure - she said they'd be out within 3 hours.

beenthere 11-24-2012 08:54 AM

They may or may not check the manifold pressue. they may only check the pressure that their meter/reg is supplying.

justplumducky 11-24-2012 01:03 PM

Ok - Not sure if he would have checked it or not (I wasn't there), but he said there was no place to hook up on the gas valve - whatever that means.

If it's of any help at all, this gas valve is not as big (not as bulky or box-y lookin') than other gas valves I've seen on older furnaces in an older mobile home park. Both, the ones on the standing pilot-type furnaces and the ones on the electronic ignition-type furnaces, are bigger and boxier looking than this one. Looking directly at this one, while kneeling in front of the furnace (gas valve installed), it does have a bit of a boxy "profile" (looking at it straight on), but is considerably shallow in depth (horizontal depth, front to back as I look at it while kneeled in front of it) by comparison (to the others just mentioned).

The only other thing he said was that there was no problem with the gas line pressure feeding the gas stove normally (just eye-balling its performance).

After minimal google-ing, manifold pressure is taken near the outlet of the gas valve? Anyway, homeowner says she's going to have to wait a bit before calling out an hvac pro, and said in the mean time if I could come up with anything else, she would welcome me back for additional fix-it attempts.

I do know what you mean, though, about Duke maybe checking pressure (in the house), but maybe not. Occasionally, I call them out for an electrical problem - some will come in the house and go above & beyond... some won't.

Thx much for your help.

justplumducky 11-27-2012 05:18 PM

I was talking to a supply house today and the rep checked this furnace model number against an ongoing recall list (Coleman). This furnace is on the list... for a possible overheat condition, strong enough to reach the drywall of the furnace closet. No injuries reported yet, but several property damage claims so far.

I then went to the recall website (address posted below), discovered they're providing free inspections and a preventative correction-type fix for the problem, via local contractors. Don't know all the details, but one of the fix's mentioned was installation of a shield to contain any overheating. Another one of the fix's, obviously, is a new heat exchanger if warranted.

I explained about the control board's flashing green light/diagnosis of ignition failure - contractor said he could take care of the recall fix as well as the ignition failure, without charge to the homeowner. Contractor is coming out tomorrow for inspection and confirmation of the the model number. Probably return the following day for the recall fix and repair, he said. I'll post here again when contractor is finished.

Model numbers of affected furnaces:

The Coleman®, Coleman® Evcon, and Red T furnaces are used in manufactured housing. On these furnaces the name plate is found mounted on the left inside surface behind the lower panel. The furnaces are a silver color with white access panels.
  • DGAT070BDD
  • DGAT070BDE
  • DGAT070BDF
  • DGAT075BDD
  • DGAT075BDE
  • DGAT075BDF
  • DGAM075BDD
  • DGAM075BDE
  • DGAM075BDF
  • DLAS075BDD
  • DLAS075BDE
  • DLAS075BDF
These model numbers, and additional information, are found at this website address:

The Recall Program Support Center at (888) 665-4640.

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