Erratic Trane gas pack operation with seemingly odd gas valve voltage

justaworkerman

I am new to your forum and have read many of its discussions but haven't seen this particular scenario. Please pardon me if I missed it. I offer a large "thank you" to those of you who have the appropriate knowledge and are willing to take the time to share it!

I have a Trane YCX030F1M0AD gas pack, 1994 mfg date, that’s been regularly maintained (until this latest issue) by the same reputable HVAC company that installed it.
They replaced the original control board, CNT1634, years ago with a CNT03457 & 06839 adapter kit and the unit has run fine until recently.



For this picture, I removed the adapter kit wiring harness and you can see the white adapter board peeking from behind the control board.

I’m trying to hold off on replacing this gas pack until I get further along on a remodel project which entails new plenum work and exterior wall modifications where the unit is located. A high efficiency unit will then replace this old workhorse. I like to fix things just for the fun of it but haven’t achieved the “fun” on this one yet.

The unit was heating properly earlier this season, and still does occasionally, but lately has often failed unless I mess with it.

The control board LED flashes slow constantly – before, during and after any heating cycles or attempts. I’ve yet to see it flash any other way.

Upon the thermostat calling for heat, the inducer runs properly (30 sec +) before the igniter starts firing. Sometimes the burners fire right up, sometimes a few cycles are required before they light and then sometimes nothing happens until lockout. As you can imagine, my disconnect has been getting many workouts due to all of my attempts at diagnosing and repair!

Initially, I thought the gas valve (24v and quite aged) was bad. However, upon checking the voltage at the valve, I was surprised to see that it initially climbs to 190VAC (+/-) before settling down to 26 VAC (+/-). Sometimes the unit ignites properly and runs its full cycle, and sometimes the valve voltage drops to 0 and nothing happens. The voltage spiking considerably above 24VAC seems abnormal to me.




Recently, after I had merely “rattled” some wires, the unit started and ran 30 minutes, raising the temperature 4°F per the thermostat without a hitch for my 1900SF main floor. It then cycled correctly for about 3 heating cycles before going into lockout.

The erratic behavior has me totally confused! Maybe I need a bigger hammer! LOL!

The burner chamber shows signs of moisture (note the valve's oxidated housing), apparently due to the exhaust chimney never having had a rain cap. I've suspected terminal corrosion and replaced those on the valve along with several others but certainly not all.

My main question - is the gas valve voltage behavior normal? Beyond that, control board and wiring issues seem suspect so any suggestions or comments on what's going on with my unit would be welcome. Thank you and please pardon the length of my post!

Jamie Hall

Those gas valve voltages are anything but normal -- though I suspect (since the gas valve has at least sort of survived) that the high voltage may be somewhat suspect.

If you jiggled some wires and the thing ran well enough for a bit, I think you have the place to start looking: bad or corroded connections anywhere along the way. Don't ignore anything -- every single connection is suspect until proven clean and secure.

mattmia2

I'd check the coil on the gas valve with an ohmmeter, i would guess it should be somewhere in the tens of ohms to maybe 10 k ohms or so depending on how it is made. If that isn't very high resistance I would look for a bad connection, bad ground, and especially some melted or chaffed wires shorting to the ignition circuit.

justaworkerman

Texting my thoughts based upon your comments which I appreciate greatly.
As the high voltage at the gas valve is not normal, somewhere along the gas valve leads the higher voltage is getting in. I'll backtrack along those leads and see what I find.
Though I've only done a rough overlook at the wiring without opening insulated bundles, I've seen no sign of bad/overheated insulation. I'll certainly have to open those insulated wraps in order to get a close look at all the relevant wires to insure their integrity.
One location that I feel warrants close inspection is the gas valve relay which I assume is on or near the board. Being the origin of the gas valve voltage makes it the earliest location where higher voltage could enter. However, given that I'm reading 190+VAC, the gas valve leads must have a 220VAC source which I don't think exists on the board. Is that a correct assumption? To get such a high voltage, mustn't both legs have to be bleeding into the gas valve leads?
If the wiring inspection turns up nothing, I'll probably work from the valve and isolate its leads out of the bundles and away from any line voltage all the way back to the board.
Please feel free to add comments or disagree with my thinking!

Jamie Hall

The reason I suspect that that high voltage may be an artifact is that I doubt that anywhere in that boiler or control there is anything higher than 120 -- and it's unlikely that there is anything which would normally act as an autotransformer or other device spiking the voltage like that.

ratio

How confident are you of that 160 volts? Is your meter something reliable, or a POS? I'm suspicious. I don't think a 24 vac gas valve would survive 160 volts; I'd certainly condemn the valve after such an event. Maybe you got onto the wires to the ignitor? That voltage is more in line with what I'd expect to see going to an ignitor. Perhaps your probes were on a spot of corrosion & your meter auto-ranged to a millivolt scale? (That has happened to me!)

EdTheHeaterMan

There should be a wiring diagram for your appliance located on one of the control panels. If you could provide a photograph of that diagram, that would be helpful. Let us know if it is original to the appliance or if it is the revised control and adaptor version.

EdTheHeaterMan

I believe this is the control you now have. https://www.emerson.com/documents/commercial-residential/install-manual-50n02a-820pdf-en-1566832.pdf
This can operate either a 2 stage gas valve or a single stage gas valve. The 2 stage operation has provisions for two speed of inducer fan and 2 speed of Blower fan.

Are you using this on a 2 stage unit or a single stage unit? Also for all concerned, this is a package unit that operates on 230-240VAC. The control transformer has a 230V primary and 24V secondary

justaworkerman

My existing control board is a replacement and required using an adapter kit. Those part numbers are listed in my initial post. The unit is a single stage unit.
The UPC on my existing control board cross references to numbers beginning the same as your link but differing after the first 5 digits. Not sure what the different numbers indicate but 50N02-495-91 is what I find. I do not have a schematic for the existing board but have been looking online.
Pictures of the original board schematics are attached even though I'm yet unsure if they're identical to the replacement board's.




As for my meter possibly being a POS and tangling wires together to get a reading, I'm taking all that as humor. Reminds me of being on jobs and swapping humorous jabs with coworkers and subs. The meter's no Fluke but it's served me quite well for several decades and I've never been known to skimp on tools. Any wiring besides the valve leads is several inches away and not interwoven with them. I tested the voltage at several different times and after replacing the terminals with minor deviation in the bizarre (to me) readings. I do appreciate the interest shown.

I'll continue to search for the existing boards schematic and update you if I'm successful.

Weather permitting, I intend to go through the wiring thoroughly tomorrow.

mattmia2

i've seen weird things happen with DMMs when the contacts on the mode switch get dirty, but if the coil in the gas valve is open or the connection to the coil is open, you could read a voltage induced in those open wires especially from the spark ignition, but if there were any load on it that voltage would drop to 0

justaworkerman

With help from you guys I no longer have a furnace problem.

I believe "mattmia2" nailed it as there was no odd voltage present with the valve terminals disconnected.

Thumping the valve would sometimes prompt the system to operate properly for only a cycle or two so I replaced it and everything's now lovely. Luckily, a shop 40 miles away had a used and exact match to the now obsolete valve.

Couldn't resist opening the valve, which I believe is the '94 original, to see inside. I was amazed as there was absolutely no oxidation whatsoever and ported surfaces still had their mirror finish.

Thank you for your patience and comments!

mattmia2

The new valve needs to be set up by someone who knows how to do combustion analysis, it has to be adjusted for the specific appliance it is installed in.