Blew another damn transformer on my Trane XB80

[clare]

Except the logic board is only on the secondary and does not provide power to the primary.

He DOES need to check the primary voltage, and also check for DC.

[clare]

>>>

Connect the transformer that tested OK on the bench to the furnace unit and see what happens - - - - - .

[trader4]

He has checked the primary voltage and reported that it's 120v. The DC component on the primary is an interesting possibility, but where that would be coming from isn't clear. However he also reported that with the thing hooked up to the new logic board, which has a 3A 24V fuse on it, that fuse blew. DC on the primary wouldn't be blowing the

3A secondary fuse. One thing is for sure, with a 3A secondary fuse he should go up to at least a 3/4 amp, if not 1 amp fuse on the primary for testing so that he can at least take some measurements.

[clare]

Connect the wires to the transformer, without the transformer screwed down (or contacting the "ground" of the furnace), and check for voltage between the transformer frame and the furnace frame (ground). There should be less than a volt (ideally, nothing, but a digital meter can read phantom voltage). If there is no voltage, the PRIMARY is not grounding. Measure voltage from each secondary wire to the transformer case. There should be NONE. (Again, a small "phantom" may appear, but touching both leads with your fingers at the same time should eliminate that)

When testing the primary voltage, you ARE closing the lockout switch, right?? And measuring from black to white?

How about measuring from black to ground, and white to ground, with and without the lockout switch closed??

[MarkK]

To the op.

here is another troubleshooting tip..

Since you have wired a fuse in series with the pri input and they blow, you can try the next thing,

instead of the fuse, use an ordinary incendescent light bulb (Not a CFL) . Wire a 40W light bulb in series instead of the fuse. (or if you prefer, wire it in series with the fuse too.

If there is a problem, the bulb will light bright, if all is well it may glow a bit.

With this in place, you can carefuly try all modes and move things etc, anytime the bulb goes bright, you have a problem.

Wire it carfully so that there isno chance for the leads to short to anything else.

You may need to use a 60 Watt or more. Note that no matter what else happens, no more then 1/2 Amp can flow through a 60 Watt bulb long term. And even if there is a fault, 1/2 Amp will not fry anything in less than a few minutes.

Mark

This is

[Steve Turner]

AHA! - progress. Yes, I did do that, but what I did not do was to also remove the compressor contactor wires when I hooked them back up. One of those (a white wire) connects to the "C" terminal on the thermostat block, the other (a red wire) connects to the "Y" terminal (along with the yellow wire leading to the thermostat). I disconnected those white and red wires (leaving the others from the thermostat connected) and BINGO; the board comes up, the relays pick and the blower motor comes on! Obviously there is a problem in the wiring leading to the compressor, but now at least I know where the hell the problem IS. Thanks for all the help!

[trader4]

On Apr 19, 7:40=A0pm, [e-mail] wrote: .com/webcache/un/furnaces%20%28furn%29/product/22-166...

True of many common thermostats, but not all thermotats. My Honeywell VisionPro, for example, has a backlight which you can power all the time by using a common wire from the furnace. Bad thermostat or short in the wire and you have an overload on the 24V transformer.

[Steve Turner]

And.... BINGO. I followed the two wires from the thermostat block which clearly led to the outside compressor unit. However, when I popped the service cover on the outside unit, I saw no evidence of those two wires. It was only when I peeked with a flashlight that I could see they were hidden by a secondary access panel (which I've yet to remove), but I *could* see they were connected to larger yellow and blue wires that led directly to the compressor relay. I concluded that the 24V signal was not able to pick the relay for some reason, but actuating it by hand (with a screwdriver, actually) proved that the contactor moves freely (no binding), and the unit comes on when the relay is manually picked. Hmm. I started moving and jiggling those wires and I could tell they had fallen down into the unit and were probably resting on something. When I pulled them upwards and peeked some more, I could clearly see that the larger yellow and blue wires were completely melted through! It looks like they had come to rest on some portion of the condenser coils. It's dark now so I can't do much to repair them, but hopefully with the light of day and some time I'll be back in business!

[Vic Smith]

I fully expect a fairy tale happy ending here. And with babies- twin control boards! You've brought AHR together as one big happy family. Thanks!

--Vic

[David Nebenzahl]

On 4/19/2011 6:41 PM Steve Turner spake thus:

[...]

And thus endeth our saga. Pretty exciting conclusion there. Seriously; I was starting to wonder if you'd ever get this one figured out. This post ought to be put on a plaque somewhere. Maybe print it up like a certificate and post it next to your furnace?

So much for those wild-ass guesses in the thread ("could be a magnet causing a saturated transformer core", etc., etc.)

[clare]

Missed th 1/4 amp - yes, I'd go to 1/2, or possibly 3/4.

Thetransformer is 40va, from previous posts. That's 1/3 amp at full load. It SHOULD hold on 1/4 amp with no load, but he SHOULD fuse it at

1/2 amp.

[Steve Turner]

I was certainly surprised and thankful for all the participation. Glad I could provide some entertainment. :-)

Yeah, I think I mentioned elsewhere that in a previous home I also had Trane A/C units with White Rodgers control boards and one of them went bad. It certainly won't hurt to have an extra on hand.

[Steve Turner]

Yeah, no kidding! A simple damn problem that was one *hell* of a bugger to find.

Well I tried to stay on some sort of course that at least made sense to me, but I certainly appreciate all the input no matter how far off base some of it turned out to be. :-)

[trader4]

orking system;

apply 115V to

current on the

not obvious to

nt I'm calling

And obviously it actually won't hold at 1/4 amp at even ZERO load. That caused most of the confusion near the end of the story, where the transformer blew the fuse repeatedly at the furnace without any load at all attached to it. I thought from the beginning it was a bad idea to use fast-acting fuses close to the limit, at least for testing purposes, to be able to get it running for current measurements.

[Steve Turner]

The culprits:

Not sure how it happened, but those two wires are completely fused together. I cut away the nasty business, reconnected them to the signal wires from the logic board and positioned them so they can't ever argue with each other again. The air conditioner is happily plodding along, and everyone lived happily ever after.

[Steve Turner]

Yes you are correct; it did cause some confusion, and a bit of tail chasing on my part. I would like to have tried the slow-blow fuses, but I couldn't *find* any rated anything less than 2 or 3 amps, and I tried two different Home Depots, two different Radio Shacks, and a Walmart.

[clare]

Ends up it was not "without a load" - just with the BOARD disconnected. He still had wires connected - that went to the AC unit, where the wires were shorted.

[N8N]

What series 260? Mine uses fuses that are similar to and only slightly cheaper than the Fluke fuses. I ended up ordering from McMaster-Carr after x-ref'ing part nos. and now I have a couple extras of both but they are Not Cheap.

What I really hated to see was when I bought my 260 (used) the fuses in it were completely the wrong size... yeah, that's a great idea, let's fry an expensive meter next time you have a brain fart because you're too damn cheap to buy the right fuse. (or kill yourself, potentially, although that's less likely.)

nate

[trader4]

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Wrong again. He made it clear he had the transformer installed in the furnace with only the primary side connected and the secondary side open and it still was blowing fuses. That apparently was because he was using the 1/4 amp fast acting fuses. Your statement makes no sense because the transformer supplies 24V to the logic board, which in turn supplies 24V to the shorted AC contactor circuit when it's time to turn the AC condenser unit on.

He clearly stated:

"One thing I forgot to do was verify that I was getting 24V on the output side of the transformer when 115V was applied to the input side. I just came back from testing for that, and now I'm confused again. I removed both wires from the output side and connected my meter to check for voltage, closed the interlock switch, and "piff", the 1/4- amp fuse (my last one) blew! "

You really need to follow the thread more carefully.

[clare]

ever since.

working system;

to protect a

installing

installed a 1/4-amp

1.5-amp fuse

myself how

apply 115V to

current on the

thermostat,

not obvious to

disconnected the

1/4-amp fuse on

board, and

approach to

point I'm calling

OK. Why did it work on the bench????? He was so vague with what he was doing/checking that NOTHING really made sense.

Main thing is, he found the problem and FIXED it without blowing more smoke.

Half the responses to the problem WERE blowing smoke.