Low Voltage Fuse keeps popping

[strawewe]

My first time trying to troubleshoot an HVAC unit, and I'm stuck. (Carrier 1 ton, can't recall the model number off hand as I'm at work)

If the AC is set to ON, the 3A fuse on my board keeps popping as soon as I push in the "dead man's switch" on the door. If it's off or set to furnace, it's okay.

Blower comes on fine, as does the furnace.

I've replaced the thermostat.

I've checked the wires from tstat to furnace, all fine. No shorts, no discontinuities.

Same thing with the wire from board to condenser, no breaks, no shorts.

Checked the capacitor at the condenser, it responds properly to the OHM meter (goes to 0, approaches infinity).

Cleaned out the contactor at the condenser, a spider made it's home there at one point. But it seems to stay open and close okay. But it never gets the chance to close because the fuse blows before the 24V can hit it.

The contactor has an internal resistance of ~1 ohm.

Transformer is putting out 26VAC, but I figure that's +/- 10% (and with no load) so that's fine.

I'm truly puzzled with this one. Again, it only happens when the Tstat is set to cool. Somewhere there's a short to ground that I'm missing. My guess is the contactor, if I disconnect it from yellow wire from the tstat it won't pop.

Is the ~1 ohm resistance normal for the windings on that solenoid?

Ideas?


Regs,

J

[Edited by strawewe on 06-30-2005 at 02:21 PM]

[faith]

Strawewe;

Help the forum help you… Provide he serial number and model number of your unit. Why the serial number? Carrier imbeds date of manufacture in this value… The same model number may include several variants over the years of production. Yes some of the variants are significantly different, hope this makes sense.

Next contact your local Carrier Wholesaler, though you better have a part number handy, the counter personnel aren’t much into one-on-one training of DIY folks. Every time I’ve had to stand in line behind one the situation isn’t pleasant for all involved, understood?

As far as what you can and can’t purchase over the counter, refrigerants are controlled you need a EPA license to handle them. BTW: All suppliers that I’m aware of do not allow return of electrical part (for any reason).

Bottom line is typically your better off paying a professional after verifying that the problem isn’t within the breaker panel or the disconnect fuse(s). If you are not experienced with handling high voltage equipment, go not further until you’re absolutely certain that your life insurance current payment is indeed paid in full and your last will and testament is up to date. When I tried to explain emergency medial procedures predicated for ventricular fibrillation (typical cause of death from electrocution) it wasn’t taken seriously. Sadly far too many members on this forum are in denial about risks they are exposed to on a job site. I’ve noticed that these same folks would rather “shoot the messenger” than benefit from any knowledge contained within.

Ben Franklin (my hero) mentioned that “Experience keeps a dear School, but Fools will learn in no other.”

God Bless...

[John Thomas]

Your reversing valve solenoid is bad.

[docholiday]

so eliminate things one by one and wring it out.

[strawewe]

Originally posted by John Thomas
Your reversing valve solenoid is bad.

Meaning the contactor?

[John Thomas]

Sorry, but if you don't know, you need to get an AC Contractor.

[Mr Bill]

Originally posted by docholiday
so eliminate things one by one and wring it out.

This is what I do.

[strawewe]

Originally posted by mrbillpro

Originally posted by docholiday
so eliminate things one by one and wring it out.

This is what I do.

It's what any of us do. Granted, I may not know all the terminology necessary for HVAC work, but I'm learning. (They didn't teach useful, practical things in electrical engineering courses.)

Like I said, I did everything methodically. Testing wires for continuity, shorts, etc. Isolated the thermostat and replaced it with a known, working model. Checked capacitors for proper behavior. Isolated the transformer and took voltage readings. Took readings of the motors to confirm their resistances. Following all the schematics and double checking every lead and termination with my trusty Fluke meter.

What I don't know is the proper behavior of the particular solenoid/contactor/widget (Furnas part no. B1360321). Sure I can take the internal resistance (1.5Ohm) and physically press in the contact (which will start the condenser fan), but that means nothing if I don't know that the proper resistance is supposed to be higher or lower than that number.

I seek advice, counsel, encouragement, even some gentle ribbing about naivety. I've gotten a mix of all of the above--the mark of a good forum. Let's try not to brazenly dismiss someone without formal education because they don't know terminology or the complete in's and out's.

On the whole, yes, I'm cheap, I don't like hiring "professionals" when I think I can fault-isolate myself first. Plumbing, carpentry, wiring; it's not hard, merely an art-form, anyone with the right mindset can learn how to do it.

So, that said, teach me, help me learn.

I remain a jack of all trades, master of none--and therein I lie content.

- J

[1972torino]

time to open a phone book. also do not condescend to us (hvac techs)by calling controls "widgets" !! you can take that crap and leave this board!!! i have no problem with diyers but you all need to show us the respect we have earned

[beenthere]

The contactor coil is shorted.

[jacob perkins]

Originally posted by beenthere
The contactor coil is shorted.

Thanks Dice!

Strawewe,
your clue was the ohm reading on the contacter coil.You see it is a direct short?It should have read...uh I dont know...about 24,999 ohms.

Try your local library or bookstore to learn some terminology and the sequence of operation,ect..It will be helpful in the future if you have this situation again.Or, come back and let us know how your doin'

[Edited by jacob perkins on 07-01-2005 at 10:05 PM]

[markco]

[i](They didn't teach useful, practical things in electrical engineering courses.) [/B]Because that's not what you went to engineering school for. Don't know how many times I've gone on calls where the furnace is completely taken apart, and the home-owner says "Well, you know, I am an electrical engineer." I just nod my head as I replace the burnt out igniter, and spend another hour re-wiring and re-assembling their furnace.

[Hroper]

Blow the fuse before you replaced the T-stat?

[git-r-dun]

You should be reading 8 to 12 ohms across a 24V contactor coil, you're definately shorted if you're only getting 1.

[strawewe]

Originally posted by Hroper
Blow the fuse before you replaced the T-stat?

Yes, actually. The unit is for the daylight basement, and as such, used infrequently (never gets too hot down there). We went to run it for a while to give it some exercise and noticed it wouldn't come on. That's when we discovered the blown 3A fuse.

From there it was trial and error, and today I think I'll order the replacement contactor from McMaster-Carr based on the help I've received here.


EDIT: I got the replacement contactor today and installed it. A/C runs like a champ, thanks to all who helped out.

[Edited by strawewe on 07-01-2005 at 07:28 PM]