yet another "broken A/C" thread

RatLabGuy

RatLabGuy

You look like a monkey and smell like one too
Joined
May 18, 2005
Location
Churchville, MD
Argh. Wife calls me at 3:30, "No AC, its hot in here".
I'm at work, walked my son through some basic diagnostics, by the time we're done... our local go-to HVAC place is closed. Damnit.
So while I'm waiting, I'll tap the resources here.

Pretty standard 12-14? year old Bryant A/c, not heat pump.
All I know is, nothing turns on. Again, I'm still at work, this is what I've gathered from my son.
- No obvious visual sign of fan motor cap being bad (first thing checked)
- flipping stat to Fan - nothing happens. Also nothing happens on "auto".
- Had him jumper R and Y, as well as R and G - nothing.

To me this sounds like it's not a stat problem, and since it affects the house fan too, it isn't just the capacitor. Possibly a control board problem?

Shouldn't I at least be able to short the house fan to run?
 
obullfish said:
Breaker?
Click to expand...
lol yes, he checked that. He flipped the A/C breaker.
Although, I just had him test for voltage at the R & G wires on the stat, and there's nothing. So maybe I need to trace it back to the transformer.
It just occurred to me the t-stat may be driven off a different (110?) line?
 
It may, but I think usually the outdoor unit or indoor unit has the LV xfrmr. Should be 24v?

Question: meter set on proper ac/dc? I think most systems run LV ac?
 
SOLVED
aparently the 3a fuse on the control board had blown. Our unit has the board in the base of the air handler below the gas furnace.
W that blown, there was no power to the t stat, etc.
Even tbe same ATC style fuses i have for the Toyotas, too bad i was out of low amp ones.... cost me a whole $3.19 for a 4 pack on my way home....

Boy that would have been a frustrating service call bill...
 
Question remains : what was the source of the fuse blowing? Check wiring by the outdoor unit if it could possible have gotten weedate etc.
 
shawn said:
Or something inside got spider-shorted...
Click to expand...
Thats my bet.
First time in the decade ive lived here that its bliwn. I left a spare in the case, sobits ready.
 
RatLabGuy said:
SOLVED
aparently the 3a fuse on the control board had blown. Our unit has the board in the base of the air handler below the gas furnace.
W that blown, there was no power to the t stat, etc.
Even tbe same ATC style fuses i have for the Toyotas, too bad i was out of low amp ones.... cost me a whole $3.19 for a 4 pack on my way home....

Boy that would have been a frustrating service call bill...
Click to expand...

$200 for $0.85 fuse and another $100 for the service call if they were honest, you probably would have been charged for the whole board.

RatLabGuy said:
Thats my bet.
First time in the decade ive lived here that its bliwn. I left a spare in the case, sobits ready.
Click to expand...

And because you’re prepared, it’ll never blow again...
 
If it does blow again, ohm out the contactor coil in the outdoor unit...if it has a low reading it can blow the fuse once it heats up... normal ohm reading is 16-20 ohms..
 
Bringing this back...
So Friday we finally had to turn the AC on.
Worked great for about an hour, then just quit.

Did some digging around... no response to anything.. lo and behold, fuse had popped again.
So I replaced it... then immediately popped.

Came back to this thread...
smith92rc said:
If it does blow again, ohm out the contactor coil in the outdoor unit...if it has a low reading it can blow the fuse once it heats up... normal ohm reading is 16-20 ohms..
Click to expand...
checked the relay (contactor). Guess what... read 21 Ohms, and one of the outgoing contacts was in rough shape, could tell it had been hovering on making a poor contact.

@smith92rc was on the money.

Amazon Primed the part, had it replaced by Sat afternoon.

I'm guessing the 7 months of sitting time was just enough to make it sticky and get past the point of working.
 
A/C fun again!

System kicks on, but isn't cold - b/c the comrpessor isn't running.
I can hear the contactor clicking on... in fact you can see it spark, but it looks like the bridge isn't staying down on the terminals. If I (carefully) push the bridge on and hold it, it sparks and the compressor spins up. But it won't stay there.
I'm assuming this means something is up w/ the coil not being able to hold the bridge in place. Since the compressor spins, the capacitor must be good... right?

Ordered (another) new contactor via Prime last night but won't get here until tomorrow (in theory). The local mom & pop HVAC place we use is swamped, the old lady taking calls is gonna see if they have one on hand, and their only service tech has too many jobs lined up already.
Supposed to be high 80s and super humid today, 90s tomorrow. Gonna be fun.
 
Put a clamp meter on the compressor wire leaving the contactor and see how many amps its pulling. Also check your xformer voltage it should be pretty close to 28v ac... you never mentioned it before but low voltage could have been the cause of the blown fuse and contactor coil not pulling in.
Then go down to your nearest electrical or hvac supply and buy a new contactor instead of having to wait for Amazon
 
braxton357 said:
Put a clamp meter on the compressor wire leaving the contactor and see how many amps its pulling. Also check your xformer voltage it should be pretty close to 28v ac... you never mentioned it before but low voltage could have been the cause of the blown fuse and contactor coil not pulling in.
Then go down to your nearest electrical or hvac supply and buy a new contactor instead of having to wait for Amazon
Click to expand...
Thanks thats a good point. I checked incoming 240AC but not outgoing.

And, yes I got the part from the local mom & pop service. Unfortunately we don't have a local electrical or HVAC supply house that will do counter sales to non-licensed yahoos like me. Frustrating.

So the good news is, just a quick swap of the contactor and we are back up and running. Done in 30 mins.
When the new cheapo comes from Amazon I'll stockpile it to have on hand as an emergency backup.
 
Just for future reference, here is a classic case of "you get what you pay for."
A year ago, I needed something but weather wasn't awful so I ordered a contactor for $13 on Amazon.
Here I am a year later, and it has failed.

On the left is the $13 unit. Right is the $60 replacement that the local guys actually use.
Now there certainly is some markup there... but note the difference in heft.
Chespo has all the windings snd contacts exposed. The clamp points are smaller.
For a part that is more or less outside, which one would last longer?
20210628_102017.jpg
20210628_102131.jpg
 
RatLabGuy said:
Just for future reference, here is a classic case of "you get what you pay for."
A year ago, I needed something but weather wasn't awful so I ordered a contactor for $13 on Amazon.
Here I am a year later, and it has failed.

On the left is the $13 unit. Right is the $60 replacement that the local guys actually use.
Now there certainly is some markup there... but note the difference in heft.
Chespo has all the windings snd contacts exposed. The clamp points are smaller.
For a part that is more or less outside, which one would last longer?
View attachment 350810View attachment 350811
Click to expand...
whats the part# on the "good un"
 
I'm really glad I got the Definite-purpose Contactor.
Because the Indefinite purpose ones.... well I just wouldn;t know what to do with.
 
Last edited:
looks like it cross references to an eaton: C25CNB125T

which you should be able to buy on amazon or ebay for ....~$20
 
Ron said:
looks like it cross references to an eaton: C25CNB125T

which you should be able to buy on amazon or ebay for ....~$20
Click to expand...
Cross reference as in, identical unit under different name" or "matches same specs"?
Of course Eaton is likely to be good anyway
Not sure what's on the box, the face w/ the label on it is now taoed to the inside of the compressor panel and I'm not going back out in the heat to look.

FYI here is the cheapo
Amazon product ASIN B00DZP2UXM
Note a bunch of recent reviews say "Works well but didn't last long..."
 
cross reference as im an eaton dealer and i throw that part number in their system and they say "use this"
haha

eaton just has a very wide distrbution network and someone always breaks the rules...
 
OK Brain Trust....

Tonight, chilling out w/ bedtime drink, and realized.. .the damn fan is blowing.
Low and behold - same thing again. Compressor not running. It worked fine all day.

New contactor sits and clicks. removed the cover, press in the switch - yup, starts up.

Checked the voltage per @braxton357. At first got excited bc its "only" read 26v, then realized, nope its supposed to be 24, not 28 :flipoff2:

Stumped, I checked the capacitor. Hm.. .resistance is infinite betweeb C and Herm AND C & fan. 4 MOhms between Herm & fan.
That, I guess, suggests the cap is bad.
OK, I can deal with that....

But if the cap is bad - why will it still turn on if I press in the bridge on the contactor? Shouldn't a bad cap prevent the compressor (or fan) from overcoming the startup resistance to spin up?
 
Have you considered letting an AC tech look at it? Not being smart azz, I diy everything myself too including installing the mini-split in this house and fixing our old central unit several times.

At some point it becomes cheaper to let a pro fix it rather than the aggravation and time/money spent trying to figure it out.
 
awheelterd said:
Where are you measuring 24v at? At the transformer or at the coil on the contactor?
Click to expand...
both.
Right off the transfermer is ~26.6v.
off the output of teh control board inside is ~26v, its the same at the wire coming into the compressor and the contactor.
ponykilr said:
Have you considered letting an AC tech look at it? Not being smart azz, I diy everything myself too including installing the mini-split in this house and fixing our old central unit several times.

At some point it becomes cheaper to let a pro fix it rather than the aggravation and time/money spent trying to figure it out.
Click to expand...
that was the first thing I did. Can't get anybody here quickly. With it being triple digits heat index I don't have time to wait.
 
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