3amp fuse blows when switching off

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  #1  
Old 07-25-15, 08:57 AM
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3amp fuse blows when switching off

I've been through a dozen 3amp fuses on the control board. Found this forum and tested for a short at the internal air handler, then tested the outside contactor. Although all seemed good, I replaced the contactor anyway.

Still seeing the same issue. With a fresh fuse, the thermostat will start the blower and compressor and the cool air will flow. As soon as the thermostat switches off the 3amp fuse blows.

Any suggestions before I submit to a call-out?

EDIT: I should also add that at the end of last summer when this started I replaced the control board and the transformer.
 

Last edited by Dutch3; 07-25-15 at 09:14 AM. Reason: Added more info
  #2  
Old 07-25-15, 09:28 AM
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That is an odd problem. The only thing I can think of to suggest is to temporarily remove the wires going to the outside unit and then set the thermostat for cooling. Obviously, the A/C won't turn on, although the inside blower should still come on. After it's been running for a few minutes, turn off the thermostat and see if the fuse blows. The idea is to try to isolate whether the problem is related to the outside unit or the blower (inside). If the fuse doesn't blow with the outside unit disconnected, then the problem would appear to be in the outside unit (or the wiring going to it). On a related note, does the fuse blow when you set the thermostat to call for heat and turn it off?
 
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Old 07-25-15, 09:34 AM
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I need to run out to pick up more fuses and will try that!

This is purely a summer issue, in the winter the internal blower/furnace works just fine. Which I guess would also point to an outside issue.
 
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Old 07-25-15, 09:41 AM
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I agree, since the fuse doesn't blow in Heat mode, it points to a problem with the outside unit. If the fuse doesn't blow after disconnecting the wires to the outside unit, you could then try the opposite: reconnect the wires to the control board and disconnect them where they connect to the outside unit (usually connect with wire nuts). It's doubtful that it's the wire itself, however that would test it.
 
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Old 07-25-15, 10:17 AM
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What make and model thermostat ?

My thought is that the stat uses a C wire and that something in the stat is creating a short on power off.
 
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Old 07-25-15, 05:28 PM
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It's a Venstar T6800. I have the same model on another circuit in the house with no issues.

I managed to find some 3A fuses (took 3 locations to finally locate some) and have 'reset' the circuit. I also stopped off at Home Depot and picked up a cheap 7 day programmable t-stat for $20. It's so hot here right now I think my wife will kill me if I tell her I'm diagnosing ... so for now replacing with a battery powered (No C connection) might give the clue that PJmax mentions. At least it's running - I'll report back in a day or two.
 
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Old 07-25-15, 07:15 PM
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Well, that was conclusive. After an hour or two the system has shut down again. With the C wire not even connected, it's not the thermostat since that didn't get to a low enough temperature to trigger an off signal. So I think it's fair to say the problem is not between the air handler and thermostat.

Prior to it giving up I checked the air and it didn't seem chilled. Checking outside and found that the large copper line was not cold and the air coming off the fans not warm which would suggest a problem with the compressor? Could it be there that the short is happening? Would that blow the 24v circuit? Possible that the 'blows on an off signal' is a red herring and coincidental.
 
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Old 07-26-15, 02:16 AM
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Many, but not all, systems have a low pressure, high pressure, or both switches in series with the 24V going to the contactor. If the refrigerant charge is too low, the pressure will be low and the low pressure switch will open to prevent possible damage to the compressor. Likewise, if the pressure gets too high, the high pressure switch will open, shutting down the system to prevent possible damage to the compressor. Usually, the high pressure switch has a reset button (often red in color). Pushing the red button will reset the switch. You will have to look at the wiring diagram for your outside unit to see if you have either or both pressure switches. They don't normally "short to ground", but I suppose anything is possible. However, if the outside unit is running, that implies that the pressure switches (if present) are closed (normal operation), otherwise the outside unit wouldn't turn on. Can you hear the compressor running or is it just the condenser fan? Most systems use two capacitors (may be a dual cap, both in one "can"), one for the compressor, the other for the condenser fan. It's possible that the capacitor for the compressor is bad. Of the two caps, the compressor one is the larger (higher value) one. They are relatively inexpensive. It might be worth replacing it.

You said that after an hour or two, the system shut down again. If the A/C unit isn't cooling, the system should have run "forever". Did the fuse blow again, this time while the system was running (not when the thermostat turned off the system)?
 
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Old 07-26-15, 08:24 AM
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First thing that I would do would be to pull a new tstat wire. I have seen several of these that have partially shorted internally due to being stretched when originally pulled. Take a close look at the wires and you will understand the problem when you see the extreme thinness of the wire insulation on these tiny wires. Many a unit has been replaced due to misdiagnosis of a bad tstat wire LOL
 
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Old 07-26-15, 08:34 AM
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The fuse was blown again and blew while the thermostat was still in a cooling range. I've replaced the fuse and removed the 24v wires that connect from the air handler to the outside unit. With thermostat set low it should just run the air handler now - will see if that keeps running.

I haven't checked the wiring diagram but there are certainly no visible reset switches at first glance. I can't be sure that the compressor is running - it has been very noisy in the past but tended to quieten down after a while. I had originally put that down to the fan being off balance and eventually settling itself since it would continue cooling. Sorry, feel like I'm drip feeding information here! The unit is less than 3 years old - a Carrier 24ABB3/24ABC6 (from the manual).
 
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Old 07-26-15, 08:58 AM
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Very hard to pinpoint a phantom short, do the basics first. PULL A NEW TSTAT WIRE.
 
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Old 07-26-15, 11:54 AM
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So then the fuse is not necessarily blowing when the thermostat is turning off but just when in cooling mode. Based on my experience..... it it's not a bare wire from the house to the condensor...... it's a defective contactor.
 
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Old 07-27-15, 06:38 AM
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Having disconnected the 24v wires at the air handler control board the fan has been running all night without issue. The next step is to reconnect that and disconnect at the outside unit before the contactor. I will then reconnect the contactor but disconnect the 240v supply, essentially adding more of the circuit bit by bit until it fails again.

@FormerMember: If I exhaust options through the above basic diagnoses, that'll be my next task, thanks! The cable itself is shielded with a pretty thick outer core but the individual strands are very thin.

I'm also going to reconnect everything and see whether the loud noise is when the compressor is running - when that noise stops, does the cool air stop then even though the outside fan continues? If so, perhaps the compressor motor is defective, possibly overheating, and causing the 24v fuse to blow due to overload after a certain time?

@PJMax: I already replaced the contactor
 
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Old 07-27-15, 06:56 AM
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If you have unused wires within the wire bundle going to the outside unit, you could try switching to a different pair within the bundle.
 
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Old 07-27-15, 07:02 AM
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Unfortunately it's a line with only two wires.
 
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Old 07-28-15, 06:06 PM
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With the 24v line to the compressor disconnected at the air handler, the fan has been merrily running for 24 hours or more.

Checked again for shorts between the air handler and compressor - all good. Measuring resistance at the air handler I get 19ohms, but then the contactor is new.

All reconnected and tried again. Everything starts up as expected, the compressor is making a racket, but it's blowing hot air outside and the copper pipe is not getting cold/wet. I switched off before the fuse would inevitably blow again.

Isn't the capacitor there to give a boost for start-up? If it seems to start would that imply the caps are good?

Given that the fuse blow isn't instant it's something that is building up which makes me think it's not a short but some overload safety kicking in, perhaps a pressure switch as Bob mentions.

I think I need to get an HVAC tech to check the pressure in the unit - if it's low could that be the problem? FormerMember said replace the thermostat wire, but if it's also happening with a battery thermostat that's not even using the 24v circuit wouldn't that rule that out anyway?
 
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Old 07-28-15, 06:31 PM
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Yes, I think it's probably time got get an A/C tech in to look at the system, particularly because the compressor & condenser fan are running but it's not cooling (if I understood you correctly). The battery operated thermostat was to rule out the possibility of a bad powered thermostat that was shorting to the Common when shutting off. While a bad thermostat wire is a possibility, that would only account for the fuse blowing, not the lack of cooling. You appear to have two problems: lack of cooling, and fuse blowing. At this point, it's difficult to say if they are related or not.
 
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Old 07-28-15, 07:52 PM
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I believe he said the unit was noisy but getting cold.


Isn't the capacitor there to give a boost for start-up? If it seems to start would that imply the caps are good?
Yes...that is correct. In any event... that wouldn't cause a LV fuse to blow.

Given that the fuse blow isn't instant it's something that is building up which makes me think it's not a short but some overload safety kicking in, perhaps a pressure switch as Bob mentions.
The pressure switches are in series with the 24v line. The switches kill the 24v to the contactor. You could still have a wire inside the condensor that is intermittently touching ground.

Many times when I get problems like this I'll put in several fuseholders in key locations to see which one blows. You could put a 2A fuse and fuseholder right at the condensor unit in the Y line. The fuse needs to be smaller than the main fuse so that the main fuse doesn't blow first.
 
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Old 08-14-15, 03:17 PM
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Follow up ...

As I'm sure you've all been waiting anxiously!

A/C tech came today to investigate. Diagnosed that there was a short between the air handler and the outside unit which was blowing the fuse. He also checked with a meter and found nothing (as I had done) but switched out one of the conductors anyway - bingo, no more blown fuse.

Next was the unit and the noise. No more noise - he believes the contactor switching off may have been causing an overload on the compressor. Bottom line, now the unit is running it's not making the noise.

Finally, the cooling. Far from being low on coolant as I had thought it is actually 'probably too much coolant' - the high pressure was high and gauges 'swinging'. Temperature difference was only 14F, he suggested 20F was what I should be looking for.

His diagnosis - probably contaminants in the system. Full evacuation needed: evac, Nitrogen fill, evac, coolant fill, new drier. Quoted just under $1,500. Calling back next week to see if I want to go ahead.

Irritatingly, with the unit being just over 3 years old, this has to be totally down to the original installation.
 
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Old 08-14-15, 03:38 PM
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There shouldn't be any contaminants in the system. The system is a closed system. The only thing that should be present in it is what was put in at installation time. Since the tech claims that there is too much refrigerant, that rules out a leak. I'm not an HVAC tech, but what he is telling you sounds suspicious to me. Also, $1500 seems rather high. Since the system is only 3 years old, your system should use R410A refrigerant, which is relatively inexpensive compared to the older R22, which is being phased out for environmental reasons. It's up to you, but I would consider getting a second opinion.
 
 

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