Hi everyone! Been browsing HVAC talk since I started in the trade about 5 years ago. Ive picked up a lot of informative information while lurking through this site & decided that the Pro Forums would be the best direction to get additional technical information/educating myself. Looking to get my post count up & apply for pro membership.
With that being said I ran into a peculiar issue on a Trane heat pump the other day and would like some insight of what is actually happening with the system electrically. I will be returning for further diagnostics.
Model XE 1000, TWR048D100A1
I do not have the indoor model at the moment.
This call started with another system that was blowing cold air, but before I arrived they reset the breaker & stated the system was operating properly. I did a visual inspection on that piece of equipment & couldn’t find anything out of the usual while on site. When I was about to leave they asked me to look at another unit that a previous technician had worked on due to it sometimes putting out cold air. Unfortunately the business was closing soon and I did not have adequate time to due a complete diagnosis.
The previous technician was a maintenance tech of ours with less than a year of experience in his own truck. He stated when he arrived the unit had a short and his fix was disconnecting the ground wire from the transformer on the low voltage side. I was given this information after I left unfortunately.
I checked the defrost board outside & noticed the ODF was not cutting off. Found the relay on the board not opening so initially condemned the board. Looking back I believe that diagnosis was incorrect. While at the outdoor I notice the Red & white low voltage insulation on the low voltage wires appeared to have melted off. I also noticed that my meter displayed 24 volts from common to orange, but when I attempted to jump orange to the reversing valve solinoid, the valve did not react. The solinoid wasn’t shorted or open.
When I go back I’ll be reconnecting the ground to the transformer & expect the short to reappear. Was wondering if you all could assist me in understanding what is happening electrically to the low voltage side of the system with the ground disconnected from the transformer. I’m assuming I was reading ghost voltage or back voltage on the O wire.
Due to the ground being disconnected at the transformer and a potential short in the system that is touching metal (assumption) is that wire acting like the ground for the circuit & completing the circuit. I’m not fully wrapping my head around this at the moment >.<
You probably won't be able to get this fast enough, but when looking for low voltage shorts, the Short Pro Tool saves a lot of hassle. Think it's made by Zebra. There's also a You Tube vid showing its ease of use.
I do a triple evac with nitro to remove non condensables.
I read through a couple times and am not 100% confused but somewhere over 50% as to what you are doing.
I got the transformer is not grounded. A red and white wire has melted insulation but not sure if you are talking unit or field wiring.
You have voltage to O and then jumpered to O why would you expect and energized valve to shift? Was the board still in defrost or out? If the fan did not shut off how did you know?
Initially when I saw the missing insulation the red wire was extremely close to copper piping at the outdoor and I suspected that wiring was the short (rub) the other technician did not find. It was blantly obvious though and I was a bit shocked that the previous tech did not see it when he was there. I’m thinking the insulation melted off some point after he left. The white wire was not near any copper, but missing insulation. I repaired those wires while on site.
I had 24 volts reading from Common to O from the thermostat wires. I jumped the O wire from the thermostat directly to the reversing valve to see if it would react, which I did not. At this point is where I suspected it was ghost voltage/back voltage.
This happened last Wednesday so I apologize if my memory is foggy regarding what was happening.
From the description from the customer I started thinking that their was an issue with the defrost cycle when I first got to the outdoor unit. I put the unit into defrost by jumping the pins shortly.
Oddly the entire outdoor unit shut off when I did that. I don’t believe this model has a quiet shift, which threw me off when it turned off when I shorted the pins.
When it came back on I noticed the reversing valve had 24 volts & the unit was in cooling, but the ODF motor was still running. This is where I mistakenly condemned the board.
I cycled the defrost several times while I was there. Sometimes the unit would go into defrost ( valve would energize ) & the fan would remain on. The unit would also sometimes stay in cooling mode.
I apologize that this is a bit jumbled up & hard to follow >.<. I was confused as to what was happening and did not make a connection to the ground wire until after I left.
1. Probably all of us have gone to the home or business where someone wants to rush you because THEY have to go. The best thing to do is give it a cursory look over (for something obvious), then tell them you're leaving and you'll return when you can have all the time you need to properly diagnose and repair their equipment. If it isn't important enough for them to stay so you can fix it, move on to someone more appreciative. If they want the heat on (or AC in summer) and the equipment fixed, they can stay. Don't let them waste your time.
And let them know if they don't want to wait, they get a charge for this waste of time, and a charge for the next visit.
2. Pro short is awesome, get one (or 2). There's also 1 or 2 videos on YouTube.
If I do a job in 30 minutes it's because I spent 30 years learning how to do that in 30 minutes. You owe me for the years, not the minutes.
Thanks for the advice. Thought it was going to be a simple diagnosis and it got a bit more complicated to figure out in a short time.
TLDR
Anothet tech unwired the ground from a split systems transformer in order to “fix” a short. Defrost board was operating intermittently & I was getting odd low voltage readings on the low voltage field wiring at the outdoor unit. Looking for an explanation of what was happening electrically to the low voltage circuit or what could be happening.
I’ll be returning & regrounding the transformer before replacing the board.
You should only get power to "O" in defrost or cooling. find out why you have power there. Or disconnect it from the house and see if it is present not being hooked up at the unit. Get a breaker on the transformer as said before. Unit likely "won't" go into defrost if it has a cooling signal.
1. Probably all of us have gone to the home or business where someone wants to rush you because THEY have to go. The best thing to do is give it a cursory look over (for something obvious), then tell them you're leaving and you'll return when you can have all the time you need to properly diagnose and repair their equipment. If it isn't important enough for them to stay so you can fix it, move on to someone more appreciative. If they want the heat on (or AC in summer) and the equipment fixed, they can stay. Don't let them waste your time.
And let them know if they don't want to wait, they get a charge for this waste of time, and a charge for the next visit.
2. Pro short is awesome, get one (or 2). There's also 1 or 2 videos on YouTube.
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If the transformer is not internally grounded or grounded ( it was removed in this example) and a short appears in the system what will happen to the circuit.
Another tech explained it like having a floating neutral, but I wasn’t 100 percent following him.
The 24 volt signal on the low voltage O wire was present in heating mode & present in the defrost cycle.
During normal operation in the heat mode if I jumped R to the reversing valve the valve would shift properly.
The voltage present on the O thermostat wire isn’t carrying amps I’m assuming as it did not make the solenoid “click” when I jumped it directly from O wire to the solenoid.
I read the original post a couple of times and it was not entirely clear.
But a couple of comments -
If there is a short the fuse should blow.
The correct way to resolve a short is to find and fix the short - not disconnect ground wires as a solution.
The output legs of a transformer when not connected to anything is floating with respect to a fixed reference such as ground. Once you ground one leg the other leg contains a 24VAC voltage relative to ground.
If there is 24V, meter probes on O and C, then you have 24V there. Just like 24V on R and C.
Low voltage side of transformer usually does not have a specific ground wire, the lower end Goodmans usually do.
I would bet of you removed O then you would have heard a shift, as you would have dropped 24V to the RV.
The RV can be energized in either heating or cooling, depending on MFR. Doesn't Trane energize opposite of the others?
You mention red and white wire. Can we presume, since it's a heat pump, there are other wires?
What are red and white connected to?
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from an excerpt by Paul Jacob in Sun City, AZ
Continuity to ground is normal on a grounded secondary circuit. The secondary (24V) side of the transformer should be grounded. I’m assuming the inexperienced tech measured continuity to ground and disconnected the ground, somehow thinking he was fixing the problem. If there was a ground short, disconnecting the ground wire will not fix it.
The voltage present on the O thermostat wire isn’t carrying amps I’m assuming as it did not make the solenoid “click” when I jumped it directly from O wire to the solenoid.
Trane does use Snubber Circuits in some of their controls, I just don’t know if this board is one of them.
The Snubber Circuit is a resistive/capacitor circuit, with a resistor and capacitor wired in series across the relay contacts. In this case, it would be the relay that closes in the defrost control when defrost is energized, and the reversing valve is put back in cooling mode.
Snubber Circuits may cause confusion because 24V will be present if the output wire is disconnected from the load (relay) and the relay contacts are open.
The voltage potential between the disconnected wire and common will be 24V, but no current is present. When the wire is placed back on the load, the 24V potential will disappear.
Using a LoZ (Low Impedance) VOM like the Fluke 116, will eliminate these ghost voltage issues.
Instead of learning the tricks of the trade, learn the trade.
If the transformer is not internally grounded or grounded ( it was removed in this example) and a short appears in the system what will happen to the circuit.
Another tech explained it like having a floating neutral, but I wasn’t 100 percent following him.
The 24 volt signal on the low voltage O wire was present in heating mode & present in the defrost cycle.
During normal operation in the heat mode if I jumped R to the reversing valve the valve would shift properly.
The voltage present on the O thermostat wire isn’t carrying amps I’m assuming as it did not make the solenoid “click” when I jumped it directly from O wire to the solenoid.
What rundawg said.
Plus, if it has a thermostat with triac relays, you will read voltage on the wire when its disconnected, and not energized for a load.
I read the original post a couple of times and it was not entirely clear.
But a couple of comments -
If there is a short the fuse should blow.
The correct way to resolve a short is to find and fix the short - not disconnect ground wires as a solution.
The output legs of a transformer when not connected to anything is floating with respect to a fixed reference such as ground. Once you ground one leg the other leg contains a 24VAC voltage relative to ground.
You’re getting to the center of my question. Sorry that my question is unclear, I suppose I’m unsure how to properly word it.
I’m 100 percent following that the short hasn’t been fixed and you hit the nail on what the previous tech said/did. He saw a wire going to ground & thought it was incorrect. He got rid of that wire and bam the fuse is no longer popping. The circuits is no longer shorting because of that or that’s how I’m atleast understanding it.
I guess the heart of my question is why the fuse is no longer popping. What is happening electrically to that circuit now.
I also have not fully understand what people mean when they say floating...I understand what you’re saying as it doesn’t have a reference point to ground, but I’m not understanding what that means electrically for the circuit...if that makes sense
Asked the same tech who told me about the floating wire & he tried to explain it that the electrons no longer have a load & are freely moving into the “short” without drawing enough amperage.
I guess the heart of my question is why the fuse is no longer popping. What is happening electrically to that circuit now.
I also have not fully understand what people mean when they say floating...I understand what you’re saying as it doesn’t have a reference point to ground, but I’m not understanding what that means electrically for the circuit...if that makes sense.
There is a short between one of the control conductors or a powered device and the system metal. Removing the jumper from one end of the transformer secondary to the cabinet ended the electrical connection of the system cabinet metal to the control system.
Floating means that the system cabinet metal is no longer referenced as electrically equal to the transformer common, and the cabinet now has a separate potential compared to the system common. This eliminates the short because there is no longer a current flow using the system cabinet metal as a short circuit conductor.
This is a dead giveaway that something is placing the system cabinet metal at a higher potential because of some transfer of power from a conductor or a device to the cabinet metal.
This is commonly referred to as a "short to ground."
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You’re getting to the center of my question. Sorry that my question is unclear, I suppose I’m unsure how to properly word it.
I’m 100 percent following that the short hasn’t been fixed and you hit the nail on what the previous tech said/did. He saw a wire going to ground & thought it was incorrect. He got rid of that wire and bam the fuse is no longer popping. The circuits is no longer shorting because of that or that’s how I’m atleast understanding it.
I guess the heart of my question is why the fuse is no longer popping. What is happening electrically to that circuit now.
I also have not fully understand what people mean when they say floating...I understand what you’re saying as it doesn’t have a reference point to ground, but I’m not understanding what that means electrically for the circuit...if that makes sense
Asked the same tech who told me about the floating wire & he tried to explain it that the electrons no longer have a load & are freely moving into the “short” without drawing enough amperage.
It helps to understand what is going on. A transformer is two or more coils wound around a common core. When there is a varying voltage on one a similarly shaped voltage is induced on the others - the magnitude is determined by the winding ratios. If the secondary coil terminals are not connected to a load there is no current flow. Although there is induced voltage in opposite phase on both legs, with reference to a fixed dc voltage the legs will be floating. For example if you can introduce some form of noise on the secondary - say due to interference - and view the waveform on a scope you will see the induced voltage riding on the common mode noise.
Once you connect the transformer legs to a load one or the other leg will be your reference. It does not necessarily have to be ground. In some equipment you can have a center tapped transformer and once you rectify the other two legs you can have a positive and negative dc rail - this sort of arrangement is common in equipment like audio amplifiers.
In your specific case it looks like the reference rail was tied to ground and some or all of the load on the 24VAC section returned current through the ground. Once the ground was disconnected some or all of the load could not return the current. So the short disappeared but that was just due to section of the 24V circuit just not operating.
So the correct steps would be to reconnect the ground, find and fix the short and proceed.
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Originally Posted by STEVEusaPA
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