Hello, new member here and love the site. I came up on a problem Friday with a Trane unit model number YCD120C3LOAB with no AC. When I arrived at the job the contactors for stage 1 & 2 wasn't engaged. I pushed stage 1 in and it engaged. Same with the 2nd stage. I checked the tranformer and found that it was at about 22v with .09 amp draw. I noticed that the transformer was very warm. I checked the wires going to the transformer to ground and found the positive wire going to main board at 150 ohms. I did the same to the contactors and got nothing to ground.
The system had a smoke detector which broke the 24v positive side to the transformer. The wires used to do so were 5 wire thermostat wire (not sure what gage). When I took the smoke detector out of the loop, the system worked properly. After doing so the transformer wasn't getting hot but it also was 7 at night and when I got there it was 5.
When I ran the system through the test mode everything work perfectly. Sorry it was so long but I am still not positive that I need to change out the smoke detector or the problem lies deeper. Any suggestions would be appreciated. Thank you...
Hello Techbob, and welcome. I believe you're on the right track, but the wiring could be shorted together or moisture at high impedence. We normally break the smoke detector through 16 and 17 i think it is, that are strapped together on the TB. I highly doubt that the detector itself is bad, this is usually just a set of dry NC contacts...But if the detector is powered up by 24 volts also, that could be a problem.
I believe the compressors were locked out and you reset them by pushing in the contactors. I can't remember by your model number what line voltage your unit is but if you have only 22 volts output on the transformer, I would hazard a guess that you have a 208 volt unit that has the transformer hooked up to the wrong tap and that is causing your low voltage to be low. See if the transformer is hooked up to 230 volts instead of 208 perhaps. I doubt the smoke detector is the cause of your problem. The previous post is correct. The contacts are normally hooked to 16 and 17 to disable the unit. Remember the detector cuts the fan and everything off, not just the compressors. Also, does this unit have a CTI? As soon as you get the opportunity, get your hands on the Trane manual titled Troubleshooting Microelectronics. It may be available on the web. It is a necessity when working on Voyagers.
Originally posted by seaboard
As soon as you get the opportunity, get your hands on the Trane manual titled Troubleshooting Microelectronics. It may be available on the web. It is a necessity when working on Voyagers.
Thanks for the info... I forgot to say that the unit is 208 and the transformer is set on 230v so that answer the 22v problem. The smoke detector is a 24v unit. Also forgot to say that I checked the coils on all 3 contactors and got the same ohm load of 9 ohms. That smoke detector does kill all the contactors. Hopefully this will help to solve this mystery.
Techbob00...check the load on the smoke detector itself, and compare to a like unit, without the unit running. If both have the same load, that will eliminate the detector. I'd bet lunch you'll find a high resistance on the one you're working on.
Could you get the Trane to repeat the original problem after you pushed in the contactors? If it hung on you again what stage hung? Were the contactor coils hot too?
How old is this unit?
You say that using the TEST on the TB all functions were fine. Did you check for shorts on the stat wiring and does it have a CTI?
Did you have the smoke detector contacts on the TB (16 & 17)?
It was wired straight off of the 24v trans for the detector operating load did you try to unload the trans and hook the detector somewhere else? Was it still 0.9 amp draw with the detector out of circuit?
Is the detector multi-volt? Could you use 208 for the load?
Did you physically check the incoming voltage with a meter and/or the Disco condition?