Trane Xl16i no fan or compressor

Our outside unit, a Trane Xl16i, 4TTX6036B1000AA stopped working a few days ago. I believe I have narrowed it down to the the LPCO. I can see the 24V measures across the blue/yellow that are coming from the control board of the blower/furnace. I do not see 24V at the contactor/coil. It looks like the HPCO (Yellow/Black) is working as I can see the 24V from when measuring the blue from the control board and the Black side of the HPCO. I am not seeing the same though for the LPCO, as I do not see 24V across the yellow from the control board and the contactor blue wire that comes from the LPCO. Could something else be pulling the voltage down or is this definitely an LPCO issue? If I engage the contactor manually I do hear the compressor start but not that fan. The schematic seems to show that blue going to the fan so that would make sense. Could it be the fan is bad and it is causing the blue to ground or such? I assume replacing the LPCO means I need someone with pressure gauges, refrigerant? As a test could I by pass the LPCO with a jumper and see if that momentarily bring the system online or is that a risk?

Thank you,

Keith

 

Welcome to the forums.

A problem with the LPCO (low pressure cut off) means you are low on refrigerant.
I would have the system pressures checked first before I assumed the LPCO switch is bad as those pressure switches are extremely reliable,

 

Thanks, I will have the HVAC guy check the pressure first.

 

Pjmax, so it was finally warm enough to have them check the system. It is basically empty so the thought is maybe a leak. They did an initial test with 1 lb of 410A but the leak detector did not find anything on the inside coils or outdoor unit. They suggested emptying the system and using Nitrogen to see if that can be easier to pinpoint a leak. A buddy of mine mentioned a color dye test. Does anyone have any recommendations on proper methods to test for a leak? Also the 1 lb that they put it seemed to hold a 5 minute pressure test. This system is probably 15 years old and I do not ever recall filling it. Any chance this is just slow degradation over time or such or does that not ever occur?

 

Approx six weeks ago it was still running. Now the system is empty.
That would have to be a pretty good sized leak.

Normally I'd test with high pressure nitrogen first looking for bubbles around fittings, valves. connections, etc.
Same around condenser coil and compressor.
If nothing found.... points to a corroded evaporator coil inside.
With age.... a corroded evap coil is the most common problem and of course the hardest to get to and work on.

In order to use dye you'd have to pretty much charge the system so that it could run.

 

Pete,

Thanks. It could be a bit longer than 6 weeks, About 6 weeks ago we had a warm day that made the upstairs thermostat trigger the AC. I am not sure the time span between that date and the last time it ran. Probably somewhere in November is my guess based on the temperatures. They are doing the high pressure Nitrogen test Thursday. I will let you know what the outcome is . He thought his wand sensed something in the evaporator coil, but it was not repeatable so maybe it just bumped something.

Thank again for all of your advice and help.

Keith

 

A one pound charge was probably not enough.

 

Hi, a word of caution! Do Not manually engage a live contactor, if the compressor was internally shorted there could be a serious flash or explosion that could cause serious injury.
Geo 🇺🇸

 

Given the system is 15 years old, if it is a corroded evaporator coil, does it make sense to replace it or bite the bullet and replace the whole system?

 

The leak is outside. The indoor coils looked good and are aluminum. The suggestion from one Trane dealer was to just replace the outdoor unit with the same model/size (but a new unit) . Is there any reason to not pursue that option?

Keith