I would be curious to know the entire model number and I am sure you are far enough into this that you know exactly what you have but if you have what it sounds like you paid for you should definitely have low and high pressure switches if they are not there and it sounds like they are not that would be a builder grade outdoor unit

Thanks, miken. I checked out the link and may order some info from it.

We purchased the top of the line 2TWZ9036A Trane heat pump and thats what it says on the unit. Its got the two compressors, so I doubt its a low-cost builder model. The only unusual thing about it is that it is a replacement unit from 2004. Our original unit was purchased in 2003, but Trane had a recall before it was even installed, so we now have the replacement unit. If Trane somehow shipped replacements out in a hurry without the pressure switches, that could partially explain our situation.

It seems more likely that the switches are there somewhere but are simply not obvious. If theyre there, though, why dont they show up on the wiring schematics? That omission makes no sense. And why do we find the two refrigerant line stubouts with nothing attached?

Someone out there must be familiar enough with the XL19i to identify the switch locations and wires. When the cover and the outdoor fan and compressor insulator blankets were removed by the last contractor, I grabbed a flashlight and examined the whole interior of the heat pump. If the switches are near the compressors they must be well hidden. This heat pumps coil is so large that you have to balance your waist on its supporting frame and dive into it headfirst to get to the lowest parts. It is just barely possible to get yourself out without help if youre tall enough.

Yes they are big units, small sheds. No such thing as a builder unit in 19 SEER, as you noted.

It is possible that the pressure switches noted that are usually installed with semi-hermetic higher end heat pumps are not there because you have scroll compressors in your unit?

Do you know which you have?

Hermetics are low and slightly ovalized, a scroll is round and taller. Can you read model # off the top of the compressor?

I decided to satisfy my curiosity and downloaded the I & O and schematics.

I can see where the HPCO & LPCO is listed in the legend, but that doesn't mean you have one on your unit. Close inspection of the schematic and refrigerant circuit diagrams both do not show any. If you did have them they would be located on the BL (blue) wire circuit, disabling the contactors, V/S fan while allowing signal to open on the ODS to shift system into backup 3rd stage heat.

According to this manual, you have the climatetuff compressors which is the infamous "snowball" that ran in a slugging condition for 20 years?

Looks like your diagram has the standard boilerplate cut-n-paste legend and no switches on your outdoor unit.

ZL, thanks a lot for taking the time on this. The compressors are climatuff compressors, and I can get their model numbers by pulling off the insulation blankets. From what Ive been told before I believe they are recips rather than scrolls. They are cylinder-shaped, though Ill bet they all are regardless of type. Ill go pull the covers and try and get a photo I can post. Ill also see if I spot anything new this time around.

Is it normal for Trane to leave things like pressure switches off a schematic? I would think they would want wires to those things to be easy to find. If it is the low pressure switch thats locked, it was probably caused by not running the system for a few months of late winter and spring. For the AC only version of the XL19i the units are only used for part of the year, so if a layoff is prone to cause a sticking switch this must have happened to someone else.

The 1st stage compressor is labeled: AP14D-YZ1-A and it also has a large number 754 and a small number 12831. The 2nd stage compressor is labeled: FH32D-FF1-GA with a large 733 and a small 12373.

I'm rechecking the wires and still believe the only wires entering the area inside the coil are the fan wires, crankcase heater wires and 3 wires to each compressor motor. But I'll re-check and see if removing another side panel shows anything else.

I'm also checking continuity on every wire connecting the heat pump to the zone control panel.

Don't bother after looking up they can only be recips.

If the pressure switches were there, you would see at least one, probably the low on the suction line near the compressors. A high one if there is one could be tucked away as it is commonly piped in the outlet of the condenser coil.

Im confident there are no switches.

With the lack of pressure switches, the only other thing besides the board that I would be looking at is the CBC (coil bottom sensor), have one? It appears to break the low voltage compressor control circuit circuit. The internal overloads which are in/on the compressor will not be seen usually unless removing the compressor terminal cover and are not connected to the control circuit.

We have the CBS coil bottom sensor. I checked its resistance at 11.95 k-ohms at just under 70 degrees. The
ambient sensor at the same time showed 12.13 k-ohms.

Weve tried swapping in a current zone controller card with new expander and zone cards, but we found that the new 3.0 controller is not compatible with our 5 year old master thermostat designed for the 2.8 controller chip. Were in the process of ordering one of the new master thermostats so we can swap it out as well.

Can you tell us which of these points weve got right and which ones are wrong? Weve been told so many different things by different Trane offices that wed like to run these by someone who actually works with their products.

Symptoms:

1. In test mode, both compressors run correctly with good temperatures and pressures.

2. In normal mode, when changed from off to cool the master thermostat display indicates the compressor is turned on for about 2 seconds, and is then immediately turned off. [Y1 is momentarily hot, then not].

3. The Discharge Air Sensor and Low Ambient Cooling sensors are working correctly. The resistance of the Discharge Air Sensor is very close to the appropriate number from Tranes table, and the Low Ambient Cooling Relay is on, correctly connecting YLo/X2 and Y at the zone controller outputs to the corresponding heat pump terminals Y1 and Y2.

Relevant Points:

4. The zone controller board is designed to ignore error conditions when in test mode, but not in normal mode. So if the compressors run in test mode but not otherwise, then the zone controller is turning them off because it senses an error condition.

5. Temperature or pressure limit conditions are recognized by sensors and controlled by switches at each compressor. The switches cycle the compressors power directly, rather than relying on the zone controller board to do so via Y1 and Y2.

6. The heat pump communicates an error condition to the zone controller via data transmitted over the communicating network by the F terminal. Defrost is communicated by 24v hot at the heat pump X2 terminal to the zone controller D terminal.

7. The zone controller indicates an error condition by flash patterns on its defrost LED. Its not flashing currently, so the zone controller is not sensing an error condition.

8. The master thermostat indicates some error conditions by flashing the Service indicator. Its not flashing, so its not sensing an error condition.

Much thanks for taking time on our behalf.

Great write up and you have a good understanding of how it works, just not whats wrong.
I would be looking at the communication interface or stat as the problem and not the refrigeration circuits as you agree it can be overridden. Being that the F circuit among others use the defrost board (DFC), I would consider that as a possible culprit. If you can and have a available stat, I would bypass the zone control with all dampers open and directly connect a HP stat to split system to try. Not the ultimate solution but a great field test. This will at least determine outdoor component problems versus indoor panel and controls.

Thanks, ZL. Im beginning to wonder about the stat more and more. Its the most likely thing to be damaged by static electric shock just by using it normally. Weve trained ourselves to touch a switch plate screw before the stat, but we might easily have screwed up and forgotten occasionally. Substituting a normal heat pump stat as a test is a good idea I had not thought of. Ill pick one up and give it a try today.

The defrost control board appears to be the only board in the heat pump itself. Weve got a brand new XL14i sitting here that may or may not use the same defrost board. I think Ill take a look inside it to check.