Coronado 83.00 Troubleshooting Manual Download Page 4

Page: 4 / 63

•

No enough refrigerant - Low high-side
pressure, low low-side pressure, poor co-
oling

•

Debris in the system - TXV plugged or un-
controllable, compressor turns hard,
receiver/dryer is icing

•

Contaminated or incorrect refrigerant - Very
inconsistent system pressures and duct
temps

Thermal Expansion Valve
Testing

General

The thermostatic expansion valve (TXV) (see

Fig. 1

)

is a controlling device that regulates the flow of refri-
gerant into the evaporator. It is actuated by changes
in evaporator pressure and the superheat of the refri-
gerant gas leaving the evaporator. Superheat is defi-
ned as any temperature of a gas above the boiling
point for that liquid. For example, when a refrigerant
liquid boils at a low temperature of 40°F (4.5°C) in
the evaporator and then the refrigerant gas increases
in temperature, superheat has been added. If this
refrigerant changed from a liquid to a gas or vapor at
40°F (4.5°C) and then the refrigerant vapor increa-
sed in temperature to 50°F (10°C), then it has been
superheated by 10°F (6.5°C). The refrigerant ente-
ring the evaporator is metered at a rate that matches
the amount of refrigerant being boiled off in the eva-
porator. In addition to metering refrigerant, the TXV
also provides a pressure drop in the system, separa-
ting the high-pressure side of the system from the
low-pressure side. To operate properly, the TXV must
have a steady flow of liquid refrigerant supplied to it
by the high side of the system. Issues such as an
excessively low refrigerant charge, or contaminates
in the system can interrupt this flow of liquid to the
valve. In these cases, symptoms may surface ma-
king it seem as if the valve is inoperative or plugged.
These symptoms can include poor performance, low
low-side pressures (even a vacuum) ,etc. Before re-
placing a TXV, it is important to verify that there are
no contaminates in the system and that the refrige-
rant charge is correct, by performing a refrigerant
identification and thorough leak check of the entire
system. The TXV is often incorrectly replaced as the
primary failed part, but is rarely the cause of a per-
formance issue. In rare instances the power element

of the TXV can loose its charge, reducing the range
of the TXV, but there is no other situation that can be
considered a failure of the TXV. Due to the similarity
of symptoms that these root causes have on A/C
performance, proper diagnosis is essential to making
the correct repairs to the A/C system.

Diagnostics

To determine if the TXV is functioning, watch the low-
side pressure while the compressor is engaged. The
influence from the TXV modulating can be seen as
the low-side gage needle "wags." As the load on the
system changes from ambient conditions, the TXV
may not wag as much, and care must be taken to
prevent false diagnosis. The function of the individual
TXVs in a dual evaporator system will be combined
to produce the total pressure needle waging that is
observed. If a TXV failure is suspected, connect the
gage set to the properly charged refrigerant loop,
and run the compressor. Watch for the low-side gage
needle to wag, indicating that the TXV is working.
Depending on the load, the low-side pressure may
progress from moderate to high, but the needle will
simultaneously wag as the average pressure chan-
ges.

11/26/2007

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