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Issue date
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Controlled document
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Revision:
WA900-3EO
FAULT ANALYSIS CHART NOTES
1. CLOGGED STRAINER OR FILTER
Occasionally the strainer or filter in the liquid
line may become clogged with foreign mate-
rial left in the system during installation.
When this happens, the liquid line leaving the
strainer will feel cooler than the liquid enter-
ing. It is badly clogged, some sweat or frost
may appear at the strainer outlet.
2. THERMAL EXPANSION VALVE LEAKS
At the end of the evacuation period partially
charge the system with R134a vapour into
the compressor suction service valve and
recheck the whole system for leaks.
3. THERMAL EXPANSION VALVE STUCK
IN OPEN POSITION
If the expansion valve is stuck in an open
position, there will be an excessive amount of
sweating on the suction line and compressor
crankcase due to the large amount of liquid
being passed into the suction line.
The expansion valve should be checked for
loss of charge or faulty thermal bulb contact
with the suction line.
4. POWER ELEMENT
The power element of an expansion valve
consists of the thermal bulb, capillary tube
and the bellows or diaphragm which actuates
the valve pin.
If this power element is defective or has lost
its charge, the valve will either maintain an al-
most closed position or may close completely.
To test for a defective power element, the
thermal bulb should be removed from the
suction line and warmed by holding it tightly
in the hand.
The valve will open if the power element is
not defective. If the power element is defec-
tive, the valve will remain closed.
5. THERMAL EXPANSION VALVE IM-
PROPERLY ADJUSTED
If the expansion valve is adjusted for too low a
superheat, too much liquid will be passed to
the evaporator. The suction line will be abnor-
mally cold and liquid may “slug” back to the
compressor.
If the expansion valve is adjusted for too high
a superheat, too little liquid will be passed to
the evaporator and the suction line will be
abnormally warm.
Superheat should always be adjusted care-
fully using thermometer and suction gauge.
6. THERMAL EXPANSION VALVE TOO
LARGE
If a replacement thermal expansion valve has
been improperly selected, and its capacity is
too great for the system, the valve will not
maintain a consistently level suction pres-
sure.
The thermal bulb will attempt to control the
flow of liquid at its superheat setting, but the
oversized valve port will pass liquid too rap-
idly.
The presence of liquid near the thermal bulb
will close the valve and the pressure in the
evaporator will drop until the valve opens to
pass another “slug” of liquid.
This “hunting” will cause a suction pressure
variation noticeable on the suction pressure
gauge.
FAULT DIAGNOSIS
