Bobcat systems use either TXV (Thermal Expansion Valve) or orifice tube depending on model. TXV provides better control but is more complex. Orifice tube is simpler but less efficient.
Variable metering device that responds to evaporator temperature and pressure. Provides precise control of superheat. More complex but better performance.
Fixed orifice that provides constant pressure drop. Simple design, no moving parts. Less precise control but reliable and less expensive.
Mounted on evaporator housing. Has bulb attached to suction line. Look for thermal bulb and capillary tube, adjustment screw, and equalizer line.
Thermal bulb senses evaporator outlet temperature. Capillary tube transmits pressure to power element. Adjustment screw sets superheat.
When evaporator gets warmer, bulb pressure increases, opening valve more. When cooler, bulb pressure decreases, closing valve. Maintains constant superheat.
Balances pressure across valve. External equalizer connects to suction line downstream of bulb. Critical for proper operation.
Usually in high-side liquid line near evaporator. May have protective cover. Look for small tube with filter screen.
Precision-calibrated tube creates specific pressure drop. Upstream filter protects from debris. Color-coded by size/orifice.
Provides fixed metering regardless of load. System performance varies with conditions. Simpler but less efficient than TXV.
Different colors indicate different orifice sizes. Check service manual for correct color for specific model.
Measure evaporator outlet temperature and pressure. Calculate superheat. Normal: 8-12°F for TXV systems. High superheat indicates undercharge or restricted flow.
Look for ice formation on evaporator or suction line. Low superheat can cause freeze-up. Check TXV operation and charge level.
Check for damage, proper bulb placement, secure connections. Verify equalizer line is connected and not kinked.
Adjust superheat using TXV adjustment screw. Turn clockwise to decrease superheat, counterclockwise to increase. Make small adjustments.
| Symptom | Cause | Diagnosis |
|---|---|---|
| Evaporator freeze-up | TXV stuck open, low charge, faulty bulb | Ice formation, low superheat, check TXV operation |
| Poor cooling, high superheat | TXV stuck closed, restricted orifice, low charge | High superheat, warm duct temp, check pressures |
| Intermittent cooling | Moisture freeze at TXV, faulty bulb connection | Erratic operation, check for moisture, verify bulb placement |
| Low suction pressure | Restricted orifice, TXV stuck closed | Low pressure, normal high pressure, check expansion device |
| High suction pressure | TXV stuck open, overcharge | High pressure, warm suction line, check TXV operation |
Always replace expansion device when compressor fails (metal contamination). Replace O-rings and verify correct type for model. TXV requires proper bulb placement and equalizer connection.