Reference
Specs & Codes
Key Specifications
| Parameter | Value |
|---|---|
| Duct temperature — normal operating | 45–53°F |
| Pressure relief valve opens at | 535 psi |
| R-134a boiling point | −15°F |
| R-12 boiling point (reference) | −21°F |
| MCC key-on delay before clutch engages | 6 seconds |
| Refrigerant type | R-134a |
| Compressor temp (operating) | Near engine temp |
Diagnosis Rules of Thumb
| If you see… | Consider… |
|---|---|
| Clutch not engaging, low pressure | Low charge, pressure switch cutout (cuts out below 28 psi on high side), or electrical |
| Both pressures elevated | Overcharge OR blocked condenser — check airflow |
| Erratic pressures, intermittent cooling | Moisture — TXV freeze/thaw cycle |
| Cold duct, low airflow | Clogged cab filters, weak blower, frozen evaporator |
| Warm duct, gauges normal, very long cycle time | Heater valve bypassing hot coolant |
| Clutch not engaging, pressures normal | Electrical — voltage drop, AC switch, EDT fault |
| Oil on hose or fitting exterior | Refrigerant leak at that point |
| Compressor running hot | Low oil quantity — check oil accounting |
MCC Flash Codes
| Malfunction | Detection Interval | Status Light |
|---|---|---|
| Temperature Sensor | Continuous | 1x per second |
| Compressor Clutch | At Cut-in | 2x per second |
| Compressor Clutch Open | Continuous | 3x per second |
| Clutch Short or Current High | At Cut-in | 4x per second |
Normal Operating Baseline
| Parameter | Normal Value |
|---|---|
| Duct (louver) temperature | 45–53°F |
| Low-side pressure (approx. 70–80°F ambient) | 25–45 psi |
| High-side pressure (approx. 70–80°F ambient) | 150–250 psi |
| Compressor housing temp | Near engine temp |
Gauge Pattern Guide
| Condition | Low Side | High Side | Duct Temp | Clue |
|---|---|---|---|---|
| Normal / Good System | 25–45 psi | 150–250 psi | 45–53°F | Baseline. Memorize this. |
| Low Charge | ↓ Low | ↓ Low | Warm | Both sides low. Clutch may not engage (pressure switch cuts out below 28 psi on high side). |
| Overcharge | ↑ High | ↑↑ Very High | Warm | Both sides elevated. High side excessively high. |
| Blocked Condenser | ↑ High | ↑↑ Very High | Warm | Same pattern as overcharge — check airflow first! |
| Blocked Evap / Cab Filters | ↓ Low | ↓ Low | Cold (no airflow) | System works, no airflow to cab. Blower check too. |
| Moisture in System | Fluctuates | Fluctuates | Intermittent | TXV freezing and thawing. Erratic cycling. |
Component Specifications
Compressor
Clutch current: 3 amps. Oil capacity: 140–170 CC (4.7–5.7 oz) — this is the capacity for the entire system. New pumps ship with a full charge of oil.
Condenser
Airflow requirement: minimum 400 CFM. Fin spacing: standard. Clean monthly in dusty conditions. Check for damage.
Receiver/Dryer
Relief valve: 535 psi. Replace when system opened. Desiccant capacity: finite. Check service manual for replacement intervals.
Evaporator
Freeze protection: EDT cycles compressor. Drain must be clear. Duct temp: 45-53°F normal. Below 40°F possible freeze-up.
Electrical Specifications
| Component | Specification | Test Method |
|---|---|---|
| AC Switch | Voltage drop < 0.5V when closed | Voltage-drop test across switch |
| Compressor Clutch | Current flow 3 amps | Use your high-current test light to substitute this load |
| Pressure Switch | High-side switch cuts out below 28 psi and above 384 psi | Pressure test with gauges |
Field Quick Reference
Tools Required
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Documentation
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Service Intervals
| Item | Interval | Action |
|---|---|---|
| Cab Filters | As needed | Replace or clean |
| Condenser and Evaporator Cleaning | Every Service | Clean fins, check for damage |