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 |
| MCC key-on delay before clutch engages | 6 seconds |
| MCC temperature control — clutch engages | −2°C to +2°C (28°F to 36°F) |
| Refrigerant type | R-134a (r1234yf is comming in late 2027) |
| Compressor temp (operating) | Near engine temp |
| Pressure switch cuts out (low side) | Below 28 psi |
| Pressure switch cuts out (high side) | Above 384 psi |
Unit Conversions
Quick conversions between the units used throughout this course.
| Conversion | Formula | Used For |
|---|---|---|
| CC (cm³) → fl oz | fl oz = CC ÷ 29.57 | Compressor oil capacity (140–170 CC = 4.7–5.7 fl oz) (every 30 CC = 1 fl oz) |
| oz → lb | lb = oz ÷ 16 | Refrigerant charge weight on the recovery scale |
| kg → lb | lb = kg × 2.205 | Scale max capacity (100 kg = 220 lb) |
| °F → °C | °C = (°F − 32) ÷ 1.8 | MCC thermostat and pressure switch temperature specs |
| °C → °F | °F = (°C × 1.8) + 32 | Same specs, reverse direction |
| in/Hg → psi | psi = in/Hg × 0.4912 | Vacuum pull spec (29.9 in/Hg ≈ 14.7 psi below atmospheric) |
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 (remember that it only takes 10% of a charge to make the static pressure look normal) |
| 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 (open or short) | At Cut-in | 2x per second |
| Compressor Clutch Short to battery | Continuous | 3x 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) | 175–200 psi (2.5 × ambient °F) |
| Compressor housing temp | Near engine temp |
Gauge Pattern Guide
| Condition | Low Side | High Side | Duct Temp | Clue |
|---|---|---|---|---|
| Normal / Good System | 25–45 psi | 175–200 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
Has to be clean. Fan must operate correctly. Bobcat calls for a 20 degree temperature differential. I find more than that normally.
Receiver/Dryer
Relief valve: 535 psi. Replace when system opened. R-Series has an interval replacement schedule.
Evaporator
Freeze protection: EDT cycles compressor. Drain must be clear. Duct temp: 45-53°F normal. Below 40°F possible freeze-up.
System Oil Accounting
Bobcat system holds a full charge of 150–170 cc (5.1–5.7 oz) of oil, allocated at 30 cc (1.0 oz) per
component. New compressor pumps ship pre-filled with a complete system charge (150–170 cc / 5.1–5.7 oz). Adjust the
base charge for whatever was actually done to the system.
| Action | Oil Adjustment |
|---|---|
| Component replaced | +30 cc (+1.0 oz) |
| Component cleaned | +30 cc (+1.0 oz) |
| Compressor replaced without flushing/replacing the rest of the system | −30 cc (−1.0 oz) per component not flushed |
| Dye added | −30 cc (−1.0 oz) per 1 oz of dye added |
| Liquid or suction hose replaced | +30 cc (+1.0 oz) |
| Discharge hose replaced | Measure oil remaining in the pump; make up to 60–80 cc (2.0–2.7 oz) |
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. Bypass test pressure switch. |
Field Quick Reference
Tools Required
Service Intervals
| Item | Interval | Action |
|---|---|---|
| Cab Filters | As needed | Replace or clean |
| Condenser and Evaporator Cleaning | Every Service | Clean fins, check for damage |