EPA 609 System Evacuation & Recharging Procedures — Questions and Answers
Question 1: What is the main purpose of evacuating (pulling a vacuum on) an MVAC system before recharging?
- To cool the compressor
- To remove air and moisture from the system (Correct answer)
- To recover the refrigerant
- To add lubricant to the compressor
Correct answer: To remove air and moisture from the system
Evacuation removes non-condensable air and boils off moisture. Moisture can form acids and freeze at the expansion device, and air raises operating pressures and reduces cooling.
Question 2: Why does applying a deep vacuum help remove moisture from an MVAC system?
- Vacuum heats the water to 212°F
- Lower pressure lowers water's boiling point, so it evaporates and is pulled out (Correct answer)
- Vacuum freezes the water so it can be drained
- Vacuum turns water into refrigerant oil
Correct answer: Lower pressure lowers water's boiling point, so it evaporates and is pulled out
As pressure drops, the boiling point of water drops too. Under a deep vacuum, moisture boils at ambient temperature and the vacuum pump pulls the vapor out.
Question 3: What is the most accurate way to recharge an MVAC system with refrigerant?
- Charge until the sight glass is clear
- Charge until the low-side gauge reads 50 psi
- Charge by weight to the amount on the vehicle's specification label (Correct answer)
- Charge until the vents feel cold
Correct answer: Charge by weight to the amount on the vehicle's specification label
Modern systems hold small, precise charges. Weighing in the exact amount given on the under-hood label or in service information is the only reliable way to get the charge right.
Question 4: After the vacuum pump is shut off, the gauge shows the vacuum rising (pressure increasing) within a few minutes. What does this most likely mean?
- The system is ready to charge
- There is a leak or remaining moisture in the system (Correct answer)
- The refrigerant charge is too high
- The compressor clutch is engaged
Correct answer: There is a leak or remaining moisture in the system
In a vacuum decay test, a properly sealed, dry system holds its vacuum. If the vacuum rises, air is leaking in or trapped moisture is still boiling off, so the system should not be charged until the cause is fixed.
Question 5: What is a likely result of overcharging an MVAC system with refrigerant?
- Lower head pressure and better cooling
- Excessive high-side pressure, poor cooling and possible compressor damage (Correct answer)
- The system automatically vents the extra refrigerant safely
- No effect, since extra refrigerant is stored in the accumulator
Correct answer: Excessive high-side pressure, poor cooling and possible compressor damage
Too much refrigerant raises head pressure, reduces cooling efficiency and can cause liquid slugging, which damages the compressor. It may also trip high-pressure safety switches.
Question 6: When charging liquid refrigerant into the high side of an MVAC system, what condition should the engine be in?
- Running at high idle with the A/C on
- Running at normal idle with the A/C off
- Off (Correct answer)
- Running with the cooling fan unplugged
Correct answer: Off
Liquid charging through the high side is done with the engine off. With the engine running, high-side pressure can force refrigerant back into the charging source and liquid can reach the compressor, which is dangerous for the technician and the equipment.
