How to Recharge your Car’s AC System

Pulling off proper air-conditioning (AC) work by yourself would require some tools. I did this recently and my cabin now feels like a trip to Antarctica! I detail the procedure for recharging an empty AC system going by industry best practices. I also cover why certain steps; often omitted during AC work, should always be followed to prevent future costly repairs.  My AC system had slowly lost its Arctic effect until it got to the point where it performed pretty poorly on hot humid days. Tests clearly confirmed it was low on charge. Furthermore, a mechanic had 3 years ago replaced the broken magnetic clutch for my compressor and didn’t pull a vacuum on the system before recharging it. My aim this time was to evacuate, vacuum and recharge the R134a AC system on my Honda Accord by weight. Simple.

Air conditioning systems can be recharged either by weight; a specified weight of refrigerant is added, or by pressure; refrigerant is added until certain recommended pressures are reached for both low and high sides. Charging by weight is more accurate as pressure can be affected by ambient temperature, thus, the service ports will read different pressures depending on the temperature of the day.

Safety Precautions for Air Conditioning work

Spilling or spraying of refrigerant into the eyes can cause blindness and on the skin; frostbite. Safety goggles and gloves should be worn.

Finally, NEVER connect these small cans of refrigerant to charge through the high side. The can explode. Also, never pump compressed air into the AC system, the mixture of air and refrigerant can ignite!


  • Safety goggles
  • Rubber gloves
  • AC manifold gauge set (with high & low side couplers)
  • Vacuum pump
  • Cans of required refrigerant
  • Can tap
  • Specified AC compressor oil
  • Digital precision weighing scale or equivalent: Bathroom scales are not appropriate as they just don’t have the precision required. An AC refrigerant scale would be the best.
How to recharge an AC system

Key tools for recharging an AC system

Getting the right refrigerant and compressor oil

Every car would usually have a sticker under the hood listing the following:

  • Type of required refrigerant
  • Quantity of refrigerant for the perfect charge
  • Type of compressor oil
AC system sticker

Under-hood AC system sticker

Another source of this will be your car owners manual. The car manual lists ND-8 as the required compressor oil


1. Prepare the AC manifold gauge set: Hang the gauge set on the locking hook of the hood. Ensure the taps for the low (blue) and high (red) side lines are in the closed position. If any of these are open refrigerant will spill out from the service line (yellow tubing) once connected to the system. Attach the respective couplers to the low and high side lines.

AC manifold gauges

Preparing the manifold gauge set

2. Locate the low and high side service ports: Follow the AC pipe work to find these. On the 2003-2007 Accord these are to the rear of the right side of the engine. High-side port caps are usually marked “H” and the high-side lines are narrower than the low-side lines which usually have their service port caps marked “L”. Screw off the caps.

Recharging AC on 2003-2007 Accord

High & low side ports covered by blue caps

Replacing AC refrigerant

Service port caps off

Once I screwed off the cap for the high side port, it became clear where my leak was coming from; it was hissing audibly. I would cover the replacement of the defective valve core shortly.

3. Connect the couplers to the service ports: The high-side and low-side ports are of different sizes and so will only fit their respective couplers. High-side couplers are typically marked red and low-side blue.  Connect to the high and low side lines to their service ports.

Quick couplers for AC service ports

Low and high pressure lines connected to service ports via quick couplers

4. Read the manifold gauge pressures: This step is more of a diagnostic step and can be omitted especially if recharging by weight. I just used it to confirm that my AC system was undercharged.

First, with the engine off both low and high side gauges should be about the same and range around 80-120psi depending on ambient temperature. Mine was low and read approximately 70psi on both gauges. This is known as static pressure.

Charging an AC system

Both low and high side gauges reading 70psi approx

Next, start the car and turn on the AC system. This next step totally confirmed the low charge in my AC system. The low side read 8psi and the high side 112psi. At an ambient temperature of  27°C, these are low. Acceptable readings for 27°C would be at least 40psi and 175psi for the low and high side gauges respectively. NB: the compressor should be engaged for you to take these readings.

Reading AC manifold gauges

Readings typical of a low charge at 27 degrees Celsius

Next, the system was evacuated. The steps are not covered in this procedure. There are special machines for this.

Recharging air conditioning systems

An empty AC system will read zero on both gauges

5. Repair any known leaks: Mine was coming from the high-side valve core. Using a valve core remover remove the valve core and install a new one.

AC valve core

Removing the leaky valve core on the high-side port

service port valve core

Leaky valve core removed

valve core

Old and new valve core side by side

6. Vacuum the AC system: This is necessary as it removes air and moisture from the system.

This is where many people fail. If the system is not “vacuumed” after being opened (e.g changed a compressor etc) the moisture and air left behind reacts with the refrigerant to form acids which corrode the metal parts of the AC system leading to leaks. Metal shavings formed from the corrosion will damage the compressor.

Attach the free end of the yellow (service) line to the vacuum pump

why vacuum an AC system

Attaching the service line to the vacuum pump

Start the vacuum pump. Then open the low-side and high-side taps on the manifold gauges. Vacuum should be applied to the system for a minimum of 30minutes. The target vacuum on the system is 30 inches of Hg (inHg).

Filling an AC system

Service line connected to the vacuum pump

Recharge automotive AC systems

30inHg vacuum applied on the system

After 30minutes have elapsed, close both taps (high and low) on the manifold gauges and turn off the vacuum pump.

Testing for Large Leaks in an AC System

Allow the the setup to stand for an hour, on an AC system without a large leak, the vacuum would remain at 30inHg and not drop. If the vacuum drops, vacuum the system again and observe for another hour. If the vacuum drops again then there’s a leak that should be traced and repaired.

7. Add compressor oil (PAG oil):  There are specific volumes of oil to add based on the work done on the AC system. If you evacuated the system, the rule will be to return the amount of PAG oil drained. If repairing a leak, for the 2003-2007 Accord, Honda recommends adding 25mls. Each manufacturer lists type and amount of oil to add in different scenarios. ND-8 oil is the specified oil for the 2003-2007 Accord.

Disconnect the service line from the manifold gauge and add the recommended PAG oil volume to the line. The oil will be sucked in with the refrigerant during the charge process.

adding oil to an AC system

I used a syringe to measure the amount of PAG oil

how to add oil to AC systems

PAG oil added to the service line

8. Calculate the required charge for the AC system: For my 2003 Accord 500-550g provides a full charge and I had 2 cans of 340g each. My target charge was 520g meaning I had to add one can plus an additional 180g of R134a.

9. Connect the refrigerant and purge air from the service line: Fully open the can tap and screw on firmly to the refrigerant can. Connect the service line to the refrigerant can, screw the can tap down fully to pierce the can. Then open the can tap again.

To purge the service line of air, gently and slightly  unscrew the manifold gauge end of the service line until some refrigerant escapes then re-tighten. Take note of the weight of the can on the digital scale after this.

Charging a an AC system

Charging about to commence

10. Charge the AC system: I describe charging as a gas as it is safer. Slowly open ONLY the low-side tap on the manifold gauge set. The vacuum will begin to draw refrigerant into the system. Shake the can continuously and turn the can between the 12 and 3 o’clock positions every 2-3seconds. 

charging air conditioning systems

Can in 12 o’clock position

How to recharge an air conditioning system

Can in 3 o’clock position

Weigh the can at intervals. Once you notice refrigerant has stopped leaving the can. Close the low-side tap. Start the vehicle. Put on the AC setting the temp on coolest and fan speed at highest. Wind down the windows for the front doors. This will ensure that the AC compressor remains on to draw in the refrigerant. Then slowly reopen the low-side tap again and continue with the shaking of the charge process. You may need to use a rag to hold the can as it gets sub-zero cold! This is normal. If you need to change to a new refriegerant can, bleed off air from the service line as earlier stated, and continue with charging.

Remember to use the digital scale to deliver the target charge amount. Undercharging will lead to poor performance while overcharging will in addition to poor cooling, damage the AC compressor.

Once done, enjoy the Arctic chill 🙂


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  • ilario

    Nice step-by-step job, thank you!
    Just before step5 you say “Next, the system was evacuated. The steps are not covered in this procedure. There are special machines for this”
    I don’t understand what you are telling about.. this is the step6 (Vacuum the AC system), isn’t?

    • Thanks for the compliment!
      There are special machines that collect the gas from the AC system. The gas causes global warming and should not be discharged into the atmosphere. The vacuuming step is different.

  • Chandler Smith

    AC systems are ancient technology anyways. Someone please please vent better car AC system.

  • Jonas E

    Nice one, Piyke.

    I observed you didn’t attach a vacuum meter to the service line (or, rather, a service line with a vacuum meter attached).

    I’ve seen procedures that require pulling down to 500 microns as the more reliable way to pull the proper vacuum, as opposed to vacuuming for a given amount of time (30+ mins) and then reading off -30 on the manifold gauges. The power of the vacuum pump (1/4, 1/2 HP…) in use would determine how long it takes to pull the required void.

    That said, our car AC techs are doing a disastrous job in this country. And don’t get me started with the home AC guys. They’re even worse. Not a single one of them owns a vacuum pump, yet they call themselves “AC guys”. It’s a disaster!

    AC jobs are typically expensive for the single reason that it requires specialised equipment (especially for the evacuation and vacuuming steps). And these are two crucial steps our AC guys never do. Air is 78% nitrogen and the compressor isn’t designed to compress nitrogen. No wonder compressor failure is so rampant.


  • Braveheart

    Great job. Thanks for this illustration.Is there anywhere I can get a vacuum pump in Nigeria?

    • Hi Braveheart many thanks for reading. I can send you the contact of the shop I bought my vacuum pump from in Lagos.
      For privacy reasons, I will send the number via the email.

  • Joe

    Piyke I want to learn from you, 1/1.

    • Hi Joe,
      Thanks for the compliment. You know I’m not a mechanic, just a DIY guy however, you can reach me through the contact us page. Lets talk.

  • Where is your shop? Need to evacuate my account system and trace for leaks.

    • Hello friend,
      Thanks for dropping by and reading through. Unfortunately, I do not own a shop for now. What city are you in?

  • Really nice Job Piyke

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