I have read on internet and in Tesla Model S service manual book that we can charge refrigerant by high pressure service port (with gas charging machines or another way) as long as there is vacuum in the system and when system is not working.

But high pressure service port is not positioned on the same line at every car. For instance, It has been positioend on the compressor discharge line where refrigerant is gas phase in BMW i3 and It has been positioned on liquid line where refrigerant is liquid phase in every Tesla models.

I am wondering, What is the considerations to choose placement of high pressure service port ?

In addition, If you could give me some additional information about charging gas from high pressure service port, It would be great :)

Kind Regards

  • 2
    "What is the purpose of high pressure service port in cars?" - It allows the connection of a pressure gauge for diagnostic purposes.
    – HandyHowie
    Apr 11, 2019 at 10:49
  • So Can not we charge refrigerant from high pressure service port ? Is it just for diagnostic purposes ? Apr 11, 2019 at 11:10
  • @engineercyprus You can both use it to check the high side pressures, both static and running, and to charge refrigerant, but you'll be able to charge refrigerant through the high side service port only with the compressor off, after vacuuming the system (if you don't vacuum, refrigerant won't get pulled inside).
    – Al_
    Apr 13, 2019 at 10:15

4 Answers 4


HandyHowie is correct, kinda sorta. When you want to understand, really understand automotive mechanics you gotta start with a close look in your wallet.

The #1 reason for the Air Conditioning (A/C) compressor to condenser hose discharge port is to save the automotive company money. Look at that port carefully. Notice it's a whole lot larger in internal diameter than the low pressure / suction hose port? That's not a mistake. Generally A/C discharge ports are located on the top of the engine, just aft of the radiator. That's not a mistake either. This location is to enable highly efficient evacuation and refrigerant fill at the automotive assembly plant. Generally a vehicle needs four minutes or so of high vacuum evacuation. Test and fill take under 60 seconds or so. That port is designed for high efficiency hook up and flow. Note that this evacuation and fill process is complete before the vehicle is ever started for the first time.

The fact that it is a convenient diagnostic location is secondary.

Yes, you can perform complete system evacuation and fill thru the discharge port. What you can't do there is add just one can of refrigerant. In fact to even try that is downright dangerous. Frankly I'd much rather see you just get used to doing all service evacuation and fill at the low pressure suction side of things. Good practice. Safety first.

Reminder: Follow Mr. Refrigerant thru the system.

  1. Refrigerant gas enters the A/C compressor. There it gets pumped/compressed to a very hot, very high pressure gas.

  2. This hot gas (200-220 deg F?) enters the condenser. There it cools down. As it cools, it changes state from a gas to a liquid.

  3. This liquid is routed to a metering system (fixed oriface tube, expansion valve or perhaps a variable control valve)

  4. A small amount of liquid is allowed to escape into the evaporator core. When the this small amount of liquid enters the evaporator core, it goes.. Whoa. Look at all this space? I think I'll get some elbow room and expand a little bit. There is a state change from liquid to gas. When it does that, it absorbs the heat of vaporization to match that state change. And losing that heat, makes the core mighty cold. Hopefully around 35 to 40 degrees F or so.

  5. As your vehicle interior cools, the now warm refrigerant gas is returned to the A/C compressor where the cycle continues.

Does that make sense to you?


"What is the purpose of high pressure service port in cars?"

It allows the connection of a pressure gauge for diagnostic purposes.

"Can we charge refrigerant into the system by high side service port?"

If the compressor is not running, and you pump a weighed amount of refrigerant into the system, I don't see why you cant charge the system from the high pressure side. I believe that while the compressor is not running, all the refrigerant in the system will be in gas form anyway.

Under normal circumstances, I would have the engine running, so that the air conditioning compressor sucks the refrigerant in through the low pressure side.

  • Thank you for your answer sir ! But Is It possible to refrigerant pass through expansion valve as a gas form ? Because most of cars have their high pressure service port on pipe where is between condenser and expansion valve. So, It would not be possible refrigerant pass through expansion valve when It charge from high pressure service port as a gas. Apr 11, 2019 at 14:41
  • I can't imagine why gas wouldn't be able to get through the expansion valve, but I am sure that there would be enough space in the condenser and pipework back to the compressor to accept the gas anyway.
    – HandyHowie
    Apr 12, 2019 at 10:11
  • @engineercyprus Most automotive expansion valves, especially ones employed with variable displacement compressors, are designed to never fully close. It's either a special bulb charge or a "nick" in the valve seat. Also, when the system is vacuumed from the high side only, the discharge valve in the compressor stays opened and the low side is vacuumed from there. The expansion valve has an equalizer line and the valve's diaphragm is exposed to the low side pressure (which acts on the underside of the diaphragm) from this line. When the low side pressure decreases so much, the valve opens.
    – Al_
    Apr 13, 2019 at 9:57
  • Also, whenever gas passes through the expansion valve, it produces a loud hissing noise akin to the one a inflatable toy makes when you fill it with air, and that's one easy way of diagnosing a system that is very low on refrigerant or that isn't subcooling properly (in that case, however, very good hearing is required).
    – Al_
    Apr 13, 2019 at 10:06

No particular consideration. Service ports are located where it's most comfortable. For example, on bus AC systems, both service ports, low and high sides, are located right on the compressor, for ease of access (here's an interesting video https://www.youtube.com/watch?v=HpVRSLg8sk0 ; compressor is a Thermo King X430-X426 by the way).

However, if you inject liquid refrigerant into the high side service port (after vacuuming the system so that the liquid will be actually pulled inside it) you can be sure that it will never slug the compressor.

Why? Due to the discharge reed valve/high side valve conformation, the same conformation that makes the valve open only when the piston/scroll/vane is progressing towards the valve (so that anything coming from the high side in a backwards fashion will immediately cause the valve to close).

If you instead inject the liquid into the low side service port, even in a vacuum, there's a chance that the liquid refrigerant will go inside the compressor and both slug it as soon as it's turned on and deprive it of lubricant (because the liquid refrigerant acts as a solvent for the lubricant, to the point that it's often used to flush the system, in a closed-loop arrangement that won't let any refrigerant escape of course). Especially if the low side service port is located very close to the compressor suction port.

Injecting refrigerant as a vapor in the low side with the compressor running doesn't require any vacuum to be done simply because it's the compressor itself that is generating the vacuum. However, it's always best to avoid the low side-compressor on-no vacuum-vapor-by pressure charging process in favor of the high side-compressor off-in vacuum-liquid-by weight, because the compressor is running with a reduced oil circulation rate while the refrigerant is very low, and compressor wear is very fast in this condition. Also, variable displacement compressors rely on a precise refrigerant charge to work correctly and can adjust running pressures, so that you necessarily need to charge by weight or you risk both undercharge or overcharge.

Never ever charge liquid in the low side when the compressor is running. Never ever charge anything in the high side when the compressor is running. For the latter, engine on=compressor running even when the compressor has a clutch... turn the engine off and keep the key in your pocket.


You must check high side pressure. If high side pressure is too high it tells you there a blockage in the system. Bad expansion valve, block filter if have one. Running with too much high side pressure will destroy your compressor over time

  • This adds nothing to the existing answers.
    – Chenmunka
    Jul 14, 2023 at 15:57

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