Car AC Service for Dummies

Let’s say you have a habit of collecting ancient cheap vehicles and you notice that sometimes the AC system doesn’t work so hot. A few examples:

  1. The jabroni offering the vehicle says the AC “just needs a recharge” (always a lie) and when you do recharge it, it lasts about 18 hours and starts hissing refrigerant and oil out of the seam of the compressor.
  2. The AC works but lacks some pep, cycles on and off when it should be running continuously, etc.

I’m going to walk you through how to do AC work!

Nick’s qualifications:

  1. A lot of time spent watching YouTube videos trying to figure out how the hell you do this.
  2. Repairing 2 car AC systems.
  3. Buying a lot of toys/equipment.

How to evaluate AC performance

The yahoos over at your Vehicle Manufacturer probably provided some helpful tips on how to evaluate your performance of your AC, particularly in the Factory Service Manual. An example is provided here for your reference from the Honda Accord 6th gen FSM:

If everything is hunky dory, you should be able to use a good old pen, a thermometer, the weather report, and a set of manifold gauges to evaluate your system performance. Measure:

  • The temperature of the ambient air, relative humidity.
  • The system high and low pressures while running at slightly elevated RPM.
  • The system air delivery temperature in the cabin with no occupants.

For example, if it’s 77 degrees and 50% relative humidity, you can draw little lines through the middle of each of those black bars and see that your:

  • Delivery temperature should be 47 degrees.
  • Your intake pressure should be something like 16 psi.
  • Your delivery pressure should be something like 170 psi.

If things are wildly outside of those limits, you have problems. Here are a list of common problems:

  • Compressor unhappy, exploding.
  • Low refrigerant charge — over time, all car systems will lose some refrigerant and will require more be added. In my Honda from 2002, only 11.8 oz was left inside with unclear previous service history.
  • Condenser completely covered with gross detritus, dents, and bug juice. This will result in the condenser getting much hotter to dissipate the required amount of heat energy across what is effectively insulation and reduced airflow.
  • For vehicles with electric fans, both fans should be spinning and not making any horrifying noises when the AC is on. If they’re not, this will destroy AC performance.

For my Honda, the delivery pressure was exceptionally high and the charge was quite low.

Evaluating AC system charge

As the book goes, the primary way to evaluate AC charge is by weight, i.e. by removing it from the vehicle and weighing it.

Low charge is not just bad for performance reasons, but the oil in the system is designed to mix with the liquid refrigerant and zip along the system. If the refrigerant level is very low, not enough oil will move through the system and you can end up with an unhappy compressor due to insufficient oil.

The major TL;DR is that it is very difficult to evaluate correct AC charge from the pressures alone. If I had been looking exclusively at the pressures on my Honda, I would have seen the high side exceptionally high and concluded that my vehicle was overcharged. It is probably a bad idea ™ to charge using one of those AutoZone cans that just has a gauge on the low side, unless your system was entirely empty before hand and you can charge by weight. At best you have to be very careful to check the internal temperature in your car as you charge; overcharging is easy when topping off which can result in bad things not to mention overall poor system performance.

If you have been convinced that you should weigh the charge using a recovery machine, skip this next section.

Superheat/Subcooling charge estimation

Best recommendations aside, you can use the superheat/subcooling method to evaluate the system charge. The Honda uses a Thermostatic Expansion Valve (TXV), whose job it is to regulate the amount of liquid refrigerant entering the evaporator coil and maintain a certain amount of “superheat.”

“Superheat” is measured by comparing the temperature of the vapor line exiting the saturation temperature on the liquid side. In a functional AC system, liquid enters through the expansion valve into the evaporator and then meanders through the evaporator soaking up heat energy as it boils. The boiling point is determined through the pressure, which is constant across the entire vapor side of the line. In a functional system, all the liquid will have boiled away inside the evaporator and then slightly more heat energy absorbed by the refrigerant in the liquid state — it is very bad ™ to have liquid making its way back to the compressor, since the compressor is not expecting to have liquid slam into it and it can be destroyed if a large quantity of liquid enters it. A TXV will basically adjust the flow of liquid refrigerant to ensure that we have 12-15 degrees of “superheat” or heat above the saturation temperature.

For this reason, you have to evaluate car AC systems with a TXV using the subcooling method, or the difference between the saturation temperature on the high side and the temperature exiting the condenser. See this video on how to do this.

The Repairs

First, go ahead and check for leaks. It’s quite easy to use a halogen leak detector and check all the valves and such on your vehicle. You’ll want to fix any leaks.

Compressor: If your compressor is oozing, it will have to be replaced. FYI cheap replacement parts can be acquired online — for my GMC Sonoma I was able to acquire a Sanden style aftermarket part for just over $100. GM probably wins the cheap aftermarket parts contest. If your compressor does fail, you need to replace the condenser and receiver-dryer and flush the evaporator and lines to minimize the chance that various detritus will meander back over to your shiny new compressor and abrading it to death. I did a video on my GMC Sonoma doing this process and it was…fun.

Valve cores: if your service ports are leaking refrigerant, you’ll want to replace the core. It is not necessary to replace the entire AC line unless additional bits are leaking. You can get pressurized valve core replacement tools on Amazon or you can drain the system and use one of the simple screwdriver type things. Bear in mind that if you opened the valve core while the system was empty, you will have to vacuum the system afterwards to remove the air.

Condenser: Probably your best bang for your buck in rejuvenating old AC systems is replacing the condenser, especially if it has been assaulted by 400k miles of rocks and bug essence. My head pressures were quite literally halved by doing this, which will extend the life of your compressor and dramatically improve performance.

Evaporator: before beginning AC investigations, it is appropriate to pray to the evaporator gods that yours is intact. Replacing these ranges from moderately painless (like in my Honda Accord requiring maybe 2 hours) to utter death-defying hell of disassembling an entire dashboard billed at 15 hours of labor alone for a $20 part in some modern GMs. Sticking your leak detector into the dashboard region should be enough to determine your fate. Make sure not to tweak your evaporator if you’re in the process of connecting or disconnecting lines (use the old dual crescent wrench strategy on either side of the fitting to avoid torquing the soft aluminum). In failed compressor cases you can sometimes get away with just flushing the evaporator as long as there is some reasonable receiver-dryer preventing the detritus from getting back to the compressor (or so I’m told…).

Recovering Refrigerant

Recovering is the major bit of work before you can just start wrenching.

Basically as a novice, just get a recovery machine, scale, and cylinder and pull out from the vapor side. The instructions should do you. I got a Robinair RG3, here’s a quick video:


Probably the simplest part. From an empty system, vacuum and then add by weight. You want to get a good vacuum on the system before and make sure it holds (to be totally PC use a vacuum gauge to measure and do a 10 minute standing vacuum test to make sure the pressure stabilizes).

The other important tidbit: when disconnecting, you want to bleed the liquid refrigerant out of the high side line back into the low side, leaving all your hoses at the relatively low vapor side pressure. To do this, you want to close the high side at the service port, cap the center fill port (or leave your can attached). open the low side all the way and then sloooooowly crack the high side valve to let the liquid flow into the low side. In this fashion you won’t horribly burn yourself with liquid refrigerant trapped in the high side line. Look up “manifold gauge disconnect procedure” on YouTube.

  1. Easiest/best for novice to charge exclusively with vapor. Things will go a lot faster if you secure a valve core removal tool, which can allow you to fill the system without the valve core restriction in place. You don’t want to dump liquid into the low side since you can trash the compressor if you’re not careful. It is possible to charge liquid first into the high side to shorten charge times as well, but if you’re doing it at home you can charge exclusively through the low side with vapor through the existing valve core, it will just take a while (perhaps 20-30 minutes).
  2. Weigh the cans before and after to determine how much refrigerant you loaded in.
  3. You should find pure R134a, not oiled or with “stop leak” in it. It’s available for like $6/can at Harbor Freight and the AC capacity is just shy of 2 cans. If you need to add oil do so separately according to the recommended manufacturer replacement volume. With the system opened you can just pour it in or use a vacuum pump to pull from one port while you add it to the other after removing a valve core.
  4. If you don’t know what was in your car, it’s probably best to use fresh R134a — it’s quite cheap to buy. If you’re going to reuse what you recovered, you want to carefully check the pressure-temperature relationships for your refrigerant after the cylinder has sat for ~24 hours and the temperature has stabilized. If the pressure is off, the refrigerant may be contaminated — air contamination is the most common, but it will settle to the top of the refrigerant and can be bled off after the tank has been sitting, and then after several more hours you can recheck the pressure-temperature relationships to make sure it’s as expected. FYI it’s common for used car dealerships to reuse highly questionable refrigerant to get their cars working (at least temporarily) so if you’re unsure, best to not reuse, especially if bleeding the top bit of the tank doesn’t result in pressures spot on for R134a.

GMC Full System

Honda Condenser Replacement

When I got the car, the AC was working but it was a little weak. I hooked up manifold gauges to it and the high side was reading something like 235 psi (which corresponds to a saturation temp of 141 degrees for R134a). That was pretty concerning because it’s well outside the usual recommendation for nominal system performance, not to mention running the AC on a hot day like that could kill the compressor.

I attempted to clean the condenser up front but 300k+ highway miles will leave 90% bugs and dents on the front, and it wasn’t very effective. I know there are stronger cleaners out there, but I opted to just get a replacement condenser unit since it was like $50. You’ll want to get a new dryer and an o-ring set while you’re at it.

I ended up getting a recovery machine, cylinder, and scale off Amazon and recovered the refrigerant, and was interested to find that the charge was actually quite low. Local shops wanted some absurd amount of money to recover the refrigerant (like $120). From what I’ve told all compressors will leak a marginal amount of refrigerant over the life of the car (some say up to 1 oz per year). The system capacity is 21.2-22.9 oz of refrigerant and I only removed about 11.8 oz for a 2002 MY. Unclear when/if it was last serviced.

The condenser replacement was pretty straightforward: after removing the bumper, there are two clamps on the front of the condenser and then it pops out. I was able to get the condenser out without removing the radiator or hoses by undoing those clamps and pushing it towards the engine — the condenser was pretty thin after the clamps are removed. After that I added a little over an ounce of PAG46 oil to the system to make up for the stuff trapped in the condenser when it was replaced.

After that I replaced all the o-rings involved in the replacement, vacuumed the system, and recharged with just shy of two 12 oz cans. While I was in there I also replaced the cabin air filter (and there was like half a tree worth of seeds inside that poor box). A quick leak check shows everything holding just fine.

The high pressure is now reading 120 psi with much better performance, which aligns pretty much spot on with the manufacturer recommendations (this is a great article)

As a side note, the service manual does recommend replacing the TXV (the expansion valve inside the cabin with the evaporator) when you open the AC system, however it’s a bit of a chore so I wanted to test everything prior to cracking that open and sure enough it’s working quite acceptably. I do have a spare just in case, but I think i’ll leave that for now 🙂

For anyone interested in getting into AC work, there are a lot of great videos by certified AC techs on YouTube which will help you understand procedures, terminology, and best practices. The equipment you need isn’t necessarily cheap but it is quite handy to have since you can use it for all your vehicles.