Understanding Basic AC Troubleshooting

Today the AC on my 2005 Legacy just felt really weak cooling down my car after it sat in the sun. Later when the outside temperature dropped it seemed like the AC was sort of working but not great. I could hear/feel the compressor kicking in and I did not notice the compressor cutting out, just not enough cold air.

I am not going to fix this myself but I want to be at least a bit educated on the possibilities. I feel like so many mechanics guess at what is the problem when it comes to air conditioning.

If the pressure is low on the system, that means a leak and something needs to be replaced, right? Put some UV dye in the system, find the leak replace the part. Recharge and good in theory?

How do you verify if the compressor is bad? If the pressure is good and the temp is low is it assumed to be the compressor? Or how do you know when it is another part of the system?

I recall hearing it is good to replace the evaporator when replacing other parts like the compressor, is that the normal rule?

When does the entire system require a flush? Only when bits break apart internally?

Last time I went through this is was with a much older crappier car, the AC never really worked so I am skeptical now.

Why not attempt to do it yourself? By the time you pay anyone to fix anything you can afford a cheap set of gauges and vacuum pump anyway!

If the pressure is low then yes there is a leak that should probably be repaired. Refrigerant doesn't get 'used up', it only leaks out.

The system consists of 4 major components. The compressor, condenser, metering device, and evaporator. Flow moves through the system (which is a loop) in that order. The compressor creates flow and the metering device acts as a restriction to create a pressure differential. The system thus has a 'high' pressure side and 'low' pressure side. The condenser is on the high pressure side and the evaporator is on the low pressure side, with the compressor and metering device being the dividing lines between the two 'sides'.

The first thing you need to do is verify what pressure is in the system. R134 gauge sets are not very expensive. About half an hour of a shop's labor rate if you want to think of it that way. When the ac system is not running the pressure will equalize throughout the system and read the same on both high and low side gauges. This is your 'static' pressure and should be somewhat close to ambient temp as a rule of thumb. If you are below 70-something degrees pressure psi will be slightly lower than ambient temp, if you're above it will be slightly above. This is your first clue as to whether the 'charge' or amount of refrigerant in the system is correct. So, for example if your engine bay is fully cooled off and it's 85f outside but you have 60psi in the system, it is undercharged.

We know you still have some pressure in the system because the compressor runs at all and it cools somewhat. There are usually both high and low pressure cutoff switches that won't allow the compressor to engage if pressure it outside a safe window. When the system is empty the compressor will not engage.

If you still have pressure in the system but are undercharged you are going to need to add refrigerant and hopefully an oil dye. They sell these in pressurized cans which you can inject through the gauge set.

Determining how much refrigerant to add when you are not starting from zero/empty-system is a game of educated guesswork. Generally speaking if the system itself actually functions properly aside from the leak, you would judge this most easily from your low-side pressure while the system is running.

There are two types of metering devices, an orifice tube and an expansion valve. They like to run at different low-side pressures. Expansion valves like 35-45psi at idle, orifice tubes 20-30psi. These are rules of thumb. If you don't have access to actual service information the easiest way to tell which one you have is to punch your year/make/model into a parts store website and see which one you can find a listing for. MOST small cars use expansion valves.

When injecting refrigerant, you will probably feed it into the low side of the system. This is because the pressure on the high side may actually be higher than the pressure in your new refrigerant can at some point which will stop or reverse flow. While feeding refrigerant to the low side of the system, be aware of where the low side service port is on the system. If it is very near to the compressor on the low side line, be especially careful not to turn your refrigerant can upside down because it will feed liquid refrigerant to the compressor, and there is some law about liquids not compressing. If you leave the can right side up (feeds much slower) or the low side service port is fairly far from the compressor, it is not as much of an issue.

Cliffnotes: Check for obvious signs of leakage. If none are found, verify you have an expansion valve, shoot a can of refrigerant oil dye into the system, then add refrigerant til you have 35-45psi on the low side at idle. Come back and recheck for green oil dye to locate leak that will eventually need to be repaired.

In reply to Vigo:

For topping off I've been told the high side is the better one to look at.

Regarding manifold gauges, they're called that because all the valves connect to a common manifold, DO NOT FORGET THAT. Generally speaking the low valve is left open and the high valve is left shut, NEVER open the high valve when adding refrigerant, the can can't handle the pressure. Do however disconnect the high hose first, with the system running, then open the high valve for a few seconds to suck down the high hose to low side pressure (watch the gauge) so you don't inadvertently lose any liquid in the gauge set.

Also, any amount of air (and the moisture it carries) in the system is bad, so don't forget to purge the refrigerant line after tapping a fresh can, crack the connection loose at the manifold and let it hiss a few seconds before opening the valve on the manifold. Same goes for the high/low hoses when you hook them to the system (with the engine off) if they don't have any pressure in them. Refrigerant is a liquid under pressure, kinda like propane or butane, so the bit of lost gas is negligible.

This is all good info. If you have a basic understanding of whats happening in the system (as spelled out above) that will help you interpret what the pressures are telling you.

Both sides low; low charge

High side high, low side low; plugged metering device

High side low, low side high or normal; bad compressor

High side very high; insufficient airflow through the condensor. Your fan should be running anytime the ac is on. If its a mechanical fan verify the fan clutch is good.

If your pressures are right it should be cold, its just that simple. If they are right and its not cold your problem is likely in the heater box, like a blend door not going to full cold setting.

Wow, that's a lot of good info. I might have to take another crack at getting the AC in my Legacy going again (and keep the SWMBO happy).

Thanks! Adam

Just going to chime in and say the best way to do this is with a real AC machine. Recover the refrigerant, pull the whole system down to vacuum and confirm there are no leaks and then refill. AC systems leak a little over time, it's normal. Eventually you'll need to recharge it. It's critical to performance that there is no air in the system btw, another reason a proper machine is best.

The compressor function can be confirmed several ways, again, the best being a machine where you can watch the high and low side pressures and monitor how they fluctuate. A high side needle that bounces a lot indicates a bad pump even if it still is functioning.

I'm a mechanic and I use these machines a lot (but can't use them on personal vehicles, state job) and it's the one job I will never do at home. You might get lucky but IMO it's better to let a shop do it properly.

Im all for ac machines but all they are is a bunch of smaller machines put in a rolling cart and hooked up to one fancy control box. Theyre nothing you cant do just by having each smaller part individually. Residential and commercial ac for example are basically always done off hand portable gauge sets and vacuum pumps and refrigerant scales and identifiers etc because if you put them all in one big box it wouldnt be portable enough. Same job still gets done, though! I draw my line in the sand at having only a low side gauge that came on a refrigerant can thats marked red yellow green and thats all you know about fixing ac. But, if you have a full gauge manifold and a vacuum pump i would say youre 80-90% as effective as if you had a $1000+ ac machine.

As far as judging refrigerant charge by the high side pressure, the reason i dont recommend that is because your high side pressure is highly dependant on how much airflow your condenser fan is creating which can vary a lot. Many cars turn the condenser fan on or off based on the high side pressure sensor, or even between low and high fan speeds. A novice may not know what to make of the 100psi swings that are normal for a system that fiddles with its condenser fans.

But, if one really wants to get into it there are usually some fairly arcane charts provided by the oems that describe the relationship between ambient temp, ambient humidity, discharge temps, low side pressure and high side pressure. On a PROPERLY functioning system. Its a rabbit hole. Even if a novice can find that chart, they may not be able to read it properly, and if their numbers dont match the chart, they may not be able to properly diagnose it. Anyone can learn this stuff, it just depends how much of your life you want to dedicate to a tradecraft you arent planning to make money off of. There IS a reason why shops charge $100+\hr. When it's easy it's easy but when it's not you have to know a LOT. When we start talking about superheat and subcool and latent and sensible and state change and btu's and blah blah blah youre going to wish you had just used the knowledge you already had to make money with your day job and just pay someone to fix it instead of picking up a 2nd underpaying profession and then not being able to bill yourself any money anyway.

So, try the basics first.. if it looks bad i'll tell you when to throw in the towel! lol

Oh man you guys are awesome and dangerous at the same time. Dangerous because you got me looking at tools. In reality I don't have the time right now to do this and this is the DD. If this were my second car or a toy I would be all over the home repairs.

This exactly the info I was hoping for, a clear understanding of how techs diagnose the problem. I used to just dump a can of O'Reilly's refrigerant in the system and cross my fingers.

BTW I did verify that the compressor clutch engages at all times even when the air temp out of the vents is not super cold. So I am guessing that means the pressure is not too low or too high.

Quote from spandak: 'Eventually you'll need to recharge it. It's critical to performance that there is no air in the system btw, another reason a proper machine is best.'

I was always under the impression if any air got in the system it means you are hosed. But do I understand you that if the AC starts underperforming after 13 years that might just be normal and I might get away with just a recharge?

Are your fans working? If you don't have good flow through your condenser you won't get cold A/C. Check that your main fan and/or auxiliary fan kick on high when the A/C is switched on.

If you have gauges hooked up you'll see a significant drop in pressure on the High side when the fan(s) run.

I literally just replaced the compressor to condensor line on my GMC. Bought a Vacuum Pump and Manifold gauges and did it all myself with the small cans of refrigerant.

My A/C is functional and gets down to about 45 degrees now at idle.

Thanks to Vigo for helping me out.

Oh and if you go the same route I did, make sure you buy the piece that connects to the gauges that you can screw on to the cans. I luckily had the little hose that attached to the can, so I was still able to make it work.

In reply to SyntheticBlinkerFluid:

"Can Tap" is what you're after, and also any adapters if needed to connect the vacuum pump to the hose.