A/C primer

[Sparky2263]

This is not a technical discussion, rather a working mans understanding of A/C to assist in diagnosing problems with the A/C in our S-10's. If you want a technical discussion, pm me. I can talk tech all day ;).

Freon- Actually a trade name for R-12. I'm gonna call it refrigerant. That way we cover all automotive refrigerants.
What is needed is a compound that changes state (liquid to gas) easily, is not combustible and is stable in a mobile environment. It's ability to change states rapidly determines its' chemical make-up.
Why do we need refrigerant to change state? Heat. We have to absorb and then shed our #1 summer enemy. Heat. Keep in mind, an A/C's job is to remove heat, not to cool the inside of the car. More on that.

The 3 primary components of mobile A/C systems, Compressor, Condenser and Evaporator. They each do exactly as their name implies. Neat, huh?

Compressor- The compressor takes a low pressure gas and compresses it to a high pressure gas. Period. But why? Heat. We need to take the heat in a large space (low pressure) and compress it to a smaller space (high pressure). That makes it hotter. Take a given volume of gas (contains X amount of heat) and rapidly compress it to a much smaller space (contains the same amount of heat, but in a smaller space leading to a significant pressure/temperature rise) and send it to the condenser.

Condenser- The condenser receives a high pressure gas from the compressor and condenses it to a high pressure liquid. Period. But why? Again, heat. We're going to change states in the condenser (high pressure gas to high pressure liquid) by removing heat from the gas. As the gas travels through the condenser, heat is released to the air traveling over the fins condensing it to a liquid. But, unlike the compressor, we have little or no temperature change from one side to the other. We have had a change of state (gas to liquid) so the release of heat has been accomplished.

Evaporator- The evaporator receives high pressure liquid from the condenser and it evaporates in the evaporator, kinda' neat also :). How? By blowing heated air over it. We use a metering device (orifice tube, expansion valve, metering block, etc. They all do the job) to flow liquid refrigerant into the evaporator. As it absorbs heat it changes state from a high pressure liquid to a low pressure gas. The result of this heat absorption in the evaporator is a significant drop in temperature of the air being blown across it. We are removing heat from this air, lowering it's temperature. This low pressure gas goes to the compressor and the cycle starts all over again.

Keep in mind the subtle, yet very important, concept of removing heat rather than cooling air. This concept is necessary in diagnosing A/C problems.

This will be a sticky, but constructive comments and questions are welcome. I'm sure I'll revise it a bit anyway as it's 2 in the morning right now ;).

[Matt 4.3 TBI]

Anyone have any input on R-414b (Hot Shot)? I've heard good and bad for it.
R-134a has NEVER performed remotely close to the R-12 in my 91's system. I'm getting desperate to get something resembling cooling on these 100+ degree days.

[Sparky2263]

Guess I'll do another write-up on conversions. I'll address the 2 most glaring issues I see with conversions first and then incorporate that into a conversion write-up later.

Matt, our biggest problem with the Blazer is airflow. We have the same flow as a truck and it has to cool more than double the cubic feet. Next time you get a chance, ck. the airflow on a 2nd gen with the blower on high. Makes our 1st gens look really wimpy. What I see making this worse is aftermarket blower motors (rebuilt) and dirty cowl screens. The rebuilt motors simply don't have the speed a new Delco does. You get what you pay for here. The 1st gens are very susceptible to debris clogging the cowl screen and even getting into the blower wheel itself. Hood modifications that reduce air pressure on the lower portion of the windshield are a big no-no also. Removing the blower and cleaning it will help greatly as will keeping the cowl screen clear.

Now, conversion problems. The biggest problem is oil. Most conversions are done without removing the original mineral oil. Then, 4-8 ozs. of esther (not recommended) or pag (synthetic, recommended) are added to a total system capacity of 40 ozs. This reduces refrigerant capacity 10-20%. Doesn't take a rocket scientist to figure out what this does to the overall btu rating (ability to remove heat) of the system. Plus the fact that you use less 134a (60-80% of system capacity) than R-12. So, by not removing the original oil, you just shot yourself in the foot.

The second biggest is system overcharge. Goes right along with the above. But, this one is easy to fix. The system is full on ours when the accumulator gets cold. Period. Anything more is an overcharge. An overcharge makes the compressor work harder and reduces the ability of the condenser to shed heat to the air flowing over it. How cold? Feel the line going into the evaporator right after the orifice? Right about there. The outlet to the accumulator will be much warmer until right at the point of a full charge. Add refrigerant very slowly when approaching full charge as the line between full and overcharge is about 2-4 ozs. Most of our conversions, with a system capacity of 40 ozs. (2.5 lbs.) will only use 20-30 ozs. of 134a. 32ozs. (80%) is the generally accepted amount but I see few that actually use that much.

Let's talk oil. R-12 systems use mineral oil. The viscosity is around 525. 134a systems use pag oil. It's viscosity (for GM systems) is 150. Just a tiny difference, eh? Here's the deal. R-12 molecules are much larger than R134a, hence their ability to absorb and shed heat (change state) is easier. It can also push (carry) the mineral oil through the system better. 134a just has a very hard time circulating mineral oil through the system. That is why it is so important to get the mineral oil out and the proper pag oil in. Esther oil has a viscosity of 100. It is commonly used as a conversion oil. Heck, I've done it. But, esther was NEVER meant for mobile applications. Given the choice, get pag 150. You won't regret it.

Evacuating the system- A definite MUST for best results. What is it and how to do it? Evacuating the system consists of putting it under a vacuum in order to lower the boiling point of water to room temperature. What? Never seen water boil at room temp? I've got a mayonnaise jar with a fitting attached to the top I use for demonstration. Anyway, we want to put it under a vacuum a sufficient amount of time to remove ALL the moisture. How long is that? I like an hour. Lower the hood and start the car to warm up all the a/c components and the moisture will evaporate quicker. 30 mins. seem to be the generally accepted minimum if you're in a hurry.

[Sparky2263]

Now to answer Matt's question (finally!),

Hot Shot is an R-12 drop-in. It works good. It also requires no conversion (i.e. it can carry the original mineral oil through the system). At my shop we use Freeze-12. Also a drop-in that seems to keep the high side pressures close to R-12.

So, here's the question Matt, what and how much oil is in your system? ;)

The other thing you can do is to install a vacuum operated heater control valve into one of the heater hoses. Tee it into the Max/Recirc. vacuum line (door at far right of instrument panel) which is the orange line coming off the vacuum dial. That'll shut off coolant flow to the heater core when Max/Recirc is selected. It'll help a bit.

[Sparky2263]

The orifice tube is the means by which high pressure liquid refrigerant is metered to the evaporator core. It is also half the name of our style system. CCOT, clutch cycling orifice tube. It is much, much cheaper to manufacture than expansion valves, expansion blocks, H-valves, whatever you want to call them. Obviously, it doesn't have the variable metering ability of an expansion valve, so it is designed for optimal cooling above 2000 rpm engine speed. Below that, it is less than optimal. So, what can we do to get better cooling when sitting at the stoplight on 100 degree days?

Enter the variable orifice valve (VOV). At low speeds (read lower pressures) it will allow more refrigerant flow into the evaporator enhancing low speed cooling. Runs around $20 at most parts stores. Here's the problem. It isn't listed for a particular car and most counter people go into spasms if the computer doesn't list it. But, they all have part number interchanges. The NAPA product line is TEMP and the part number is 207325.

It will drop outlet temps, at idle, about 5-10 degs. A very noticeable difference, especially when it's 95+ degs. outside.

[Matt 4.3 TBI]

Turns out they did charge it with Freeze-12, that's what the work order says. Unfortunately I can't find the sheet which said how much oil and charge they put in. I imagine they use Alldata for their specs tho.

I had to replace a cracked high pressure line. I bought a new accumulator and a variable orifice tube, I left it for them to install so they could put the right amount of oil in. They evacuated the system for about an hour before they charged it. I know the spec sheet said 68* F at around 2000rpm, very disappointing.

[Sparky2263]

68* on normal mode (not max or recirc) would be considered okay with an ambient temp of 98* or above. A 30* drop from ambient is about average. Now, on recirc/max, it should just keep getting colder and colder all the way to about 40-45* cooler than ambient, depending on humidity and cabin sealing.

Post up how it all works out.

[Matt 4.3 TBI]

I only run mine on max, since the bi-lev and norm a/c settings seem to be a waste of time. Sometimes it's only borderline cooler than running with the windows down :(

I'm going to assume they used Ester oil, since I told them that's what was in the system. When I converted it to 134a in 2003, I change the accumulator, o-tube, compressor and flushed the lines, condensor and evaporator. I used 2 lbs of 134 and 8oz of Ester oil. Worked great until the line cracked.

The real question is, should I look into getting it charged with 414? Is that going to cool better than Freeze-12?

[Sparky2263]

I don't honestly know if one is better than the other.

I'm wondering, is the recirc. door opening?

I'll add this in. These 95*, 90+ humidity days we've been having? It'd take a good 15-20 minutes of steady driving to get the Blazer cool. Air distribution just flat sucks.

Also, R-12 is still avail. I keep a 30lb on hand. Got a few people willing to cough up $50 a lb for it. Maybe an option for you?

[Matt 4.3 TBI]

Okay, new problem that has cropped up. Now the A/C is acting like it has low charge, and I'm getting a 'ting-ting-ting' coming from the front of the engine. I'll check it with my stethoscope when I get home, but I suspect it's the compressor clutch plate. Compressor engages for a second, disengages, repeat, repeat. Doesn't compress long enough to get anything cool. It's blowing 100 degree air on a 93 degree day. I'll check the pressure with my gauge when I get home, too. It was checked for leaks before it left the shop, I suppose one could have cropped up since then tho. Might be time to get the UV dye in it again.

[Matt 4.3 TBI]

Hey how many pounds of freeze-12 should be in our Blazers' system? Sheet says they put in 2.

[Sparky2263]

That'd be 80%. I'd just charge it 'til the accumulator is cold though. That'd be a good indicator if there was a high side problem. I'm pm'ing you my phone #.

[Tech10]

Another problem with our S10's is there isn't enough air flow over the condenser.It just doesn't keep as cool as it should.I don't know if you have a electric fan or not but a good one will keep the condenser cooler in stop and go traffic than the OEM clutch fan.Slow to no movement let's that condenser get hot and pressure rise decreasing efficiency.R12 in my opinion is the best but 134A can get the job done if done right. The receiver-drier should always be changed each time the system is empty regardless of the reason for loss of refrigerant. It should also be changed every three years, because the desiccant pellets will break down and clog the system.

[Sparky2263]

The receiver-drier should always be changed each time the system is empty regardless of the reason for loss of refrigerant. It should also be changed every three years, because the desiccant pellets will break down and clog the system.


Our systems have accumulators. The dessicant is contained in a bag that, withstanding extreme circumstances, doesn't pose any problems so long as it is evacuated properly. The dessicant releases it's accumulated moisture when put under a vacuum. Accumulators also don't do the filtering in our systems (as opposed to a receiver/drier, which is installed in the high side) as that is the job of the orifice tube.

I saw a posting giving the info you gave above. It also contained these tid-bits, which are also wrong;

Compressor
The compressor receives the Freon in a gas-like state and compresses it into liquid and circulates it into the condenser where the heat is taken out of the (R12 or R134a). The compressor is lubricated with special oil. It is very important to use the correct oil to the compressor and system. Some seals and gaskets May not work with some oils.

Why Replace the Receiver-Drier?
The receiver-drier must be changed each time a system is empty regardless of the reason for loss of refrigerant. It should also be changed every three years, because the desiccant pellets will break down and clog the expansion valve. This will in turn cause the system to become inoperable and May damage the compressor.

The above info is flat wrong and/or doesn't apply to our systems.

[jjwalker]

guys, I know this isn't tech related really, but be glad you have A/C. Hell, be glad you have a heater as I have neither at this point.

[Matt 4.3 TBI]

Heh, who has AC? I had working AC for one month this summer, and haven't had it before then since 2003. Original compressor went out in late 2002, replaced it and did all the work to switch to R134. Worked great until my engine rebuild in late 2003. Ever since then it won't hold refrigerant for more than a few weeks. I think the leak is in my evaporator now but due to time and $$ constraints I don't know when it'll get fixed. Heat works well, a little too well, especially in the summer.

[Sparky2263]

A/C season is coming back around. Figured I'd bump this to the top.

[blanz]

My compressor went in my 2000 S-10 and I replaced the accumulator ($45.00), the expansion/orifice tube ($3.00) and a bottle of PAG 150 oil ($8.00) when I did my compressor swap. For that little money and effort it was well worth it. Hell, the damm evacuation and recharge cost me $145.00 bucks! Wish I could have done that myself too. Besides, a friend who does A/C on the side said it is a must when you repair a system to change out those two parts.
Now my system blows ice cubes with no more oil/freon leaks.
Believe me, the S-10 A/C system is very easy to work on yourself. Don't do it on the cheap!
Be safe, Be cool!!!
Bill.

[Abaddon]

Well, my A/C is having problems. It appears I have a leak back by the evaporator. This isn't something I want to do myself but would this also be a good time to switch over to R-134a? I've heard good and bad results but at least I could pickup the recharge stuff at the store if I ever developed a small leak after the repair. C'mon cold weather!

[Maximus]

I am running 134A and get temps as low as 49 degrees on these days we have had that were close to 100 degrees.It doesn't cool as well as 12 did in my system at idle even with variable orifice tube.But it gets the job done.And it usually takes a few miles of steady driving to get it cycling through and get good and cold.And also I noticed a significant difference in cooling when I went to a electric fan than I did with the clutch fan.Especially at stops when the clutch fan would be barely moving air across the condenser the electric fan on high moves plenty of air all the time which really did help out a lot.

[Abaddon]

So you are generally happy with your A/C converted over to 134a? I may make the switch. My A/C doesn't work at all when I'm idling because it's low on freon so it wouldn't be much of a change for me. Thanks for the input.

[Maximus]

Im happy with it but R12 did cool it better but the cost for R12 just wasn't worth it to me to keep cool when 134A could do almost as good a job.Hell even 134A is going up in price from what it used to be.I remember when you could get it for just a few dollars a can.