Let me give you the run down. I'm a beginner at this, and i'm looking to learn more. I have a manifold gauge set, vacuum pump, and access to recovery equiptment.

The temp coming out of my vent on my 88 F150 was around 50-60 degrees with windows up a/c on max. The previous owner has allready done an R134a conversion on it. (I dont know if it was a full conversion with replacing everything, or just the 50 dollar "kit")

I figured it was low so I hooked up my gauges. The high side was at about 260 psi, and the low side at about 42 psi. The outside air was about 85-90 degrees.

My inexperience tells me this is okay, possibly a little high even? But it should be enough to cool my truck lower than 50 degrees shouldnt it (all temps are in F). I though i remember reading somewhere that it should be around 35-40 or so.

I actually now i believe that a 50 dollar "kit" was used to convert.

My next question is where to go from here? Do I start replacing parts? Or are there other alternatives?

Also, a side question, at the parts stores, are there any diferences between brands of R134A (besides the color of the can)? Or is it all the same stuff?

Thanks to any help in advance.

 

Those pressures are both a bit high, but pretty close at those temps. You should be seeing better outlet temps.
It's possible that the system is actually low on charge, but poor airflow over the radiator is keeping the pressure up. There could also be heat leaking from the heater core.
First, try clamping off the heater hoses and see if that helps the vent temps. You'll notice that the low side pressure will fall if this is a problem.
Also, mist water from a garden hose over the condenser. If the high side drops like a rock, there is an airflow problem. If it drops slowly, your airflow is ok.
If neither of these things tells you anything, you'll have to dig a little deeper.

 

thanks for the detailed info. I'll give that a try. It seems to get pretty good airflow over the radiator, the ol 300 flows pretty good.

 

Allrighty then....

I took a stab at both tests today. I clamped off the heater hoses, no change. It was cooler out today, and for pressures when I first hooked up I got about 30-32 low side and about 200 high side. I didnt have a garden hose handy, but I did have a bottle of water, and i shot the full 20 oz of water on the condenser and it went right from 200 to 140, the compressor kicked off, then the static pressure went up, kicked the compressor back on, and it kept running at about 150 psi high side, 25 psi low side. After a while (about 4 min) it rose back up to 200 high side, and 30 low side. During this test the vent temp went down from about 50 to 45 degrees F. I think i'm on the right track.

What is the best way to clean the condesner? Like a pressure washer at a carwash? or will that flatten the fins? How about compressed air? or would I have to phsically remove the condenser and clean it that way? The condesner doesnt seem like its clogged, but i guess that really doesnt mean anything.

Thanks for the help!

 

Cleaning the condenser may help. I use oven cleaner and flush it with a garden hose. Cleaning it certainly can't hurt.
The most likely problem, though, is the fan clutch is very weak. If it's over 8 years old or more than ~70K miles, replace it. There's no good way to test them. The engine will tolerate a bad fan clutch, but the AC will not. Poor cooling at idle is the first sign of a failing fan clutch. You missed this symptom because your system is also low on refrigerant.( It could very well be that the bad fan clutch caused the high side pressure to rise and open the pressure relief valve, causing the undercharge).
Just a note: If the system has been converted to R134a, use only a Motorcraft fan clutch. The aftermarket ones don't seem to work well on converted systems. This is just my experience.

Most of that 20 oz of water probably ran off quickly and the High side dropped to 140, so with good airflow the high side would be about that psi or lower. Figuring 80degrees ambient, you would want to see about 200-220psi on a fully charged system. Your system is low on refrigerant. You'll need to replace the fan clutch to get good readings and determine your next steps.

Since you have the gauges, a vacuum pump, and want to start learning about AC, here's a good basic rule of thumb for you:
- The low side pressure will be roughly equivalent to the evaporator temperature. (the outlet temp will be a bit higher). The lower the low side, the lower the outlet temps.
-The high side should be roughly 2.5 times the ambient temperature.
- These pressures are on a fully charged system, Max AC, High blower, doors open, 1500 RPM, with good airflow over the condenser.
Those are rough guidelines for a "normal" system that will get you in the ballpark when diagnosing problems.

Check out www.autoacforum.com . It's a forum aimed at DIYers or beginners like yourself. There's a lot of MVAC professionals there that contribute to help those folks fix their systems right. It's probably the best MVAC forum out there. (IMHO, it's one of the best online forums, period. It's right up there with FTE). It's sponsored by Arizona Mobile Air (www.ackits.com), a very good source for quality AC parts and equipment. I'm not connected with AMA. I've just been reading that forum for many years and am a very satisfied customer.

 

Properly done (no walmart parts ) your AC should blow 36-38* out of the vents.

That is why DIY'ers shouldn't do the job.
AC professionals should.

 

Howdy from hot and humid Texas!

You have gotten some pretty good advice there. However, when you are taking the pressure and temperature readings, there is more that you should know.

Most pressure and temp readings are taken with the engine at normal operating temp, running at about 1500-1800 RPM. You may need to provide airflow across the condensor via a fan blowing from the grill across the condensor to provide the equivelent to travelling down the hwy at cruising speed. The doors on the vehicle must be CLOSED, windows rolled up, the A/C on max/recirc, blower motor on high, and the thermometer in a central vent.

The outlet temps partially depend on the ambient temp, humidity, the condition of the metering device (FOT or TXV) and the amount of refrigerent in the system.

The outlet temps should read about 4-5 degrees warmer than the evap temp, and the evap temp should be no lower than 33 or 34 degrees, to prevent evap freeze-up.

The high side pressures should be roughly twice the ambient temp, plus the humidity, plus 10%. So if the outside temp is 80 degrees and the humidity is 60%, then the high side pressure should read about 80 x 2 + 60 + 22 = 240 PSI.

The low side depends upon the type of system you have. If you have a CCOT(FOT) system, the low side should cycle between about 45 PSI and 22 PSI or so. If you have a TXV system, the low side should hover around 28 - 31 PSI.

Sometimes in very high humidity and temps, you may not get down to 41 degrees outlet temp. maybe only 50 or so. That would be normal if the temps are around 90 with high humidity.

You can tell what type of A/C system you have by locating the drier/accumulator. If you have an accumulator, big cylinder near the evap, you have an FOT system. If the cylinder is located near the condensor, (receiver/drier), you have a TXV system.

Stay cool,

Dan Harriman 84 F150 lwb
Orange, Texas

 

What a great post by KC5GXL. I've asked these questions only to get so many evasive answers and he "hit the nail on the head". Now I can go to work on my A/C and feel confident that I have accurate info to analize my system. Thanks Dan for your post!

 

Yep. Excellent post.
Howdy from Hot, Humid northern Ohio, kc5gxl.

 

I just want to take the time to thank you guys for the help. It really makes me angry when someone posts "A/C is for professionals only, take it to a shop".

People arent just born professional A/C tech's. It takes time and practice, which is why I appreciate the help!

I'll be printing off a few of these posts to keep for reference. Thanks alot!

Oh, and does it make any difference what R134a I use (no stop leak additives etc, just straight R134a), like what brand? at Sams Club they have a case (12 12oz cans) for about 33 bucks, which is just under 3 bucks a can (I think the brand name was johnson), compared to the cheapest house brand stuff at the auto parts stores for 5.99 a can!

Oh, and Kc5gxl (or anyone) could you elaborate more on what TXV and FOT systems are. I know one uses an orfice tube (like Fords and GM?) and another uses an H bock (Chrysler?) Maybe i'm completely wrong on that, but I figured i'd take a guess! I was just curious as to the operating principals of both systems. Or if you could point me in the direction of a good source for information on them. I'll have to visit that a/c forum!

Thanks again
-Peter

 

Hello Peter et al;

<Oh, and does it make any difference what R134a I use (no stop leak additives etc, just straight R134a), like what brand? at Sams Club they have a case (12 12oz cans) for about 33 bucks, which is just under 3 bucks a can (I think the brand name was johnson), compared to the cheapest house brand stuff at the auto parts stores for 5.99 a can!>

R134a is R134a. Period. Of course, there is the R134a that has oil in it, stop leak in it, and so on, etc., etc. But, the bottom line is this: straight R134a is R134a. If it had other components in it, then it would be something else. There are probably very few companies that actually make the stuff, but the cans have several different brand names on them. $3.00 a can is great. You should buy a case or two if you can. It doesn't go bad with age.

<Oh, and Kc5gxl (or anyone) could you elaborate more on what TXV and FOT systems are. I know one uses an orfice tube (like Fords and GM?) and another uses an H bock (Chrysler?)>

The TXV (H-block in some makes of vehicles) or Thermostatic Expansion Valve system uses the temperature of the evaporator to control a valve that opens and closes the passageway for the refrigerent to enter the evaporator. Most use a sensing bulb that is attached to the outlet, or suction, line from the evap. The bulb is filled with a gas that expands and contracts as the temperature of the vapor coming out of the evap rises and falls. The bulb is attached to the TXV and opens or closes a pintle inside the valve to control how much refrigerent goes into the evap. It is an active system.

The FOT system (Fixed Orifice Tube) uses pressure to control the compressor operation. Located on the accumulator, or near the outlet to the evap, there is an electrical switch (low pressure cut-off switch) that will make or break contact, turning the compressor on or off. It is a normally closed switch that allows electricity to go to the compressor clutch if the pressure is above a certain amount...say 45-47 PSI. As the compressor pulls in vapor from the evap, it compresses it into a high temperature, high pressure gas. It then goes to the condensor, where it is condensed into a high pressure, high temperature liquid. (This is true of all A/C systems, whether TXV or FOT). It then goes to the FOT which is nothing more than a screen with precise size holes in it to allow refrigerent to flow through it, but at a much slower rate than the compressor is pulling it out of the evap. The FOT is on the inlet side of the evap (as is the TXV). It acts as a restriction to the flow of refrigerent to the evap.

As the compressor pulls more vapor out of the evap, the pressure in the evap goes down. Once it gets to a certain point, let's say about 22 PSI, the contacts in the Low pressure cut-off switch open, cutting the electricity from getting to the compressor. As that happens, the pressure in the system starts to normalize (go towards the same pressure in both the high side and low side of the system). As the low side pressure rises, it soon gets to the pressure that will close the contacts in the low pressure cut-off switch, and the whole process starts over again, and again, and again. The FOT system is a passive system. In other words, no moving metering valve like in the TXV system.

Hope that helps a bit,

Dan Harriman 84 F150 lwb
Orange, Texas

 

This is GREAT stuff! Dan explains A/C operation so that even a novice like myself can understand it. Thanks again for your post.

 

Alright, now let me take a stab at diagnosing a problem then. This is a true problem that i'm working on fixing right now.

My GF's sister has a dodge neon. The compressor doesnt run. The actual compressor will spin freely when turned by hand. There is about 110 PSI high and low side (static pressure). I was actually doing some repairs on the car, and one day the A/C just stopped working. Being a dodge, it uses an H block, so its a TXV system.

My next step would be...

Test the voltage at the clutch for power.

If power, replace clutch,

If no power, check low pressure switch, high pressure switch, possibly H block (dont exactly know how). Also could I try jumping the low pressure switch to see if I can get the compressor to turn. Main reason being it looks like it is much harder to get to the compressor than it would be to get to the low pressure switch (on H block right?)

 

All good post from a bunch of very knowledgeable guys, but I would like to add one thing.

134 is not as efficient as the old R12, so the condenser is sometimes inadequate, although it does look like peter94 has an air flow problem. You can get parallel flow condensers from ackits that are a heck of a lot more efficient than the old condensers that came stock on these old trucks. Plus the fact that they are more compact and you can put a much larger one on than the old factory unit. I changed mine out and the temps went down by 10 degrees even at idle.

Also peter94 you might try an electric pusher fan in front of the condenser to increase air flow.

 

Hmmm...I have my brothers old flex-a-lite laying around from his ranger...Might not be a bad idea!