Low Pressure Question

[Flopster843]

I have a quick question for the experts here in TDR,

My AC started getting a little warmer than normal so I hooked up my gauges for a recharge. It was low on both sides so I added a little 134a to bring the low side back up into spec, by the high side is still pretty low. It was around 90 here today. The manual says I need 30-40 on the low and 250-350 on the high. The truck is still cooling inside, but isn't quite where it was. I can't find my thermometer right now to take a vent temp reading on the inside. Condenser was changed a few months ago due to it blowing apart and I put a new compressor line and switch on it as well. Think my compressor is dying and needs to be replaced, or is this high-side reading acceptable?


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[B.G. Smith]

During the test you need the AC on max, the fan on highest speed, engine at operating temp. and running at 1000 rpm. All doors and windows closed and hood mostly closed. My chart shows @90 deg ambient suction at 50 to 60 or less, discharge 215 to 270. You probably need more refrigerant? bg

 

[Motorhead]

You can also use a 134a temperature-pressure chart that checks the system NOT running and at AMBIENT temperature (Outside air temp). So, basically if you have a working system, you can test the "Charge" level statically. http://www.advantageengineering.com/fyi/289/advantageFYI289.php

You can check this way on the low side as your gauge will read out better. With the AC off, the pressures will or should equalize.
I have personally used both, reading the pressure differentials to see if the system is working properly and the static method for checking proper refrigerant charge.

Look on the bottom side of your compressor close to the back of the compressor and you will see a small pressure relief valve. If it is wet and oily, it has been releasing over pressure. You may need to have your engine cooling fan function and speed checked. At idle on a warm to hot day, you should be hearing the engine cooling fan running at partial engagement. If you have been leaking R134a, you are also losing small amounts of refrigerant oil. That loss also needs to be replaced.

 

[sag2]

To answer your question the high side is too low. If you are using 12oz cans I would discharge (properly and legally of course) and add back approximately the correct amount so you know where you are. Then you can properly diagnose from there.

 

[Tuesdak]

In order to get the best performance out of the CCOT systems you should evacuate and charge the system by weight so you know exactly whats in them. Before you bother use a leak tester to search for any leaks. (This is also the procedure in most service manuals.) The systems are charged by weight with any shortage or excess refrigerant changing the liquid (oil refrigerant mix) level in the accumulator. (After the accumulator runs dry with lower charges the level in the evaporator starts to drop.) The evaporators spit out just enough liquid with the gas into the accumulator to move the oil in the system. Low levels cools great but starves the compressor for oil and too much reduces performance. OEM systems have a leakage overcharge reserve figured in, but, in newer systems this is less due to EPA concerns.

As B.G. noted fan speed being low directly controls high side pressure. See the temperature in the low side gauge being in the 30's? It's right on the edge of kicking out so it doesn't ice up the evaporator. This means there isn't much load on the system boiling refrigerant off to raise the high side pressures. It also could be low on refrigerant with a quick and dirty test being to feel both evaporator inlet outlet lines. They should be the same temperature. If the outlet is way warmer you are still low on refrigerant.

 

[Flopster843]

I do believe I had a fan problem due to the truck getting a little warmer than normal while towing and very poor AC performance when the truck wasn't moving very fast. I have swapped out the fan clutch today to try to alleviate any issues with it. I have noticed what appears to be an oily residue around the pressure relief valve previously. I believe that fan clutch wasn't engaging enough upon startup which might have led to the system over-pressurizing and hitting the high side relief. I've been chasing a slow refrigerant loss for a while now and can not find any leaks. I've been over it several times, and it's been in the shop several times getting them to look it over and nobody can find anything.

I'm going to evac and recharge the system to spec in the next few days as a good base line for the pressure issue. I can understand a higher than normal pressure, but this extra low pressure has got me puzzled.

Motorhead, I wasn't aware that a static pressure check could be accurate. I'll check it out and report back.

Can anybody explain why the service manual specifies a lower pressure than just about every 134a chart on the internet does?

 

[PWong]

The high side reading was way too low and you are thinking in the right direction.

 

[Flopster843]

Sorry it took so long to reply to this thread, been crazy busy. So it took about a week or so before I got around to looking at the truck again. When I hooked up the gauges again, the high side pressure was really high and was causing the compressor to cycle often due to the high pressure switch. I hooked up my recovery unit and pulled a little refrigerant out and got the pressures back down to an acceptable level and everything seems to be working again.

What baffles me about this entire thing is the crazy low pressure and then everything just started working again. The compressor was running the entire time when I got the low pressures the first time. I guess I'll just have to watch everything for a while and see what happens. Thanks for the help.

Here's a link to the current gauge readings. Ambient temp was about 93 when this was taken.
https://drive.google.com/file/d/0B-gGPBwSwsYWTnpYcnhtY2Fvc00/view?usp=sharing

 

[xcc_rider]

First let me say I know absolutely nothing about our truck ac systems but I did read a post on another forum posted by a tech that partial loss of refrigerant can be caused at start up with a non functional fan (lack of cooling). I don't know if this applies to your truck now since you did the fan conversion but it was recommended if you had a viscous clutch fan that you install an electric fan to cool the refrigerant at start up to avoid the high pressures.

Sorry but that's the best I can remember from the post.