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A/C help ,,, less than optimum performance


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Hello all. I am welcomimg myself to the forum by asking for some tech advice on the a/c in my 2000 Silverado 2500. (2x4 ext cab 6.0L)

Here we go ... I just replaced a leaking compressor with a new Delphi compressor. (I know, not the best choice of compressors. I've got my fingers crossed. Lol.) Also replaced the condinsor, accumulator and orfice tube. Flushed evaperator and lines prior to intsalation.

Pulled vacuum for close to an hour. Yes, I have vac pump and manifold guages. Purged yellow fill line and charged the system with the recommended 30oz of r-134.

High side pressure went sky high. (425+psi) Low side was closer to normal at 37psi. When high side got that high I would turn off the engine. For whatever reason I started and stopped the engine numerous times with no change in pressures. At that point I decided I had missed something when flushing so I evacuated the system and reflushed the lines and evaperator again.

Second charging resulted with no change ... at first. Again the high side shot up to 425+ so again I turned off the engine to prevent damage from over pressurizing the system. Started engine again but this time pressures were closer to normal. (???) High side never went over 200psi and the low side was still about 37psi. (pressures at idle)

With a themometer in the center vent, a/c on max with windows closed I was only getting air at 45*.Outside temp was 77* with high humidity.

Here's where I'm having trouble ... Off idle (1500rpms) the low side drops to 30-32psi and the compressor starts cycling. Once off the low side rises to 45psi and the compressor comes back on. Then back down to 30-32psi and off again. I didnt time the cycles but my best guess would be about 30 seconds on and 15 seconds off.

Decided to go for a test drive down the freeway so the condinsor was getting plenty of air. Air temp rose to the mid 50's. Exited freeway and sat still for a couple of minures at idle and air temp went back down to mid 40's.

Its obvious the cycling compressor is effecting my off idle air temps. The question is why is it cycling prior to reaching optimum air temps?

Reconnected guages and decided to try putting a little more "freon" (134) into the system. My intent was to raise the low side pressure a couple of pounds and stop it from cycling at 30-32psi. Nope. No change. I now have 36oz of 134 in the system but its still cycling at 30-32psi.

Tomorrow I will replace the low side pressure switch. 30-32psi seems kinda high for the switch to be cutting out the compressor. 25psi or even a little less sounds closer to being right. If I'm right the cycling problem will likely be fixed.

However, even if a new switch fixes my premature cycling ... at idle when the compressor is not cycling I should be getting air that is cooler than in the mid/upper 40's. And I did have a fan blowing through the condensor.

It was hot today. Mid to upper 90's. Idle air temps and freeway air temps traded places. Upper 40's on freeway and upper 50's at idle. Considering the 95+ outside temp I expected my idle and freeway air temps to change but almost 50* at 65mph?. Needless to say it isn't exactly freezing me out.

After I install the new pressure switch I'll put the guages back on and post back what there reading. Hopefully the new switch will have fixed my cycling problem.

 

FYI ... At 57 years old this is not my first rodeo. I've had great results in the past on countless vehicles but this one for whatever reason is giving me a head ache. Lol.

 

I hope I've given yall enough info to make some recomendations as to where I've gone wrong or what I've missed. Any/All help would be appreciated.

 

Oh, as for oil in the system ... I put 8oz's of Pag 150 in it. Four in the new compressor and 4 in the new accumulator. I rotated the compressor countless times both sitting level and turned up on its front end supplying oil to the front seal. .

 

 

Thanks in advance.

Robert

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Update ... Replaced low side pressure switch. New switch settings are 20psi off and 40psi on. No more cyclimg at any rpm range.

 

New guage and air temp readings.

 

Ouside temp - 91*

Static system pressure - 88psi

Run time - 30+ minutes

Low side at idle - 38psi

High side at idle - 205

Low side at 1800 rpms - 29psi

High side at 1800 rpms - 215

Cabin vent air temp at idle - 52*

Cabin vent air temp at 1800 rmps - 38* (feels great!)

No cycling.

 

Obviously the new pressure switch fixed my cycling problem. And the pressure readings and air temps are much better off idle now. (1800rpms) Still think the 52* air temp is kinda high at idle but I'll live with it considering I dont do a lot of sitting.

So ... it appears as though I've answered and fixed my own problems. Sorry If I've wasted any of yalls time.

 

Thanks anyway and have a great day. .

Robert

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The compressor should be cycling depending on airflow thru the condenser, ambient temp, and compressor rpm. The compressor cycling chart is in degrees C but it's easy to convert to degrees F.

 

I would also check you fan clutch.

 

These other tables are not particular to the Silverado but they give you an idea of what your readings should be like.

 

DEWFPO

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Thanks guys. I appreciate your input.

 

Now, about that compressor cycling chart. Why is it supposed to cycle several times a minute? I know its supposed to cycle when or if the evaporator reaches about 36* to keep it from freezing. Other than that, why other than pressures being too low or too high is it supposed to cycle?

 

The numbers on the second chart and the numbers I'm getting are quite different. Living in the Houston area where the humidity is rather high (70-80% is faily normal) that charts says at 70% humidity and 90* outside temp the max low side should be 53psi and a max high of 405psi. Really? Normal max high at 405psi? That sounds awfully high and really hard on the entire system. Compared the those numbers I'm way off.

That being said, since my compressor isnt cycling anymore I don't know what my low side might climb to when cycled off. When it was cycling the highest low side pressure I saw was about 45psi. I'll put the gauges back on and unplug the low side switch turning off the compressor clutch and see what it reads.

I would actually like to see my high side a little higher at about 275 or so. Any suggestion on how to get it to come up? (keeping in mind I have an additional 6oz of refrigerant in it) Wouldn't taking a little out raise both the high and low psi?

 

 

Thanks again for your help.

Robert

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Hi Robert,

 

I should have said this in my previous post and I was remise. I am not a professional mechanic. The charts I provided are just examples of charts provided by different manufacturers for their r134a systems. I've lived in a dry climates unlike Houston and my experiences with R12 and r134a systems are a bit different.

 

In my experience, most systems need at least 42-45 psi to turn the system on and turn it off at 23-27 psi. Depending on the ambient temp, humidity, rpm and airflow thru the condenser and evaporator; the pressure on the low side of the orifice tube can rise to reach the 'turn on' point. This happens at idle if temps are low. Around here in the dry heat (10% or less), even at 95F, and the engine idling for 30 mins with the hood down the compressor will cycle on and off.

 

The system cycles when the pressures on the low side are below or above the set points. The systems are designed such that these set points are equivalent to having an evaporator reach no lower than about 36 degrees so that no frost occurs on the evaporator. This should be the case only if the system is operating as designed and has the correct amount of refrigerant in the system. If one of the components is not functioning properly or if there is too little or too much refrigerant in the system then you get sub-par performance out of the system.

 

The point of cycling the compressor is several fold. It saves fuel (and wear and tear) by only using the compressor when needed and it is a much simpler design than those used years ago when the compressor ran all the time.

 

I my limited experience, overcharging a r134a system causes higher than normal high (and low) side pressures, increased wear on the compressor and hose and sub-par cooling performance. The compressor wouldn't cycle much at all because the the compressor can't bring the low side pressure down to the 25 psi or so needed to cut it out. If it does cycle, it likely won't stay off for long since the low side pressure will build up more quickly. This of course all depends on how far overcharged it is. The fixed orifice tube is designed for a certain refrigerant to be metered (allowed to expand) over a certain pressure range. If that range is exceeded on either side sub par cooling is the result.

 

Having an undercharged R134a system is a better place to be than having an overcharged system.

 

I had a GM chart around here that showed low side pressure readings and center vent outlet pressures but it was less detailed than any of the others I have. It seemed purposely vague and I found little to no use for it.

 

I don't see how removing refrigerant would increase your high & low side pressures.

 

With a cold engine, can you measure your static pressure and tell us what it is and at what ambient temp?

 

Again, I am no professional mechanic and there are plenty of far more skilled and experienced folks on this list that will likely help you far better than I.

 

DEWFPO

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Thank you for your explanation on the importance of the system cycling. And I agree that my pressures are too low. Starting with the low side, the new switch I bought has working pressures of 20 to 40psi. (20psi off and 40 on) Even with the "low" low side pressure I have (30psi) I don't think it will ever drop down far enough to reach the 20psi cut off point. And that switch was the only one O'rielly offered for my truck. My question is why would the only switch offered have such low off/on points if the system is designed to run higher pressures? Again, if my low side pressure was closer to "normal" at 40+psi it would never drop all the way down to 20psi and cycle the compressor. I'm not sure it would reach the 23-27psi you mentioned much less 20. How and why would a system designed to run in mid 40's drop down far enough to cycle in the mid 20's?

 

You ask about the cold engine static pressure and ambient temp. Yesterday when I hooked up the guages the engine was cold and the ambient temp was 91*. The static pressure was 88psi.

 

So why are my pressures so low? As stated in my first post, the compressor is capeable of reaching pressures well over 400psi. I kept turning off the engine thinking it was going to blow something up. So it's obviously not the compressor. Nor do I think I have moisture or air in the system.

 

Just for comparison we hooked up the guages to my sons 98 Corvette. The engine was at operating temp and although I dont remember his low side pressure I know his high side was 275psi. Thats still a long ways from 400 and his ac works fine. In fact, in all the sytsems I've ever worked on I've never seen high side pressures over 350. And that was on my E250 7.5L van on a hot day, high humidity, engine idling, no fan blowing thru condenser. Everything was hot that day naturally making the pressure rise.

 

I'm not disagreeing or arguing with you in any way. I appreciate your help. Just stating past experiences.

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A few questions:

 

How are you measuring the weight of the refrigerant?

 

What are you using for the refrigerant supply? 30# bottle or disposable cans?

 

Is the condenser the same type and size as what was removed?

 

Have you confirmed the fan clutch is working optimally? Without enough airflow, you won't get your pressure numbers down or vent temps. Just because your radiator doesn't overheat doesn't mean the clutch is fully locking up anymore

 

^^^^^Do not dismiss this

 

 

The way I charge is to weigh the charge in from a bottle on a electronic scale. I monitor vent temps as well as watching pressures. You're looking for the lowest vent temp possible. If you overcharge vent temps will begin to rise.

 

Usually the high side pressure is about 2.2 times ambient temp. as a general rule of thumb. Many variables though

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With a cold engine at 90F, static R134a pressures should be around 104 psi. But this is only a ballpark figure.

 

I've seen high side pressures as high as 538 psi (before the high pressure blow-off opened on the compressor) on a properly charged system that was malfunctioning. (HIgh pressure sensor on the condenser failed so the compressor did not cut off when the electric engine cooling fan failed to energize to provide additional cooling is 105F heat and stop and wait traffic).

 

If you raise the engine up to 1500 rpm with the fan on high will the low side pressures go down far enough for the compressor to cycle off?

 

 

The way I charge ……………………….... I monitor vent temps as well as watching pressures. You're looking for the lowest vent temp possible. If you overcharge vent temps will begin to rise.

 

 

I've charged systems this way also and it's worked very well for me. It's a shade tree approach that ignores pressures and gauges but it seems to work if you follow a few important steps.

 

- Max A/C

- Fan on High

- Digital thermometer in center dash vent

- Add refrigerant until the center vent temp stops going down. (I typically wait 1-2 mins without further drop).

- Close the valve on the refrigerant and disconnect, your done.

 

It's not precise but works O.K.

 

DEWFPO

 

Attached is another chart of general Compressor OFF cycle times vs. ambient temp.

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Going to start by answering TXAB questions.

 

No I do not have a 30# bottle. I am using disposable 12oz cans. I put in one and a half cans in which was as close as I could get to 30oz. (12+12+6=30) However, since the compressor was cycling I thought it was asking for more charge so I put the remaining 6oz into the system totaling 36oz. I should have written down the pressures prior to adding the additional 6 oz but failed to do so.

 

The condenser I bought was an aftermarket from O'Rielly's. Its the right one for the truck but not OE. The only reason I replaced it was because I wanted everything new other than the lines and evap. That and for a lack of better words I wanted my ac to kick cold ass. Lol. I did however check the refrigerant passage by blowing air thru it. Needless to say the new one had much better flow.

 

I will be sure to check my fan clutch.

 

 

DEWFPO ... Exactly how ball park is that 90* 104psi static pressure? By that number my static pressure is about 16psi low. (88psi) So what exactly is that telling me?

 

No, the pressure does not drop low enough at 1500rpm to cycle the compressor. Remenber the new switch O'Rielly sold me has a rather low 20psi cut off. The lowest reading I've gotten was 29psi.

 

Drove the truck around today and boy was it hot outside.Don't know exactly but had to have been in the upper 90's. Going down the road my vent temp got down around 38-40* Felt great! But as soon as I stop it climbs up to almost 60* pretty quickly. Start moving again it goes back down. I definitely need to check my fan clutch.

 

Back to you TXAB ... You said, [Without enough airflow, you won't get your pressure numbers down or vent temps. ]

Getting pressure number down isn't my problem. I'm only getting about 210-215psi on the high side. And my low side is only at 30psi. If anything I need them to go up.

If I raise my low side by 10psi or so I don't see any way my compressor is going to cycle with a 20psi switch on it. It will simply never drop that low. Its not cycling now and my low side is only 30psi. at 1800rpms.

 

Obviously something just isnt right.

 

I think I'll go to the chevy stealership and see what pressures their low side switch has. If their switch switches off somewhere in the upper 20's I'll buy it and replace the 20psi switch I just bought from O'Riellys.

 

So how do I get my pressures to come up from a low of 30 and a high of 215? Half of ambiant in the heat of the day here is about 45-48*. Todays humidity was "only" 58%. :<

 

 

Robert

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Using your method of charging, I'd say you do not have 36oz in your system. You cannot accurately measure weight with your method. You realize there is some left in each can? As you fill with each can you're leaving a larger amount in each can. Plus their are several ounces left in the gauge hoses. Does your gauge setup allow you to seal off the your charge refrigerant hose at the source (can) so as not to allow refrigerant in the line out and air into the line each time you swap a can?

 

I asked about the condenser because if it is larger or smaller then the 30 oz GM guideline is out the window more or less because you've changed the size (capacity) of the system. I've built custom systems for all kinds of vehicles, large and small, and have to calculate an initial charge to get the system running then use the evap temps and pressure method to finish it off

 

Some aftermarket switches have a set screw to adjust the on/off range

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I put in one and a half cans in which was as close as I could get to 30oz. (12+12+6=30) However, since the compressor was cycling I thought it was asking for more charge so I put the remaining 6oz into the system totaling 36oz.

 

I will be sure to check my fan clutch.

 

____________________________________________________________________________________________________________________________________________________________________________

 

 

 

Hi Robert,

 

DId you mean you added two and half cans of refrigerant and not one and a half?

 

I hate guessing at things but that is exactly what I am doing here……. so take these comments for what they are worth.

 

Based on the low static reading my gut would say that you are actually low on refrigerant. That may or may not be your only issue though.

 

I'm wondering if the control valve in the compressor got buggered up and is stuck. (I am not a compressor expert and have never rebuilt one so I don't know if your compressor even has one).

 

I'm also wondering if there is a slight restriction in the orifice tube? Unfortunately there's no way to tell unless you remove it.

 

txab is correct. Some low pressure switches have a set screw in them to adjust the switch but you shouldn't have to do that on a new switch.

 

SInce your getting better cooling output when your going down the road that tells me either your not getting enough air movement thru the condenser at idle, the compressor is not operating up to par (not pulling or pushing enough), or you have a restriction in the system. It also sounds like your refrigerant level may not be within spec.

 

The easiest and cheapest thing to check is the fan clutch. That could eliminate one possibility quickly.

 

If it were me, I would then add a little more refrigerant using the method txab and I mentioned earlier in this thread and check the pressures and vent temps again and see if things get better or worse.

 

After that, I'd be pulling the orifice tube to check for a restriction and try to measure the size to make sure it is the correct one.

 

If that's fine, I'd look at the compressor.

 

I know this may not be much help.

 

DEWFPO

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Again starting with txab ... Yes, I am aware that I loose a little refrigerant both in the can and hoses. But to be honest I never really thought of the loss in ounces. Good point.

 

Yes and no to being able to isolate the fill line stopping it from getting air into it while changeing cans. I'm sure I dont need to explain to you how guages work. Knowing that, each time I change cans I bleed/purge the air out of the fill line using the schrader valve located on the guage manifold. Doing so I loose a little refrigerant each time. I can see where my losses of refrigerant here and there are quickly adding up to ounces.

 

Another good point about the capacity of the new condenser. Although the physical size was the same, the original one had square tanks on the sides and the new one has round ones. I can see where there could be a difference in capacities. Now that I'm thinking about it, I'd venture to say the new one having the round tanks would have more capacity than the old square tanks.

 

 

dewfpo

Lol. Sorry about the typo. Yes, I meant to say I had put 2 1/2 cans of refrigerant in.

 

I'm going to try adding more refrigerant very slowly while watching pressures and air temps.

 

Cant answer your question about the compressor control switch. Simply dont know.

 

The orfice tube is new so I cant imagine it would be restricted. That and wouldnt a restricted o-tube create higher, high side pressure?

 

I didnt see an adjustment screw on the switch but will look for one.

 

I checked my fan clutch and it feels fine. I compared it to two other vehicles and they all felt about the same. If I do ultimately end up with clutch problem, I will replace it with duel electric fans. No more guessing if its working or not and a little better gas mileage to boot.

 

Short of evacuating the system again, I'll check everything a third, fourth ... hell I've lost count how many times I've checked everything. I'm going bald scratching my head. Lol

 

 

 

Thanks again to both of you for all your help. Please bare with me as I may not have time to work on it tomorrow. (Sunday) You'll be the first to know when I do.

 

 

Robert

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