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Perplexed at vent temps when driving, help please

rspreng on Mon July 25, 2011 7:59 AM User is offlineView users profile

Year: 1995
Make: Chevrolet
Model: K1500 Silverado
Engine Size: 350 5.7
Refrigerant Type: R134A
Ambient Temp: 80
Pressure Low: 25
Pressure High: 180
Country of Origin: United States

Help please.

I just installed a new compressor, old one the belt jumped and cut the electrical point on the clutch so it was easier to install a new compressor. Purchased a new one not reman.

So I installed compressor, pulled vac. for an hour, vac. held, charged enough to cycle compressor, evacuated and held under vac. for 45 min then charged. System is 32 oz, I might be 3-4 oz over no more, was having an issue with scale.

So here is my question, at idle on 80* high humidity morning I have a vent temp of 40° after truck has run for a few minutes. As soon as I take off down the road vent temp goes to 49 and I get compressor cycling, (most likely reason for not keeping vent temp 40°)

When vent temp is 40° low pressure reading is ~25psi really close to the cycle switch limit, so when RPM goes up, low pressure drops and cuts off.

Same type thing happens when ambient gets to 100°, vent temps usually warmer.

Truck has cold vent temps at idle and increases has RPM/speed increases. Note: At 70mph on highway 2000RPM vent temp is 49° HVAC controls - recirculate and high speed selected on blower speed, manual controls.

What is causing higher temps when cruising?

Thanks

iceman2555 on Mon July 25, 2011 9:34 AM User is offlineView users profile

It appears the system is undercharged. Adding refrigerant to a system in the manner described is not an acceptable method to insure a proper charge rate. The only true method of knowing how much refrigerant is in the vehicle is to weigh the amount and be precise with the weight. Add sufficient refrigerant to 'pre-charge' you hose assembly. This is not only necessary for cooling performance but for compressor longevity. An undercharge of 10-15% will have an adverse effect on lubricant migration. Also, there was no notation of new orifice tube, accumulator, flush or new lubricants. There are some serious issues in this repair that should be addressed.

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The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

rspreng on Mon July 25, 2011 9:56 AM User is offlineView users profile

Quote
Originally posted by: iceman2555
It appears the system is undercharged. Adding refrigerant to a system in the manner described is not an acceptable method to insure a proper charge rate. The only true method of knowing how much refrigerant is in the vehicle is to weigh the amount and be precise with the weight. Add sufficient refrigerant to 'pre-charge' you hose assembly. This is not only necessary for cooling performance but for compressor longevity. An undercharge of 10-15% will have an adverse effect on lubricant migration. Also, there was no notation of new orifice tube, accumulator, flush or new lubricants. There are some serious issues in this repair that should be addressed.

Maybe I did not expain myself, I weighed in the charge of 32 oz the 3oz over was a scale issue I thought at first but was fine. I added another 3 oz because I thought the system was undercharged based on the pressure readings.

The pre-charge? Is that to get the system to 0psi from vacuumn state?

The trucks AC system was cooling fine before the belt issue. Orfice tube and accumulator were not touched. No flush was performed. 3oz of compressor PAG oil was installed in new compressor, (put in what was drained from old compressor) wanted to inspect orfice tube but there was no way to loosen connection at condensor w/o breaking or cutting the line. Nut was seized.

iceman2555 on Mon July 25, 2011 11:00 AM User is offlineView users profile

The statement that 'having a problem with the scales' was an issue. A standard set of gauges requires...app 3-4 oz of refrigerant to 'pre-charge' the hoses. Not allowing for this may result in an undercharged system. Adding the extra 3-4 oz as stated should be sufficient for this....if the scales were correct at the offset of this procedure.
That being said...your pressures seem a bit low for the temp range stated. The low pressure indicated is very close to the LPCO cut off for this system. Elevating the RPM increases the efficiency of the compressor and result in a decrease in this pressure.....the result is excessive cycling of the compressor.

Your description of performance and cooling issues and pressures would indicate an undercharged system. Insure the scales are correct and working properly. Test the scales with a known weight and adjust according. It is important with these systems to insure the correct charge. As the charge rate decreases...the need for more precise monitoring is required.
Good luck.

-------------------------
The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

rspreng on Mon July 25, 2011 11:22 AM User is offlineView users profile

Thanks for the help, much appreicated, borrowed scale from work, as it is certified monthly.

The pressures stated in orginal response were after truck was cooled in shade, with a steady vent temp of 42°, windows/doors were closed.

Late yesterday with an ambient of 97°, my low side was 40psi and high was 230psi, both dropped just a tad as truck was cooling.

I bet I might still be 3oz off because of the hose, did not think about that, thank you.

Will put the gauges back on and see what I am getting. Might even tape them to the windshield and drive around.

Also I might put a fan in front of the radiator as I am relying on the mechanical fan (belt drive)

iceman2555 on Mon July 25, 2011 11:30 AM User is offlineView users profile

The 'second' pressures seem to be more in line. DO NOT PUT A FAN IN FRONT OF THE VEHICLE! Make it work with the engine supplied cooling.
Test the inlet and outlet temps of the evaporator. Max Cool/Hi blower/Doors open/Idle speed. Allow the system to operate for about 5-7 minutes. Measure the evap inlet (after the orifice tube or at the fire wall) the evap outlet (short distance of metal tubing before the accumulator inlet). These should be the same. Use your fingers if nothing else is available. The inlet may be slightly warmer than the outlet...but the outlet can not be warmer than the inlet. If a temp measurement tool is available the temps should be the same or within 3-5 degrees of each other. The inlet being slightly warmer is better.

-------------------------
The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

tvlunn on Mon July 25, 2011 11:58 AM User is offline

Hello Iceman2555,

At the moment, I do not have any AC issue .......... however, I have been lurking for the past couple of weeks and noticed your statement to the original poster:

"A standard set of gauges requires...app 3-4 oz of refrigerant to 'pre-charge' the hoses"

I would not have thought that much refrigerant would be in the manifold gauge hoses. So, as a learning to me also, whenever one is weighing in a charge to a vehicle, are you suggesting we add an additional 3 or 4 ounces. The few times I have added refrigerant to my vehicles, I have always added exactly what the manufacturer recommends. Is this going to cause me issues in the future?

Thanks

iceman2555 on Mon July 25, 2011 12:22 PM User is offlineView users profile

We have tested various manifold to determine the amount of refrigerant necessary to 'pre charge' the hoses. This amount varies by the type manifold and the length of the hoses. We have found that the average is app 4.0 oz. As recharge rates become more critical due to the decrease of system total recharge this amount becomes more and more important.
The number one failure of replacement compressors is directly related to lubricant. The flow of lubricant within the system is controlled by the amount of refrigerant or the flow of refrigerant within the system. An undercharge of 10-15% can reduce lubricant flow substantially.

-------------------------
The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

rspreng on Mon July 25, 2011 12:24 PM User is offlineView users profile

Quote
Originally posted by: iceman2555
The 'second' pressures seem to be more in line. DO NOT PUT A FAN IN FRONT OF THE VEHICLE! Make it work with the engine supplied cooling.

Test the inlet and outlet temps of the evaporator. Max Cool/Hi blower/Doors open/Idle speed. Allow the system to operate for about 5-7 minutes. Measure the evap inlet (after the orifice tube or at the fire wall) the evap outlet (short distance of metal tubing before the accumulator inlet). These should be the same. Use your fingers if nothing else is available. The inlet may be slightly warmer than the outlet...but the outlet can not be warmer than the inlet. If a temp measurement tool is available the temps should be the same or within 3-5 degrees of each other. The inlet being slightly warmer is better.

Gotcha, scrap the fan idea, no problem

I did pick up the RPM's to around 1800 and watched the gauges, low dropped to around 35 and high was close to 300 IIRC ambient was 97°

On to evap inlet and outlet temps, I have a thermocouple on my fluke meter, however not sure if I can clamp it on the outlet, not alot of pipe there before the accumulator. However, by observation and feeling the outlet it is sweating and cool/cold, the inlet is not sweating and feels just a tad cooler than ambient. There is a temp difference of what I would think is greater than the 3-5°'s.

On a side note I did pinch the heater hose lines and that has no affect so blend doors are working.

bohica2xo on Mon July 25, 2011 3:27 PM User is offline

Time to look at the fan clutch. that 1800 rpm test shows a lack of airflow on the condensor.

Make sure the air path through the condensor stack is clean. On a cold start the fan should howl. After idling for 5 or more minutes with the engine at operating temperature, and the A/C on high with the doors open - the fan should engage. Increasing engine rpm should make the fan howl. It may slow down again if you hold the engine speed up long enough.

Even in the city, dirt / bugs / grime build up in the closely packed condensor fins. A deep cleaning with a strong surfactant is recommended.

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

rspreng on Mon July 25, 2011 5:39 PM User is offlineView users profile

Thanks. I will check the fan. The condensor is clean. No dents or bugs stuck in it. Fins are straight.

I added 4 ounces of R134a and that brought the low side to 30, high side was 195 and ambient was 78. I drove it around and it seems to be behaving properly. What are thoughts on those pressures?

This site is very helpful, many thanks.

tvlunn on Mon July 25, 2011 5:45 PM User is offline

Thanks Iceman2555!

I shouldn't be "stealing" this thread, but I do have one other question for you"

Your statement has got me thinking: An undercharge of 10-15% can reduce lubricant flow substantially .............. based on this comment, can one then conclude that the AC system will fare better with a small overcharge versus a small undercharge? In other words, if one has to err on weighing in a charge, err on the side of a slight overcharge. Thanks so much!

TVL

tvlunn on Tue July 26, 2011 9:56 AM User is offline

bump

iceman2555 on Tue July 26, 2011 12:19 PM User is offlineView users profile

Thought I posted a reply. Must be this old freon clouded mind!!!

A while back, we did several test to determine this possible under charge condition. This amount of possible undercharge was as much as 4.3 oz. Even back in the early days of R/R machines, several manufactures of these machines stated in their directions to add '2 oz ' for each of the hoses. A one point in time this possible undercharge probably did not have a serious effect on the system. Certainly not with pressures and vent temps. However, as the system requirements for recharge becomes more stringent....the ability to charge a system with cans or containers and no monitoring system becomes more and more difficult. This issue is also an important aspect of those shops that have older, possibly, outdated R/R machines. Many of these machines are so far out of tolerances that they are not charging the systems as they should.

Several weeks ago had the opportunity to visit a shop with a jobber salesperson. This was a prospect call for new business. Several questions were asked concerning his current compressor supplier and possible failure concerns. The installer voiced his displeasure at several compressor vendors and a list of problems. All the issues stated could be directly related to lubricant issues. When asked to 'see' his recovery equipment......well...lets simply state that this equipment was severely lacking in service and the ability to properly charge a system.
A comment to the sales person after leaving the shop was that if they did begin to sell this customer to expect a high percentage of compressor returns. This was not only validated by the condition of his AC service equipment but to other statements made by the installer.

One of the most important aspects of an AC repair is to insure that the system is properly and completely recharged to OE specifications. Unfortunately, this is almost impossible to accomplish with the use of cans and a set of gauges and a perceived pressure range. Systems were not designed to be charged to a perceived pressure....but the use of dedicated recharging equipment to insure that the systems were completely recharged. Operational pressures are a diagnostic tool to determine performance concerns with a fully charged system.

As a personal note...yes, I would like to see the system over charged by a very small amount. It is critical that the modern system be properly charged. Compressor longevity or the lack thereof is the result.

-------------------------
The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

tvlunn on Tue July 26, 2011 2:59 PM User is offline

Thanks Iceman2555 .................... just the information I needed.


As a side note: a month or so back I had to work on my wife's 2006 Jeep Liberty. The issue was resolved, but in the process, I had to remove the R-134A refrigerant, vacuum and re-charge (28 ounces was the manufacturers recommendation). After looking over my maintenance log (yes ...... I keep notes for whatever reason), I see that I added 28 ounces and then 2 additional ounces. I don't know what made me add those 2 extra ounces that day, but according to my notes, I did.

Do you feel that gets me close enough to a precise charge to ward off any possible problem in the future? OR would you recover the refrigerant, vacuum the system and recharge with an additional 4 ounces. I just hate to mess with the AC as long as it is working so well. But, I do value your input!

Thanks again!

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