Automotive Air Conditioning Information Forum (Archives)

rebar on Sun July 19, 2015 9:40 PM

Year: 1995
Make: Ford
Model: E350
Engine Size: 5.9
Refrigerant Type: 134a
Ambient Temp: 80
Pressure Low: 22
Pressure High: 175
Country of Origin: United States

Hello.

I just replaced the compressor, condenser, accumulator and O-tube and drained all oil after removing the evap. Added the factory 5 oz of oil in the compressor , spinning it by hand 15 times, and 2 oz in the accumulator inlet then assembled with nylog. Pulled 250 micron vacuum, triple evac. Added the factory 2.75 lbs/44 oz of 134a as a liquid before starting the engine.

After warm up I turned on max a/c . I checked idle pressures. 33/180. 52 from vents, 84 ambient. 900 rpm.
I took the van for a drive on the interstate since my condenser has a little air leakage/bypass issue.
At 65 mph I turned the AC to max and was disappointed with the 60 degree vent air. I have more air flow now since I replaced the blower motor, so it was slowly cooling the interior to a tolerable temperature after 5 minutes. But I hoped for cooler vent temperatures after all the effort and time to do it right.

Today I put a huge fan in front of the van, and set my idle to 1700 rpm to recheck pressures. I had 25 low and 175 high at 80 degree ambient with 52 degree vent air and dropping. Slowly, the low side kept dropping lower, and I saw 23 psi right before the compressor was cut off by the low pressure switch. It came back on, but is that normal operation?

To me, if the system ran with a lowside pressure a little above the cut out pressure. 29 psi/34 degrees, It would remove more btu's without the short cycling interruptions, and have less wear and tear on the clutch. Unless , the high side was so high, the compressor was stressed out pumping it with nowhere for excess refrigerant to reside with no receiver.

Should I leave the factory 2 3/4 lbs refrigerant alone and live with the low pressure cycling? Since for a big leaky van without rear a/c and sheet hung behind front seats, it was tolerable on a 85 degree day.
Or add enough 134a to prevent dropping lower than 29 at 2000-2500 cruising rpms? How much that would be , and how high the head pressure becomes is the question.

Thanks



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95 E350 6bt/nv4500

GM Tech on Sun July 19, 2015 9:48 PM

Rare to cut out with blower on high- but 23 psi is a good pressure to shut off the compressor to avoid evap freeze-up- mid 20's suction pressure is desired and ideal- so your cooling is doing fine- vent temp will vary with fan speed- and by all means run it in "recirc" air mode- like the way a/c does in your house- it will cool much better than to try to cool 80 degf air one time and blow it out your door vents- I always test evap air temp at low fan recirc after a high fan recirc run of 15 minutes or more- 50 deg vent on high fan outside air is pretty good- and your suction pressure says you should be able to get low 40's on low fan recirc- and the compressor will start cycling.

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The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

rebar on Sun July 19, 2015 10:10 PM

Id be happy with mid 20's pressure to keep my compressor running, but I'm dropping lower.

I'm running Max a/c which is recirculation and the dampers are working. Pulling air from Inside.. But if its cycling, its not removing btu's while off.

It was 52 vent in the driveway because it was 33 psi and not cycling. As soon as I drove the pressure dropped and cycled the compressor giving me the 60 degree vent air.

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95 E350 6bt/nv4500

GM Tech on Mon July 20, 2015 7:34 AM

It is working as designed by Ford for extra humid weather- But if you want to experiment- go ahead and jumper the cycling switch connector- but be careful- suction pressures down into the teens will limit oil flow and return back to the compressor and destroy it- or if you have a massive loss of refrigerant, you will have no low charge cut-out and again, destroy your compressor- been there done that when I was in your situation.-- I'd keep a suction pressure gage on it (routed inside the cab) as you experiment.

I used to intentionally fail units- and the easiest way was to make it continuous run at around 10 psi suction-- lots of frost on pipes and compressor lasted about 10 miles until the front thrust bearings welded themselves together.

-------------------------
The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

rebar on Mon July 20, 2015 12:13 PM

Quote
Originally posted by: GM Tech
It is working as designed by Ford for extra humid weather- But if you want to experiment- go ahead and jumper the cycling switch connector- but be careful- suction pressures down into the teens will limit oil flow and return back to the compressor and destroy it- or if you have a massive loss of refrigerant, you will have no low charge cut-out and again, destroy your compressor- been there done that when I was in your situation.-- I'd keep a suction pressure gage on it (routed inside the cab) as you experiment.

I used to intentionally fail units- and the easiest way was to make it continuous run at around 10 psi suction-- lots of frost on pipes and compressor lasted about 10 miles until the front thrust bearings welded themselves together.

Yeah, no thanks.. I'm leaning toward adding a few oz of refrigerant to keep me above 23 psi and skip the experiments.

Aren't experiments when you don't know the end result?



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95 E350 6bt/nv4500

jsmitty on Mon July 20, 2015 11:54 PM

That's how we all learn.

rebar on Tue July 21, 2015 7:16 AM

Quote
Originally posted by: jsmitty
That's how we all learn.

Id rather learn from other peoples mistakes.

Now I'm being told I pulled vacuum to low at 300 microns. That I pulled air through the shaft seal and now its leaking because my system acts like its low on charge..

Cant win for losing



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95 E350 6bt/nv4500

GM Tech on Tue July 21, 2015 7:27 AM

No way can you pull too much vacuum-....

-------------------------
The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

rebar on Wed July 22, 2015 7:29 AM

Adding 2 oz made a big difference. I'm more satisfied now.

78 ambient. My pressures remained at 28/160 at 750 rpm with no cycling.

At 1900 rpm ,65 mph, the compressor started cycling again. But this time the cycling slowed, and the lowside pressure stayed lower (high of 42) during the off cycles. On cycles were longer. Vent air dropped to 48 ! From 60.

So adding two oz's dropped vent air 12 degrees. But ambient was 4 degrees cooler today.

With a highside that never went over 180, I think there might still be room for lower vent temps and less cycling.

Id like it to run at 29 and hold like it does now at 750. But at 2000 rpms while I'm towing. But also considering leaving it alone with 48 degree vent air.

I downloaded the video of how the system cycles at higher rpm after adding the 2 oz. First at idle 28/160 without cycling, and then cycling at cruising rpms.




video

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95 E350 6bt/nv4500

Edited: Wed July 22, 2015 at 7:41 AM by rebar

mk378 on Wed July 22, 2015 2:10 PM

You need enough refrigerant to "flood" the evaporator with liquid that is in the process of boiling. If it is undercharged, some of the evaporator metal will not get cold, even though the low side is low and what liquid that is in the evaporator is boiling at a low temperature. Thus the air temperature rises.

The way to tell is to accurately measure the temperature of the outlet pipe of the evaporator and compare it with the temperature of the inlet pipe and / or the saturation temperature corresponding to the low side pressure. Some liquid refrigerant leaving the evaporator and making the outlet pipe and accumulator cold is a characteristic of CCOT operation.