Just Curious / Now Guidance

[akaDigger]

Never trouble trouble until trouble troubles you !!!

What's to sort out on that truck?????????????

I hear ya!
The 1986 Ford truck is now a R-12 to r134a conversion. I went to adjust the low pressure cut off because of freezing. The switch, seat of the pants adjusted down from 40psi, was too set low at about 20psi. I think I managed to sort that with the whacked gauge. IIRC, 25psi should be good there.

A 52oz R-12 fill originally, it now has 35oz of r134a. Also went with red tube versus the original blue. I had been waiting for milder weather. When I put the charge in it was 95°F outside.

[akaDigger]

Got the new gauge. It is as accurate as the high pressure gauge. Now curious about the wide splits between low and high pressure readings.
Ambient starting out 80°F. Outside air and vehicle in the shade. 80psi static pressure. Pressure switch opens at 25psi, closes at 45psi.

On MAX at 700rpm idle: 25 (cut out) low / 190 high.
MAX at 1400rpm: (rpm at 55mph) 25/210

NORM at 1400 started at 30/250 and gradually climbed to 40/325 before levelling off. With hood open, probably getting warm engine compartment air in through cowl vent. AC draws outside air from cowl but has air also coming from behind front inside fender.

Everything good and warm, switch back to MAX: Eased back to 25/240 but needed an rpm bump to get it to cut out. Seems the more rpm, the wider the the split.

Center vent 40° on MAX, 60° on norm.

If I slow the fan in MAX it cuts out and in a lot.

[tbirdtbird]

Have to wonder why you are running with outside air entering.....

There is confusing inconsistency being posted here.
You give this
MAX at 1400rpm: (rpm at 55mph) 25/210
then several readings that are higher....what's with that

[akaDigger]

AC draws some outside air when in NORM. Is this not correct? Higher evaporator temperature due to outside air results in warmer refrigerant/higher pressure. Is this not correct?

I used 1400rpm because that is cruising RPM.

The later, higher reading in MAX was after everything was warm. Higher system temp. Condenser now hot. Only had engine fan for cooling. New Hayes fan clutch BTW.

[tbirdtbird]

I am not familiar with your car's AC controls and exactly what they do. For that matter we do not even know what car you are working on, you never said. What repairs have been made and why?
In any event, try to find a setting that does not allow outside air in. Can't think of more foolhardy engineering. Kind of like leaving your refrigerator door open all day. No recirculate setting? I have never owned a car that did not have a recirculate feature.

That is quite a split (Pressure switch opens at 25psi, closes at 45psi.)
Is that how it is rated or is that your observation. As far as I am concerned that is too wide a split, you may have to swap it out.

You have some non-standard behavior going on with your system the way it is designed, or at least the way you describe it as being designed.

[akaDigger]

I am not familiar with your car's AC controls and exactly what they do. For that matter we do not even know what car you are working on, you never said. What repairs have been made and why?
In any event, try to find a setting that does not allow outside air in. Can't think of more foolhardy engineering. Kind of like leaving your refrigerator door open all day. No recirculate setting? I have never owned a car that did not have a recirculate feature.

That is quite a split (Pressure switch opens at 25psi, closes at 45psi.)
Is that how it is rated or is that your observation. As far as I am concerned that is too wide a split, you may have to swap it out.

You have some non-standard behavior going on with your system the way it is designed, or at least the way you describe it as being designed.

Perhaps you missed the the post immediately preceding the post you are responding to.

The 1986 Ford truck is now a R-12 to r134a conversion. I went to adjust the low pressure cut off because of freezing. The switch, seat of the pants adjusted down from 40psi, was too set low at about 20psi. I think I managed to sort that with the whacked gauge. IIRC, 25psi should be good there.

A 52oz R-12 fill originally, it now has 35oz of r134a. Also went with red tube versus the original blue. I had been waiting for milder weather. When I put the charge in it was 95°F outside.

Other info: All components are new except compressor. It is a dismantled and cleaned, resealed 1988 FS-6. The condenser is a serpentine replacing the OEM fin/tube.

All vehicles I'm aware of have a provision for make-up/outside air. For that matter, all commercial buildings too.

Running in NORM on my vehicle prevents freezing of the evaporator due to the extra heat from outside air being transferred to the refrigerant. I can run the fan at medium or low speed in this mode without icing. On MAX and high fan, it gets uncomfortably cold in the cab not to mention the noise of the high fan speed. I can modulate temperature in cab by adding reheated air but this does not help with the fan noise.

ac ford 2.JPG (89.39 KiB) Viewed 5593 times

Looking at specifications on clutch cycling switches, it appears my sampling runs from19psi range to 25psi range. Thunderbird, you state that you believe a twenty psi range is too wide. What should it be on my application? What is it on your vehicle?

My original question: Why are my low and high pressure readings so wide? Barring any revelation from this forums panel of experts, I will turn my attention to the orifice tube. On the net, there is no consensus as to whether red or blue should be used in my application / R12 to 134a. Like Neo, I chose red.

red blue 2.JPG (25.76 KiB) Viewed 5593 times

[tbirdtbird]

Oops, my bad, sorry I missed that post.

Any vehicle I have ever owned had a fresh air option (option, not mandatory) and a recirculate function. I have never owned a Ford, so I have no direct knowledge. But again, if there is no recirc option, then the Ford engineers must keep their refrigerator doors open all day.
My cars have been Toyotas and Mercedes, and BMWs. Oops I owned a F250 Super Duty for a while. Frankly I thought it was under engineered. My next pickup was a Silverado. Anyway all cooled properly. The thermostatic control has been a probe in the evap that would also prevent frosting.

"Why are my low and high pressure readings so wide?"
Is this because the readings are taken with the compressor engaged and not engaged? Can't be, you are smarter than that

The controls I have seen have about a 10° split

[tbirdtbird]

I have re-read your scenario a dozen times.

See if I have summarized this correctly.

MAX at 1400rpm: (rpm at 55mph) 25/210 (Engine cold)
NORM at 1400 started at 30/250 and gradually climbed to 40/325

So as the car warms up the pressures rise, especially the high side? Pressures always read when clutch engaged?
Can we get complete set of readings both at MAX and NORM. Don't need idle. Include vent temp
Can you lay a blanket over the cowl vent while doing so to eliminate that as a variable

It is not clear to me how the OT could be held responsible here.

Perhaps JohnHere or Bohica could also respond

[akaDigger]

See if I have summarized this correctly.

MAX at 1400rpm: (rpm at 55mph) 25/210 (Engine cold)
NORM at 1400 started at 30/250 and gradually climbed to 40/325

So as the car warms up the pressures rise, especially the high side? Pressures always read when clutch engaged?
Can we get complete set of readings both at MAX and NORM. Don't need idle. Include vent temp
Can you lay a blanket over the cowl vent while doing so to eliminate that as a variable

It is not clear to me how the OT could be held responsible here.

Perhaps JohnHere or Bohica could also respond

Yes, starting fresh, on MAX (recirc) fan on high, doors closed windows open, the pressures 25/210 seem good at 1400rpm except the low is a bit low. The clutch switch is cycling now at 25psi and I have constant cycling. The high is going up and up while the low side is dropping. The high side is at 210psi when the low side hits the cut off at 25psi.

Just ran it a quarter mile down the highway at just above 1500rpm. Clutch was cycling six to eight times per minute. Seat of the pants measurements. I can hear the clutch each time it goes out or comes back in. Trying to count clutch clicks and count seconds simultaneously isn't easy.

The switch setting now prevents icing at 25psi. It was icing everything when it was set at 21psi. Ice from OT all the way to and including compressor suction manifold.

On the OT, my thinking is it is acting as too much of an obstruction. High keeps going higher, low going lower. Also thinking low charge but the high high side reading says otherwise. And it gets plenty cold.

Hope this is not clear as mud.

[tbirdtbird]

If you read my thread, maybe we are both having the same problem.......restriction in low side