Steve83, sounds like you know the Ford A/C's pretty well. Comments please:

I've been working on A/C and refrigeration for years. My 2002, as with many other folks' 2002's and even earlier models, the compressor cuts on and off when in Max A/C. The cab never cools down any better in Max then normal.

I've checked pressures, superheat.....and it appears to me that the suction pressure just flat runs too low. My truck has 31k miles on it. I've had it back to the shop once and they checked the screen and put back the specified charge amount. Basically there was nothing wrong they said.

I don't see any temperature drop across the accumulator and can only come to the conclusion that the orifice that Ford spec'd is too small. On a hot day in normal a/c mode, the pressure will run in the low 30's to high 20's. As soon as I go to max a/c, the pressure will work it's way down in a matter of a minute or so to the cutout of 22 psi and the cab temp is mid to upper 70's.

I think the orifice is the brown one which is the smallest orifice tube from what I find on the web. What do you think about experimenting with the largest orifice size or somewhere in between until I get good cooling and the suction pressure up some? By the way, I saw a tech bulletin on the 99's I believe and it said that Ford has spec'd the charge too low and to add a pound.

My 2002 power stroke is spec'd at 2lbs 10 oz. I added about 8oz and the compressor stays on a little bit longer but still cuts out too soon.

What is the pressure cutout supposed to be for this vehicle?
Is it adjustable?

If I do experiment with different sized orifice, what head pressure range should I work toward and at what rpm in park on a 90+ degree day? I don't want to get the head too high.

Thoughts?

 

Are you keeping the blower on hi? If there's not enough air going across the evaporator core to boil the refrigerant, the pressure WILL drop and the clutch will cycle, even if the charge is right. Do those trucks have as much of a problem with leaves in the ventilation system as the older ones? That will reduce the airflow, too.

I'm not as familiar with the new trucks (OK, not at all ), but all R-134a systems are basically the same. The low side should be in the upper 20s and the low cut off switch should be set at 24psi. On mine, there's a tiny flathead screw between the terminals of the low switch to adjust it, but I don't know about yours.

"Max A/C" just means "recirculate" - nothing more - so when you're in "Normal", you're cooling the warm outside air and then blowing the cool air out of the cab. When you choose Max, you're REcooling the cool air in the cab, so the clutch should EVENTUALLY cycle, but only after there so little heat left in the air that the refrigerant won't all boil in the evaporator. (There's not supposed to be a noticeable temp drop across the accumulator, because the orifice is in the evap inlet.)

On '80-96 trucks, we use a red orifice for 134, but I've never seen what your truck uses so I can't give you any advice.

What's the high side doing when the low side drops? I've always added refrigerant until the low side stays at ~26psi with the blower on high, the idle at ~2000, and the selector on Max, but if the high side goes over ~280 or won't come above ~190, there's a problem.

You don't really have to worry about the high side going too high since all OE 134 systems have a high cut off switch, too.

 

I think i would go with the red ford orifice or a VOV and then charge useing the temp to pressure chart and the high side numbers use what you need to get that right and egnore the ammount set for the factory charge ive allso known of a case where they went with the white GM orifice tube and add some good sucess .

 

Thanks all for the comments. Steve, yes I understand the refrigeration stuff and physics of freon. That was my second career. I ran some tests on the ac system, knowing that the suction was going too low. This is in Max a/c, high fan speed and about 2k rpm. The compressor would cut out, like you say, at 22 or 24 psi suction, can't remember exactly. The cab wouldn't even be that cold.

Then I drove the vehicle and put my Fluke digital temp probe in the vent and you can see the temp go down and then go back up, repeatedly which meant the clutch was cycling. Can watch it cycle even at curb w/2krpm. Thinking that Ford must have designed the orifice for the right size, I decided to test by putting a piece of cardboard over the condensor. I don't remember what the high side went up to but it never kicked out the compressor due to high head pressure, and the suction pressure never rose. That really baffled me.

The only conclusion I can come to is that the orifice is just too small. It is the brown one which looks like the smallest orifice Ford makes. I just can't believe these trucks are coming out of the factory like this. This topic has been on this board before or over on the power stroke forum.

I see that there is a bulletin about the charge being too low in the '99 models and to add another pound. I added 12 oz/a can and it only cools slightly better with the clutch still kicking in and out. I'm going to check and see if the low pressure switch is adjustable. If it isn't, I'm going to try the red orifice and see if I can keep the suction pressure up more with that one. I tried the VOV and it didn't help, compressor still cycles.

This situation is so extreme that I can cool the cab off better in normal ac then max.

Will let you know what I find out.

 

Have checked out the bulliten board at
http://www.aircondition.com/ ?
some good A/C talk there

 

Well, I've done some experimenting and tests. Following are readings before any changes:
orifice is the red one
Normal AC:
low - 36
high - 225-235 psi (these readings at 1500 rpm)
8 degrees superheat
80 degrees outside, 60 degree vent temp

Max AC:
Took readings right before cutout at 21 psi on low side
low - 21
high 190
vent temp 47

Readings vary as radiater fan varies speed

Removed the red orifice and put in a blue one
Vacuumed then charged system to about 5 degrees superheat and after going to max ac the coil froze up.

This makes me realize that you can't run the superheat too low on these systems or the coil will freeze and really drive the low side down quick. I recovered freon to get about 10 degrees superheat and ran the truck. I was getting about the same cooling on max but on normal ac, the air is warmer so I went back to the red orifice.

Recharged to about 10 degrees superheat on normal ac. I'm basically back where I started.

One thing I noticed when removing the red orifice was a few metal chips on the screen. This bothered me but not as much as when I removed the blue orifice I only ran for a day. That one had a couple of chips on it too.

I sure hope the compressor isn't breaking down, this truck has 32k miles on it and the compressor shouldn't be going out already. Have you ever seen small chunks of metal on the screen and the compressor still last a long time? I wonder if there is some junk in the condensor or something?

Anyhow, I'm just going to live with the darn thing. If there isn't some slight clog somewhere in the suction side, maybe these compressors are a little too big capacity for the system, I don't know.

Let me know if anyone has anymore pointers? I just remember my older vehicles had much cooler air in max ac and the compressor never cutout like this one does.
Anyone know what the low side cutout should be for a 2002 f250?

Main thing that was reinforced in this experimentation is that the charge is critical on these systems. Since the cutout psi is ~20, that means the saturation temp is about 21 or so. That means that you need about 10 degrees superheat to keep the coil from freezing and causing even more low pressure cutouts. If you try to add enough freon to get the suction pressure up and keep it above cutout, the coil will just freeze up.