Anyone know the specs for super duty a/c components? I have been adding to the long thread on Super duty a/c after doing some investigation on my system which has the same conditions as those posted. See my adds for details.

The basic problem is that the design of this a/c system causes the low side pressure to run very low, say 40 psi to 20 psi. The low pressure cutout on my 2002 is cutting out at 20 psi. I removed the low pressure switch connector and jumped it out and the low side pressure hangs around the 20 psi area on a 93 degree day in max a/c, fan speed 3. Since the system wants to run around 20 psi, the compressor is constantly cutting out.

So I'm curious if anyone knows the spec for the low cutout pressure on a super duty crew cab, 2002.
I added 8 oz more then full charge and it still cuts out, so I don't think it's wise to add enough freon to keep it from cutting out due to possible damage to the compressor on cooler days.

Another thing I've noticed is that the fan clutch on these new models is very reactive to temperature changes unlike their predescessors. The fan kicks in and out much more often, at idle. So this "feature" also contributes the high pressure staying lower and consequently the low pressure side being lower.

 

Greg,

I am following your posts and am anxious to see what you find out. I have a 99 SD CrewCab and the AC performance is poor. I have contemplated putting in a heater bypass which I believe would help but is not the real cure to the AC problems.

My system worked better when the vehicle was new than it does now. My system has been gone into twice by the dealer; for evaporator core replacement and accumulator replacement. I wonder how low and for how long they evacuate the system.

One piece of advice I got from an AC guy here was that the R134 systems require the system be pumped down to a very low vacuum so that all moisture is removed before adding freon. I may try this if I can find a source of cheap freon and a set of R134 gages/fittings.

I will probably wait until the spring as the weather has cooled off now.

Pete

 

Thanks Pete, yeah I wonder how long they pump down too. I was in or around the shop when they were doing the a/c work and they didn't pump it down for long. I would have pumped it down at least an hour. the 134 isn't too expensive per can.

I just found a website that advertises a Smart VOV which is a variable orifice instead of the fixed one that comes with. I sent an email and will probably call them tomorrow to discuss this more. I found a website that shows the different orifice colors and corresponding sizes and guess what, my 02 has the brown orifice which is the smallest.

Here's what I'm going to do:
Either use the Smart VOV if I feel confident about what they tell me in terms of it raising the low side pressure at highway speed, or I'm going to get a larger orifice and reduce the charge. This is so crazy. When my truck is idling, it will cool down really well in max a/c after a while, then when I get going down the road it heats up. I think the designers tried to make the truck cooler for idling, which they did but unfortunately, the pressure gets too low at highway speed.

More to follow.

 

By the way, I wouldn't do the heater bypass thing. You are right, you still haven't fixed the problem and once we do, you'll be really happy. For people that do the heater bypass thing, they will get a little cooler air but the compressor is going to cycle even more because of the cooler air. This whole problem is the low operating pressure. The other falacy to this whole thing is that the low pressure cutout was originally intended to keep the compressor from sucking in air when you ran out of freon because it will pull a vacuum with no freon. I don't understand why they didn't leave the cutout at around 10 psi.

 

The "heater bypass thing" is not intended as an A/C fix. It is a solution to the vent air being hotter than the outside air temp. There are days when just circulating the outside air without resorting to A/C is all that is necessary to cool the interior. If you don't do the "heater bypass thing" then the vent air is going to be 10º hotter than the outside air temp. So, on a day that it is 70º outside you would have to run the A/C to cool the interior to 70º.

super duper

1999.5 F250 XLT SuperDuty SuperCab ShortBed
7.3L PowerStroke AutoTrans 3.73LSD 4X4

 

The purpose of the pressure switch is to keep the evaporator from icing up. The boiling point of liquid refrigerant is proportionate to the pressure. If they set the switch to 10psi, you'd have a solid block of ice in the evap. case and no airflow. In order to diagnose, we need the low side pressure, and the temperature at the evap. outlet.
BTW, I don't know if they intended for the pressure switch to prevent operation w/o refrigerant, but it does a good job of that. The reason that its not good for the compressor to run w/o refrigerant is that it will eventually pump all the oil out to the condensor, but you need the refrigerant to mix with the oil and bring it back around to the suction side of the compressor.
I was also going to put in the heater core shutoff valve, but I think I'll just wait 'till next spring. its cooling off here.
mike

 

I have found the ultimate solution for the heater shut off valve. This eliminates raising the hood and manually opening or closing an in line valve. It is an electronicly controled heater valve, including wiring and a switch. It is universal for 12v applications, and has 5/8" hose connections. What I intend to do is: Remove the cigarette lighter assembly (non-smoker) and use the existing wiring to power the switch. The switch assembly looks like it will fit nicely with minor modification (could even get a Ford ****). Cool!

http://www.fix.net/~mbarrett/valve.jpg

super duper

1999.5 F250 XLT SuperDuty SuperCab ShortBed 7.3L PowerStroke AutoTrans 3.73LSD 4X4

 

That electic controlled heater valve is what I had in mind too. Where did you find it and houw much does it cost?

 

I found it online at truckair.com. It's a bit pricey ($126.40) but it sure beats getting out and raising the hood to open the valve because the little lady is cold. You can call them at 800-338-6626.

super duper

1999.5 F250 XLT SuperDuty SuperCab ShortBed 7.3L PowerStroke AutoTrans 3.73LSD 4X4

 

lugnut, in max a/c, fan speed 3, the outlet temp at the vent is 47 degrees and the suction pressure is right near 20 psi. If I've been running for a while on normal a/c and switch to max, the temp will go 20 47 and then rise to 47.5 or 48 and then go back down to 47. The compressor is cycling because the suction pressure is operating at the 20 psi area. I jumped out the low pressure switch and observed the suction pressure on max a/c and it stays right at 20 psi, of course it will go lower when the inside cools off more.

I added more freon above normal charge to try and get the pressure up some but after adding 8oz and it not helping, I stopped. So it appears that the system wants to operate right at the cutout pressure when in max a/c. I think I stated this previously but the dealer removed the orifice and it wasn't clogged, replaced it anyhow, no help.

I'm going to try the Smart VOV and see if that will keep the pressure up more. The orifice from the factory is the brown one which is the smallest orifice, .047. I think that Ford Engr tried to improve the low speed/idle cooling by reducing the orifice, thus the suction pressure but it is hurting highway speed cooling due to pressure running too low. No temperature drop across the accumulator either so that isn't clogged.