2001 379 ac problems

Improper charge, weak expansion valve.

 

Most peterbilt trucks that I have charged will not cool properly unless you have the exact amount of freon in it. Usually 5.5 LB. No more. No less. Some take 5.0 LB. There should be a tag on there so go by it. Your high pressure should not cause the fan to kick on at 190. It should get up to 300 before the fan comes on. You have an orifice tube in the cab portion of the truck and a expansion valve in the sleeper (which is a PITA to get to).

I would drain it and then replace the orifice tube maybe even put a new accumulator on it (I would). Run a good long vacuum on it and then charge to label.

The orifice tube is located at the lowest metal line that enters the cab near the turbo area (usually a wiper fluid tank there that blocks access). After draining the freon you can unscrew the metal connection and the orifice tube is in there. It is difficult to remove and difficult to reinstall usually. They make a tool that does both that will help. Make sure you put some A/C oil on the o-rings before reinstalling.

 

Air dryer could be plugged also

 

It sounds to me like you have two problems, low on Freon and something going on with the expansion valve. As stated above Pete uses an orifice for the expansion valve in the cab. If you had a failure a year ago, (the reason the entire system was replaced), you might have a very small piece of trash randomly getting in the orifice. This will make them do eratic thing's. I would suggest changing the dryer, the orifice that takes the place of the expansion valve, a good vacuum out, and then recharge the system properly and you should be good to go.

 

You have two options, an orifice tube or an expansion valve. There's no such thing as an orifice expansion valve.

Heavy Pete, if you're only getting 50 psi on your low side, I'd say you're undercharged. Low side pressure should be approximately the same number in psi as ambient temperature in degrees F... so if it's 80 F outside, your low side should run 80 psi +/- 10 psi or so. If you haven't replaced the dryer or orifice tube recently, it wouldn't be a bad idea to do so. Neither are high-dollar items.

 

I was just wondering if the problem was remedied? I too am having the exact same issue. Yet, I have replaced the orifice tube, dryer, compressor, o-rings in all lines. I removed the cab evaporator and cleaned it off. I blew every line out to insure no blockage. Everything blew out debris free. I did vacuum the system for 30 minutes then allow it to sit for 30 minutes w/vacuum pump off to verify zero leaks. I recharged system to 5.5 lbs and still blows @ 70 degrees. If it is the expansion valve, how do you access this little animal in a 379? Thank you!