Peterbilt a/c help

Give gearhead a shout. I'm sure he'll have an idea.

 

the fan should kick on at 350 psi I believe, and 20 psi is extremely low...

 

This is common with pete. Is the evaporator clean? If so try adding 1oz. of 134a at a time until the compressor runs right. The tag on the truck that tells you how much to put in is not always right on the petes for some reson. I was told this at pete's ac class I took in baltimore when they started using the orfice tube. Now they went back to expansion valve what's that tell you. Just add a little at a time there is a thin line between it working right and the system freezing up. While your adding watch and make sure your engine fan still cycles. Then turn fan speed for blower motor to 2 and make sure the compressor will cycle off at some point. The system cycles off of pressure because there is no freeze switch. So the system is very sensitive to proper charge. The system uses a pressure switch which is gray in color and located in the low side To cycle the compressor.

 

I put 3# in an expansion valve 379.

 

Can someone point the location on the expansion valve and orifice tuve on these 2007 Peterbilts 379 thank you

 

No expansion valve, orfice tube is located @ evaporator inlet.

 

Orifice tube is inside the evaporator high side tube. Accessed from the outside at the firewall. Disconnect the lower line from the evaporator and pull the tube out with needle nose and make sure you have a firm grasp. Pay attention to arrow direction of refrigerant flow. The cab does not have an expansion valve on orifice tube system.

The bunk may have expansion valve and it is mounted inline at the evaporator inlet.