My A/C works fine while driving down the road, and in fact it still gets very cold, but during stop and go tracffic or at red lights, it will start blowing warm air. I've been told it could be the pressure cycling switch from the service guy at the dealer? And it's at the firewall? Well I have looked, asked at three different auto parts stores and yet to find this culprit. All I can find is the high presure valve. I have checked the fuse for the fan and it's still good. Any suggestions? It's an 03 with the 5.7

 

Your condenser fan is not running under the hood. Usually the motor fails.
Start the truck, turn on the a/c, open the hood and look at the electric fan on the driver side. If it is not turning, try to smack the motor and see if it starts. If so, the motor is bad. If not, you need a test light or meter to see if there is power getting to the fan.

 

What he said ^^^^^^ make sure the fans turning with some guts/torque to it as well.
The fan brushes wear out and causes either intermittent/no fan operation.
This cause your AC to overheat and the pressure cycling your hearing.
Al.

 

Have you checked the pressure in the system? If the low side drops below 30 PSI the sensor will kick the clutch out till it equalizes and returns to normal.

 

Also chec out this link

A/C Condenser Fan Motor Info

 

This is true, but in his case, at idle, the low side pressure rises. This would make it colder at idle and short cycle at drive rpms.

 

i have this same problem with my 02 with the 5.9l. it did the same exact thing. n i didnt not check the fan either. but i hooked the gauges up. n the high side was like. pegging the guage. i think it was around 3 er 400psi er so. so i was thinkin maybe a blockage somewhere?

 

When mine did this it was the condenser fan, I cleaned out the motor from the two slots on the bottom ( with the ignition off ). Work it loose with wd40 or other electrical safe cleaner/lube. Turn ignition on & test it to see if it comes on properly at idle. Keep trying, lube it again after a little use.

 

Yeah thats really bad. I would pull your Orpheus tube and clean it or preferably replace it. You could also run the system without the tube to see if it clears the blockage. If It does I would also change you're drier and Orpheus tube.

 

If your highside is that high you have a blockage somewhere. Possible condenser or evaporator. If you open the system at all, replace the drier. If your orifice tube is filled with debris and metal shavings you might want to prepare to replace your A/C system.

Be careful with A/C. It is very sensitive to charges, lubrication, etc... the smallest thing and you can kiss it all good bye

Best wishes pal!