2006 Dodge Ram 2500 Diesel 5.9 Cummins
There is no relay. The pressure sensor sends signal to PCM, then the PCM sends signal to the TIPM or total integrated power module which is sort of self contained fuse box but really a micro computer that controls many things. The Module has had known issues that can affect one of many things. I lean to bad module. Here is the diagram for compressor. 
The Integrated Power Module (IPM) is a combination of the Power Distribution Center (PDC) (1) and the Front Control Module (FCM) (2). The PDC mates directly with the FCM to form the IPM. The PDC is a printed circuit board based module that contains fuses and relays, while the FCM contains the electronics controlling the IPM and other functions. The IPM is located in the engine compartment, next to the battery and connects directly to the battery positive via a stud located on top of the unit. The ground connection is via electrical connectors. The IPM provides the primary means of voltage distribution and protection for the entire vehicle.
The molded plastic IPM housing includes a base and cover. The IPM cover is easily opened or removed for service access by unscrewing the cover retaining nut and has a fuse and relay layout map integral to the inside surface of the cover. This IPM housing base and cover are secured in place via bolts to the left front fender support assembly.
Replaceable components of the IPM assembly are broken down into the following components:
All of the current from the battery and the generator output enters the integrated power module via a stud on the top of the module. The integrated power module cover is removed to access the fuses or relays. Internal connections of all of the power distribution center circuits is accomplished by a combination of bus bars and a printed circuit board.
2006 and later Dodge Trucks have a Totally Integrated Power Module, TIPM. This module is used instead of a fuse box, and there is no relay controlling the A/C as in previous generations. This TIPM has been very problematic for Dodge, particularly with the 5.9 Diesel trucks. The TIPM controls not only the air conditioning, but also brakes, lights, and various other items. There are several flashes and updates available from the dealer for this module. If the vehicle is experiencing erratic or no A/C clutch engagement, it is necessary to examine the TIPM. Keep in mind, the clutch coil on this compressor has an embedded diode which also commonly fails, many times in conjunction with the TIPM, so don't forget to test resistance.
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Please see link below for the illustration of where it is located.
http://autorepair.about.com/library/images/bl136a-lib.htm
It is also possible that there is some wiring issue should the relay be OK.
The AC relay should still be under the hood fuse box. On the cover there should be the labels for each. It is one of the big black ones. You can swap it with the horn or the ABS to see if it works. If it does then it is a faulty relay. You should also see fuses and you may have them checked if blown.
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This is coming up for a Dodge caravan. I need to locate this in my Dodge Ram 2500 diesel.
I am having issues with my a/c in a 2006 Dodge 2500 diesel. It stopped blowing cold. I need help locating the relay switch for the a/c. One Fixya expert advised "The AC relay should still be under the hood fuse box. On the cover there should be the labels for each. It is one of the big black ones. You can swap it with the horn or the ABS to see if it works. If it does then it is a faulty relay. You should also see fuses and you may have them checked if blown." However my fuses are clear not black. Please I need more. The heat index in Louisiana yesterday 7-11-11 was 117 degrees. Anxiously awaiting.
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