I have a 1998 dodge 3500. The high pressure side is about 200 and the low side is about 40. When the compressor cuts in it takes the low side down to about 20 then cuts out ( which takes less than 10 sec) I replaced the cycle switch (on top of the dryer) and it made no difference . When I unplug the switch and put a jumper wire across the plug the the system runs constantly and cools well. The low side stays about 15 lbs. Any ideas?

 

Your pressure is low. Need to add some refrigerant. Welcome to DF!

 

How high should the high side be on a hot day then?

 

Most of the "Self fill" cans go on the low side, and it will have a chart on the can telling you what low-side pressure should be vs. ambient temp. Compressor needs to be running when check the pressure as well.

So long as you don't run into the high pressure switch killing the compressor, I don't pay that much attention to high-side when just topping a system off.