If its kicking on and off or what you would call "short cycling", its because of one of two things. The compressor has a low-pressure switch and is kicking on when the thermostat is activating the start windings on the compressor terminals and when it cranks up the low pressure switch kicks off, the system 1. could be low of 134a refridgerant,run between 25 and 32 pounds or whatever your truck requires, 2. could have a restriction somewhere in the system, maybe in the liquid line dryer or the a moisture build up in one of the line sets due to a leak or past leak in the system that never was evacuated fully. Now hear me this, you may know or not know, but before you fill it back up again make sure you pull a deep vac on it to rid the system of that moisture, you never know it might fix your problem.